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OK.  Today we're going to get into
some stuff where we're kind of

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peering way back in evolution about
how life first learned to make

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energy.  But before we do that I
just want to finish up talking a

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little bit more about enzymes,
the biological catalysts that are

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critical for life to exist and about
how energy is stored.

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I want to clarify a point that
clearly confused a couple of you in

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the last lecture.
So the Gibbs free energy that we

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talked about can tell us that a
reaction could go and,

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in this case, would actually release
energy if it occurred,

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if these reactants were converted to
these products.

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But the problem for most reactions
is that in order for the reaction to

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take place there's a state in the
middle, some chemical state known as

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the transition state which is
energetically less favorable than

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either the reactants
or the products.

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And if A and B are going to convert
to C and D they have to probably

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start coming together in some kind
of way, and that becomes

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energetically unfavorable.
And this gives this activation

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energy --

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-- or delta G00.
And if the cell wants chemical

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reactions to take place at 25
degrees Centigrade aqueous solution

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it has to do something about that,
and so it employs biological

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catalysts.  And what a catalyst does,
as you've heard in chemistry,

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is it lowers the activation energy
in some way so that the molecules

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have enough energy just within their
normal energy distribution at that

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temperature to get over the hump.
These biological catalysts come in

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two flavors.  As I said there are
enzymes which are made of protein

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with all of those amino acids,
the side chains that we talked about

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when the thing folds up in
3-dimensional space form a little

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chemical environment that enables
that activation energy to be lowered.

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There are also few --
Not so many.  But we now know that

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there are catalysts made out of RNA.
These are called ribozymes.  There

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are not so many of them but they're
important.  Now,

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the characteristics of these that
are important is the specificity --

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Each enzyme or ribozyme is highly

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specific for a given reaction.
So that means the reaction probably

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will barely go unless that enzyme or,
in some cases,

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ribozyme is present.
And so that's really the secret to

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how cells control all of these many,
many, many hundreds or thousands of

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chemical reactions that take place
that are necessary for life.

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Because what they need to do,
and if they want to control whether

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reaction takes place or not is they
control the availability or the

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activity of an enzyme.
And when we talk about gene

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regulation you'll see,
for example, one way a cell might do

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it is to not even bother to make the
enzyme unless it wants a particular

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reaction to take place.
Or it could take an enzyme that's

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there and put little bells and
whistles on it that make it more

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active or less active.
And we'll see an example of that

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pretty soon.  That is the secret to
how cells are then able to

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regulate metabolism.

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And these biological catalysts use a
whole variety of different molecular

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mechanisms, although all of them
follow this principle of what

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they're trying to do is lower the
activation energy.

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So I'll just give you an example.
I showed you of how one particular

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enzyme does it just in sort of
cartoon form.  I gave you the

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example of glutamate being converted
to glutamine.  Now,

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both of those are amino acids that
are critical for making proteins.

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The cell has to make both of them.
But as I showed you converting

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glutamate to glutamine is
energetically unfavorable.

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It's got a delta G plus 7.
And then I showed you if you had an

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ATP going to ADP at the same time
you could actually drive the whole

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reaction forward because there was a
net gain.  But how is that actually

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accomplished?  And it's the enzyme
that carries this out.

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And I'll just show you,
as I say, in sort of cartoon form.

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The way the enzyme works it has one
binding pocket for glutamic

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acid or glutamate.
It fits in here.

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It makes lots of specialized
contacts, all those sort of

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molecular interactions we're talking
about.  And it also binds this

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molecule adenine triphosphate or ATP
which is an adenine,

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a ribose and then three phosphates
joined together.

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And it makes interactions along
here that enable it to bind very

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specifically.  Now,
by providing all this binding energy

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for ATP and for glutamic acid what
the enzyme has done is positioned

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the carboxyl group of glutamic acid
right next to the last

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phosphate on the ATP.
This enables this to form a bond

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here which liberates ADP and leaves
you now with the glutamic acid with

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a phosphate on.
That reaction goes forward because

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you broke the bond of ATP,
but this is still a pretty unhappy

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molecule.  It's got a lot of oxygens
at very close proximity.

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So the enzyme has another binding
pocket that's absolutely specific

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for ammonia.  It won't fit water
which is very close,

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which is a good thing because that
would just reverse the process.

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Ammonia gets in there and then it
attacks here and liberates the

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phosphate.  And that then gives you
glutamine and the inorganic

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phosphate.  So the enzyme has
provided this binding surface that

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makes the reactions go under
biological conditions.

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But it's also managed in the same
process to have it go by a mechanism

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in which it sort of temporarily
captured that energy that's in the

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ATP bond and then used it to drive
the rest of the reaction.

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I mean it's the magic of how all of
this developed.

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It's really amazing but that's how
every single biochemical step in

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your body takes place.
Virtually of them require an enzyme

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that in some way is highly tuned to
do just the one single reaction.

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As I said, the principle of how
these enzymes work is they lower the

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activation energy.
And the way they do that in general

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is they provide a binding pocket
that resembles the transition state.

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So as things approach here then it
fits best into the pocket and

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therefore you get some energy back
and kind of lowered the energy hump

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that's necessary to go over.
And here's a reaction I'll be

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showing you in today's lecture.
It's going to involve the transfer

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of a phosphate to a glucose.
And the first thing that happens is

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this enzyme interacts with ATP and
takes one of the phosphates and

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attaches it to one of its aspartic
acid carboxyl groups.

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So you've got actually a mixed in
hydride if you know chemistry.

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But again it's captured that
phosphate.  This is a very unstable

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bond.  And so if you break it you
will release energy.

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And what the enzyme does is it
allows the hydroxyl of here to come

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and attack this phosphate,
and that then releases the aspartate

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of the enzyme and you end up
affecting the transfer of the

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phosphate that began life on ATP.
And now it ends up on the glucose.

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But, as you can see here, phosphate
interacts with four atoms.

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But as this hydroxyl comes in it
has to attack the phosphate.

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And somewhere in the middle there's
an intermediate where all of these

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things are interacting.
And some crystallographers actually

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managed to capture that in a crystal
structure.  And here you can see

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this is the oxygen coming from the
sugar, this is the oxygen of the

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aspartate and here is the phosphate
where it's now,

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as the attack is taking place the
thing is sort of pushed out,

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and it's caught right at that
transition state.

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And that's what the enzyme is
providing a binding pocket for and

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thereby lowering the activation
energy.  It's a really beautiful

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piece of structural work.
The second thing then I want to

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clarify was this molecule ATP which
is, as I say, like energy money for

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the cell.  When there's a reaction
where it can extract energy it tries

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to make ATP.  And when there's a
reaction that doesn't want to go it

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will somehow figure out a way to
spend that energy and make the

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reaction go forward.
And the molecule, just to put it

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again, because it's a pretty
important one in biology.

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That's adenine which you already saw
when we talked about nucleic acids.

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And it's got three phosphates like
this.  You can see it's probably a

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pretty unhappy molecule because it's
got all of these oxygens stuck

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together.  And if you break this
bond then you release some energy.

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So you could think of it in this
kind of way.

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That if we have ADP,
which is adenosine diphosphate plus

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inorganic phosphate,
and ATP is here.  And if you were to

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break the bond and make it back into
ADP and inorganic phosphate then you

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would have gone energetically
downhill.

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But in order to make this you could
think of it as taking an inorganic

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phosphate ion and this ADP,
if you start pushing them together

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the negative charges are going to
repel and you kind of go up an

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energy hill.  But if you ever get
them close enough then they start to

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share electrons and they fall into
this sort of energy well.

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And this is what ATP is.
And so it's sort of like taking a

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spring and pushing it together.
And then when you form the bond

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it's like you put a little hook on
it.  And now you've got this spring

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that's compressed.
And it's stable, it won't do

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anything, but there's energy stored
in there that you can use.

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And it's the same principle in
terms of how the cell stores energy

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within ATP.  And this
energy is stored --

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-- if you think of it in bundles of

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about 12 kilocalories per mole.
That's about how much energy is

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released under physiological
conditions when you hydrolyze that

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bond.  So hydrolyzing ATP to give
ADP plus inorganic phosphate will

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have a delta G of minus 12
kilocalories per mole under

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physiological conditions.
Now something in terms of evolution,

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which I know a number of you said
you were interested in,

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here's a really interesting thing.
This is the main energy storage

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molecule for the cell,
but you've heard about it before

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because adenosine,
that's the nucleotide that we find

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in RNA.  And, in fact,
ATP is also the precursor,

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as we'll learn, for making RNA.
And one of the things that puzzled

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scientists for many years is how did
life ever get started

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in the first place?
There seemed to be a chicken and an

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egg issue that proteins did the work
and DNA stored the information and

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RNA was kind of a messenger in
between, and we'll talk a lot about

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that in between.
So how could you ever get life

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started?  So the current thinking is
that sometime,

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if you remember in that first
lecture, we had about 4.

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billion years ago the first
organism, something like today's

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bacterium showed up here about maybe
3.8 billion years ago.

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That somewhere in between there was
what people are now thinking of as

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an ìRNA worldî where RNA managed to
act as a ribosome and catalyzed

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chemical reactions,
but it also had the capacity to

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store information.
But it's sort of intriguing,

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although no one has proven that.
It's just a hypothesis.  We also

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see that the major energy storage
molecule found in all living things

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is also a building block of RNA.
It certainly sort of fits with that

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kind of idea.
Now, there's one other kind of

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reaction I'm going to have to tell
you about.  Penny will talk quite a

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bit about this when you're thinking
about how organisms make living.

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But this is a set of reactions
known as ìredox reactionsî.

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So the loss of one or more

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electron(s) is called an oxidation.
And the gain of one or more

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electron(s) is called a reduction.
If you're going to take away an

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electron somebody else has to get it.
So these things always happen

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together.  And therefore they're
given the term redox reactions where

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electron(s) from somebody goes to
somebody else.

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So somebody gets oxidized and
somebody gets reduced in

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the same reaction.
And you can think of them as a

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transfer of hydrogen atoms,
not hydrogon ions.  And the most

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familiar kind of sequence that you
will see over and over again in

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biology is the sequence you go from,
let's say, a methyl group to an

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alcohol with a hydroxyl to an
aldehyde or a ketone with the double

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bond oxygen to a carboxyl group.
You go one more step then it's CO2.

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So going in this direction it's an
oxidation.  If it's going in that

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direction the molecules are getting
reduced.  Just the same way that the

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cell and life have molecules that
store energy in ATP,

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they also have an important molecule
that stores electrons.

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00:16:03,000 --> 00:16:14,000
And that molecule is known as NAD or
nicotinamide adenine dinucleotide.

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00:16:14,000 --> 00:16:37,000
NAD+.  And its structure is,

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00:16:37,000 --> 00:16:46,000
let's say a ribose, a five carbon
sugar.  And it's got this entity on

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it.  This is in your book so don't
worry if you don't get

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00:16:55,000 --> 00:17:03,000
the structure down.
There's a positive charge on the

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00:17:03,000 --> 00:17:11,000
nitrogen here.
And it's joined through a

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diphosphate linkage to,
guess what?  Another molecule of

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00:17:19,000 --> 00:17:27,000
adenosine.  Here we find again a
piece of a thing we find in RNA is

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00:17:27,000 --> 00:17:35,000
now part of this system
for storing electrons.

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And the way this works is if you
have two hydrogen atoms transferred

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00:17:45,000 --> 00:17:55,000
to here then this entity right here
goes to this plus a hydrogen ion.

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And this we would know is NADH.
I left out an oxygen here.

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00:18:02,000 --> 00:18:08,000
Somebody picked it up.  [LAUGHTER]
Just too excited by the annual

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00:18:08,000 --> 00:18:14,000
Valentine's Day visit here.
I wish the rest of you had a song

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for you, too, but we didn't have
time to set that up.

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00:18:20,000 --> 00:18:26,000
So there's an important thing here,
too, because actually a lot of

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00:18:26,000 --> 00:18:32,000
energy is stored in there.
This is a bundle of energy in this

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00:18:32,000 --> 00:18:38,000
molecule that's actually about 50
kilocalories per mole.

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00:18:38,000 --> 00:18:43,000
And especially when we get to next
week's lecture you'll see how cells

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00:18:43,000 --> 00:18:48,000
go about extracting the energy out
of that and making that energy into

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00:18:48,000 --> 00:18:53,000
ATP, which is sort of a universal
currency the cells can spend.

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00:18:53,000 --> 00:18:59,000
Now, somebody asked about
memorizing all of these structures

220
00:18:59,000 --> 00:19:02,000
I mean really,
As Julia says in her thing,

221
00:19:02,000 --> 00:19:05,000
we're trying to get you to focus on
the concepts here.

222
00:19:05,000 --> 00:19:09,000
You won't have to memorize the
structure of everything.

223
00:19:09,000 --> 00:19:12,000
It would be helpful if you
recognized that glutamine and

224
00:19:12,000 --> 00:19:15,000
glutamate are of the 20 amino acids,
but we'll give you the structures

225
00:19:15,000 --> 00:19:19,000
and we'll give you the structures of
something like NADH if you needed to

226
00:19:19,000 --> 00:19:22,000
do something with it.
But the important thing is to

227
00:19:22,000 --> 00:19:25,000
remember that energy is stored in
that high energy bond of ATP,

228
00:19:25,000 --> 00:19:29,000
that electrons are stored in this
NADH, and they can be used in

229
00:19:29,000 --> 00:19:33,000
reactions that oxidize or reduce.
NAD and NADH can be used in

230
00:19:33,000 --> 00:19:38,000
reactions that remove or give
electrons to biomolecules.

231
00:19:38,000 --> 00:19:44,000
Now, the same thing goes for what
I'm about to tell you now because

232
00:19:44,000 --> 00:19:49,000
one of the first things that had to
happen as life evolved was there had

233
00:19:49,000 --> 00:19:54,000
to be some mechanism of getting
energy made.  And the reaction I'm

234
00:19:54,000 --> 00:20:00,000
going to tell you about
is called glycolysis.

235
00:20:00,000 --> 00:20:07,000
And it's a way of taking a molecule
of glucose through a whole series of

236
00:20:07,000 --> 00:20:15,000
biochemical transformations and to
end up yielding --

237
00:20:15,000 --> 00:20:26,000
-- two molecules of something that's

238
00:20:26,000 --> 00:20:32,000
known as pyruvate.
And it also makes two molecules of

239
00:20:32,000 --> 00:20:37,000
ATP and two molecules of NADH.
So it's a way that was invented in

240
00:20:37,000 --> 00:20:42,000
evolution of making ATP by carrying
out a chemical transformation.

241
00:20:42,000 --> 00:20:48,000
And this is basically the same
chemical transformation that we've

242
00:20:48,000 --> 00:20:53,000
been talking about that Lavoisier
and Pasteur studied except that,

243
00:20:53,000 --> 00:20:58,000
as I'll show you, you do a little
bit to convert it either to lactate

244
00:20:58,000 --> 00:21:02,000
or to ethanol.
I'll get to that in a few minutes.

245
00:21:02,000 --> 00:21:06,000
Remember the point, also just to
remind you, the reason I gave you

246
00:21:06,000 --> 00:21:10,000
that historical thing is because
what it turned out when people

247
00:21:10,000 --> 00:21:14,000
started out to study something,
was winemaking of great interest to

248
00:21:14,000 --> 00:21:18,000
French scientists,
was what they actually learned was

249
00:21:18,000 --> 00:21:22,000
how cells made energy.
And, in fact, here we're looking at

250
00:21:22,000 --> 00:21:26,000
a sort of biochemical fossil in a
way because this pathway of

251
00:21:26,000 --> 00:21:30,000
glycolysis, which you'll
see is kind of awkward.

252
00:21:30,000 --> 00:21:35,000
It's got ten different biochemical
steps, it needs ten different

253
00:21:35,000 --> 00:21:40,000
enzymes, and what the cells got out
of it is two molecules of ATP.

254
00:21:40,000 --> 00:21:45,000
But this system developed
apparently way,

255
00:21:45,000 --> 00:21:50,000
way back in evolution before life
forms got into these various

256
00:21:50,000 --> 00:21:55,000
Kingdoms because it's in virtually
in every living creature no matter

257
00:21:55,000 --> 00:22:00,000
what it is and it's essentially
biochemically identical.

258
00:22:00,000 --> 00:22:03,000
Now, it's possible we could go back
nowadays and devise a better method,

259
00:22:03,000 --> 00:22:07,000
but once that something like that
gets fixed in evolution,

260
00:22:07,000 --> 00:22:10,000
if something mutates to try and
change it most of the time it's a

261
00:22:10,000 --> 00:22:14,000
disadvantage.  And so if something
gets locked in,

262
00:22:14,000 --> 00:22:17,000
and this is true of many,
many of these very complicated

263
00:22:17,000 --> 00:22:21,000
biochemical pathways.
So you won't have to remember all

264
00:22:21,000 --> 00:22:24,000
these structures I'm going to put on
the board, but try and stay with me

265
00:22:24,000 --> 00:22:28,000
because I want to sort of show you
one of these.  This is probably the

266
00:22:28,000 --> 00:22:31,000
most ancient of these pathways.
And it's still in all of us.

267
00:22:31,000 --> 00:22:35,000
It's in the bacteria in our guts.
It's in the plants in the field.

268
00:22:35,000 --> 00:22:39,000
If you go out in the open ocean
organisms still can carry out

269
00:22:39,000 --> 00:22:43,000
glycolysis.  So one thing,
though, I want to try and put it in

270
00:22:43,000 --> 00:22:46,000
this way, if I came to you and said
I've got the greatest idea.

271
00:22:46,000 --> 00:22:50,000
This is going to be how we're going
to make energy and evolution as part

272
00:22:50,000 --> 00:22:54,000
of this entrepreneurship,
I think you'd be right to be

273
00:22:54,000 --> 00:22:58,000
skeptical so I'll probably sort of
tell you in that way.

274
00:22:58,000 --> 00:23:02,000
So I've already shown you how to
write glucose in a linear form,

275
00:23:02,000 --> 00:23:07,000
although I then told you that most
of the time in solution it's

276
00:23:07,000 --> 00:23:12,000
cyclized into a pyranose ring,
a six membered ring.  But for the

277
00:23:12,000 --> 00:23:17,000
moment we can think of glucose as a
stick.  And I'll get you to just

278
00:23:17,000 --> 00:23:22,000
focus on the one position,
the two position and the six

279
00:23:22,000 --> 00:23:27,000
position in that linear thing.
If you look back at your notes you

280
00:23:27,000 --> 00:23:32,000
can see what the full structure of
glucose looks like.

281
00:23:32,000 --> 00:23:39,000
But this is how the process of
glycolysis starts.

282
00:23:39,000 --> 00:23:47,000
This is if your body is going to
take a molecule of glucose and make

283
00:23:47,000 --> 00:23:55,000
energy out of it,
this is the first thing it does.

284
00:23:55,000 --> 00:24:03,000
It takes an ATP.  It converts it to
an ADP.  It puts the phosphate down

285
00:24:03,000 --> 00:24:08,000
here to give glucose-6- phosphate.
That's the only thing that changes.

286
00:24:08,000 --> 00:24:11,000
Isn't this just like most young
entrepreneurs?

287
00:24:11,000 --> 00:24:14,000
Give them some venture capital.
The first thing they do is spend it,

288
00:24:14,000 --> 00:24:17,000
buy a nice potted plant for the
company they're building.

289
00:24:17,000 --> 00:24:21,000
It doesn't seem to be, if you want
to make energy,

290
00:24:21,000 --> 00:24:24,000
starting out here spending energy is
the first thing that the cell is

291
00:24:24,000 --> 00:24:27,000
doing.  It's using up an ADP,
although the overall goal is to make

292
00:24:27,000 --> 00:24:32,000
ADP.
It then does a little shuffle,

293
00:24:32,000 --> 00:24:38,000
reverses the position of the double
bond and the hydroxyl.

294
00:24:38,000 --> 00:24:44,000
This is an energetically something
without much cost,

295
00:24:44,000 --> 00:24:51,000
but this sugar is different because
this is now fructose-6-phosphate.

296
00:24:51,000 --> 00:24:57,000
It's got a little bit different
arrangement of the double bond and

297
00:24:57,000 --> 00:25:04,000
the hydroxyl, but energetically it's
pretty much the same thing.

298
00:25:04,000 --> 00:25:10,000
Then the next thing that happens the
cells spends another molecule of ATP.

299
00:25:10,000 --> 00:25:25,000
It gives now --

300
00:25:25,000 --> 00:25:31,000
-- fructose-1,
, this is the sixth position,

301
00:25:31,000 --> 00:25:37,000
the one position to two position,
1,6-diphosphate.

302
00:25:37,000 --> 00:25:41,000
It doesn't look like we're on our
way to make energy yet.

303
00:25:41,000 --> 00:25:46,000
Cells invested two molecules of ATP
and what it's done is it's got this

304
00:25:46,000 --> 00:25:50,000
glucose transformed to fructose 1,
-disphopshate.  Well, what happens

305
00:25:50,000 --> 00:25:55,000
now then is the cell splits this
into two three carbon units.

306
00:25:55,000 --> 00:26:00,000
There were six carbons in glucose.
Yeah?

307
00:26:00,000 --> 00:26:10,000
Well, it's a linear molecule.

308
00:26:10,000 --> 00:26:14,000
There's a phosphate here and a
separate phosphate down there.

309
00:26:14,000 --> 00:26:19,000
They should be.  Yeah.  I'm
probably dropping charges and

310
00:26:19,000 --> 00:26:23,000
hydroxyls, OK?
But check your book if you notice

311
00:26:23,000 --> 00:26:28,000
something like that.
So what we get --

312
00:26:28,000 --> 00:26:32,000
What the cell gets out of this then
are two three carbon units,

313
00:26:32,000 --> 00:26:45,000
one of which is this --

314
00:26:45,000 --> 00:26:49,000
-- known as dihydroxyacetone
phosphate.  And you can find these

315
00:26:49,000 --> 00:26:54,000
names in your book.
You don't have to, as I say,

316
00:26:54,000 --> 00:26:58,000
remember the structures.
What I've done is basically taken

317
00:26:58,000 --> 00:27:02,000
this molecule and I've flipped it
over so that the phosphate will be

318
00:27:02,000 --> 00:27:05,000
down.  And you'll see why I've done
that in a second.

319
00:27:05,000 --> 00:27:09,000
And from the bottom half of the
molecule then we get --

320
00:27:09,000 --> 00:27:37,000
This is glycereldahyde-3-phosphate.

321
00:27:37,000 --> 00:27:41,000
So this is three carbons.
This is three carbons.  This was

322
00:27:41,000 --> 00:27:45,000
six carbons.  So the cell has split
it into these three carbon units

323
00:27:45,000 --> 00:27:49,000
that are very similarly related
except where the double bond is.

324
00:27:49,000 --> 00:27:53,000
And there's an enzyme that actually
catalyzes the conversion of those

325
00:27:53,000 --> 00:27:58,000
two.  It's a catalytically perfect
enzyme that goes.

326
00:27:58,000 --> 00:28:01,000
It's just limited by the rate of
diffusion.  And it can do something

327
00:28:01,000 --> 00:28:05,000
of the order of ten to the eighth
molecules a second.

328
00:28:05,000 --> 00:28:09,000
It's a really, really efficient
catalyst.  So what happens then is

329
00:28:09,000 --> 00:28:13,000
this, since these are in equilibrium
the cell is going to now

330
00:28:13,000 --> 00:28:24,000
start to pull these --

331
00:28:24,000 --> 00:28:28,000
This.  But these will be converted
into that and will be able to get

332
00:28:28,000 --> 00:28:32,000
here.  So we're going to follow the
fait then of these --

333
00:28:32,000 --> 00:28:46,000
-- two glycaraldahyde-3-phosphate

334
00:28:46,000 --> 00:28:56,000
molecules.  Excuse me.
Sorry.  OK.  Now, at this point the

335
00:28:56,000 --> 00:29:04,000
cell is at the aldehyde stage.
And it's going to carry out an

336
00:29:04,000 --> 00:29:08,000
oxidation reaction.
So it's going to take a couple of

337
00:29:08,000 --> 00:29:13,000
electrons away from here,
and it's going to therefore be

338
00:29:13,000 --> 00:29:17,000
carrying out an oxidation.
If the molecule is getting oxidized

339
00:29:17,000 --> 00:29:22,000
something else has to be reduced.
What's going to get reduced is NAD+.

340
00:29:22,000 --> 00:29:26,000
We'll need two molecules of that
because we've got two molecules of

341
00:29:26,000 --> 00:29:31,000
glyceraldehydes phosphate.
So we end up with two molecules of

342
00:29:31,000 --> 00:29:35,000
NADH plus a hydrogen ion.
And this is an energetically

343
00:29:35,000 --> 00:29:40,000
favorable reaction.
So the cell is able to sneak a

344
00:29:40,000 --> 00:29:45,000
phosphate in and make a molecule and
still have the reaction go forward,

345
00:29:45,000 --> 00:29:49,000
have a molecule that's not very
stable, but it can make it because

346
00:29:49,000 --> 00:30:00,000
the overall thing goes forward.

347
00:30:00,000 --> 00:30:04,000
And there are two of these.
And what we have now is

348
00:30:04,000 --> 00:30:09,000
1,3-phosphoglycerate.
What the cell has basically managed

349
00:30:09,000 --> 00:30:14,000
to do is to get two phosphate groups
very, very close together.

350
00:30:14,000 --> 00:30:19,000
So you're probably getting,
hopefully, the concept that if you

351
00:30:19,000 --> 00:30:24,000
stick a bunch of negative charges
together and hold them together that

352
00:30:24,000 --> 00:30:29,000
molecules, if you break one of those
bonds are going to go energetically

353
00:30:29,000 --> 00:30:35,000
downhill.
And you can do work.

354
00:30:35,000 --> 00:30:41,000
And the way it does that then is in
breaking this bond it uses it to

355
00:30:41,000 --> 00:30:48,000
make two molecules of ATP.
So you've now got, this is up at

356
00:30:48,000 --> 00:31:02,000
the acid level or a carboxyl group.

357
00:31:02,000 --> 00:31:07,000
And we've got three
phosphoglycerates.

358
00:31:07,000 --> 00:31:12,000
So at least from the point of view
of this as a plan for making energy,

359
00:31:12,000 --> 00:31:18,000
we've now managed to get back those
two ATPs we invested.

360
00:31:18,000 --> 00:31:23,000
So up until now we've got our,
the venture capital money we put in

361
00:31:23,000 --> 00:31:29,000
has be recovered,
and we've got a couple of molecules

362
00:31:29,000 --> 00:31:34,000
of NADH out of it.
But what the cell now does is finish,

363
00:31:34,000 --> 00:31:39,000
to carry out some more steps that
let it make a couple more molecules

364
00:31:39,000 --> 00:31:44,000
of ATP.  So the first step then is a
kind of just a switcheroo between

365
00:31:44,000 --> 00:31:50,000
where this hydroxyl is and this
phosphate is.  So it brings the

366
00:31:50,000 --> 00:31:55,000
phosphate up to here.
As you might guess this is

367
00:31:55,000 --> 00:32:04,000
energetically not much of a change.

368
00:32:04,000 --> 00:32:10,000
However, what it does now is it
enables the cell to eliminate a

369
00:32:10,000 --> 00:32:16,000
molecule of water from here so we
get two molecules of water come out

370
00:32:16,000 --> 00:32:22,000
because we had all along here we're
carrying on two molecules from up

371
00:32:22,000 --> 00:32:28,000
there because we have two of these
three carbon units.

372
00:32:28,000 --> 00:32:34,000
Then the molecule that
we then get here --

373
00:32:34,000 --> 00:32:44,000
-- is this molecule which is known

374
00:32:44,000 --> 00:32:52,000
as phosphoenolpyruvate.
And several of you are saying you

375
00:32:52,000 --> 00:33:00,000
don't remember much from chemistry.
So this is a keto group, which I

376
00:33:00,000 --> 00:33:06,000
know you were introduced to.
But it's in an equilibrium with

377
00:33:06,000 --> 00:33:10,000
what's termed an enol form where you
have an OH here,

378
00:33:10,000 --> 00:33:15,000
a double bond like that,
and that's known as an enol.

379
00:33:15,000 --> 00:33:20,000
Now, this is energetically greatly
disfavored.  So normally most of the

380
00:33:20,000 --> 00:33:24,000
time you find something in a keto
form, but occasionally you find it

381
00:33:24,000 --> 00:33:29,000
in an enol form.
And what's happened here really is

382
00:33:29,000 --> 00:33:34,000
the cell has trapped what would like
to be a keto at this position in an

383
00:33:34,000 --> 00:33:38,000
enol form.  Again,
this is a very energetically

384
00:33:38,000 --> 00:33:43,000
unstable molecule.
You've got all these oxygens

385
00:33:43,000 --> 00:33:48,000
together, two of these,
and so the cell is once again able

386
00:33:48,000 --> 00:33:53,000
to take ADP and make two molecules
of ATP.  And we end up with --

387
00:33:53,000 --> 00:34:05,000
-- two molecules of pyruvate.

388
00:34:05,000 --> 00:34:13,000
And extraordinary amount of work.
What do we get out of it?  Well,

389
00:34:13,000 --> 00:34:21,000
we've got a total of four ATPs now
plus two NADHs.

390
00:34:21,000 --> 00:34:29,000
What did we invest?
Two ATPs.  So the net yield from

391
00:34:29,000 --> 00:34:37,000
this reaction is two ATPs
plus two NADHs.

392
00:34:37,000 --> 00:34:43,000
So strange as it seems this was one
of the first sequences of

393
00:34:43,000 --> 00:34:49,000
biochemical steps that were put
together in a pathway that we're

394
00:34:49,000 --> 00:34:55,000
capable of letting an organism
generate molecules of ATP,

395
00:34:55,000 --> 00:35:01,000
or sort of form of energy money,
but metabolizing something it could

396
00:35:01,000 --> 00:35:06,000
find like a molecule of sugar.
There are two enzymatic steps.

397
00:35:06,000 --> 00:35:10,000
That means that there has to be a
separate enzyme for every step in

398
00:35:10,000 --> 00:35:15,000
the pathway.  Now,
the ATPs, as I said,

399
00:35:15,000 --> 00:35:19,000
have energy in bundles of about 12
kilocalories per mole.

400
00:35:19,000 --> 00:35:24,000
There's a lot of energy here in
NADH.  And in the next lecture I'm

401
00:35:24,000 --> 00:35:28,000
going to talk to you about
respiration, which is something

402
00:35:28,000 --> 00:35:33,000
you're aware of.
You know we respire,

403
00:35:33,000 --> 00:35:38,000
but chemically what that we'll see
means is basically it's a way of

404
00:35:38,000 --> 00:35:43,000
extracting the energy that's in the
NADH by transferring electrons to

405
00:35:43,000 --> 00:35:47,000
oxygen.  And that's a wonderful way
to make energy.

406
00:35:47,000 --> 00:35:52,000
It's far more efficient than this
ancient pathway,

407
00:35:52,000 --> 00:35:57,000
but at the time life started there
wasn't any oxygen in

408
00:35:57,000 --> 00:36:02,000
the environment.
And, in fact, it didn't reach,

409
00:36:02,000 --> 00:36:07,000
as I said, I think it was something
like 20% of today's levels until we

410
00:36:07,000 --> 00:36:12,000
were about a half a billion years or
so ago in evolution.

411
00:36:12,000 --> 00:36:18,000
So organisms had to learn to make
energy without oxygen being around.

412
00:36:18,000 --> 00:36:23,000
And this was the way that they did
it.  And it was such a success in

413
00:36:23,000 --> 00:36:28,000
evolution that our bodies do,
the bacteria in our gut do it and

414
00:36:28,000 --> 00:36:32,000
just virtually all living forms.
So it's sort of a biochemical fossil

415
00:36:32,000 --> 00:36:36,000
but it was so successful it took
hold.  It's sort of like legs.

416
00:36:36,000 --> 00:36:40,000
Those appeared in evolution.  And
there are all sorts of organisms now

417
00:36:40,000 --> 00:36:43,000
that use legs,
and they've evolved into wings and

418
00:36:43,000 --> 00:36:47,000
everything, but it's all the same
basic idea.  You could imagine a

419
00:36:47,000 --> 00:36:51,000
life form that started with wheels.
And maybe if it had been the first

420
00:36:51,000 --> 00:36:54,000
thing to do maybe there'd be some
sort of organisms with wheels,

421
00:36:54,000 --> 00:36:58,000
but legs were such a success at some
point that that's what got used and

422
00:36:58,000 --> 00:37:02,000
then evolution made various
embellishments on it.

423
00:37:02,000 --> 00:37:07,000
But there is a problem here.
I don't know if anybody can see

424
00:37:07,000 --> 00:37:12,000
what it is.  If I'm going to be able
to use ATP to make energy and I want

425
00:37:12,000 --> 00:37:18,000
to keep generating more and more
molecules of ATP so I can build

426
00:37:18,000 --> 00:37:23,000
stuff, I cannot give those electrons
in NADH to oxygen.

427
00:37:23,000 --> 00:37:29,000
So what would happen if I just kept
running this system?

428
00:37:29,000 --> 00:37:33,000
Anybody see what the problem would
be?  Yeah.  You'd run out of NADH,

429
00:37:33,000 --> 00:37:38,000
exactly.  We need to somehow recycle
that NAD so it can take place.

430
00:37:38,000 --> 00:37:43,000
If we could give it to oxygen,
oxygen as I've told you in

431
00:37:43,000 --> 00:37:48,000
respiratory, that would be cool.
But organisms didn't have that

432
00:37:48,000 --> 00:37:52,000
option.  And so they worked out ways
of doing things with pyruvate.

433
00:37:52,000 --> 00:37:57,000
And this is where you'll see this
coming together with what we talked

434
00:37:57,000 --> 00:38:02,000
about the other day.
So let's take those two molecules of

435
00:38:02,000 --> 00:38:07,000
pyruvate.  And there are basically
two strategies,

436
00:38:07,000 --> 00:38:12,000
two major strategies you find in
nature.  One is to take the two

437
00:38:12,000 --> 00:38:18,000
NADHs plus two hydrogen ions and
convert it to two molecules of NAD+

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00:38:18,000 --> 00:38:23,000
so that regenerates it.
And what do you get if you do that?

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00:38:23,000 --> 00:38:31,000
You end up with molecule.

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00:38:31,000 --> 00:38:35,000
Two molecules of that which I
introduced you to the other day.

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00:38:35,000 --> 00:38:40,000
That's lactic acid or lactate, the
organisms that make yogurt carry out.

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00:38:40,000 --> 00:38:45,000
That's what they do.
That's why yogurt goes sour.

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00:38:45,000 --> 00:38:50,000
What the organisms are doing when
they're making the yogurt that you

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00:38:50,000 --> 00:38:55,000
had for lunch,
I love those pictures.

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00:38:55,000 --> 00:39:00,000
I found them on the Web and put
them in.

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00:39:00,000 --> 00:39:04,000
What they're doing really is they're
getting rid of that NADH so that

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00:39:04,000 --> 00:39:08,000
they can do another cycle and make
more energy.  Now,

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00:39:08,000 --> 00:39:13,000
I mentioned that this happens to us,
too.  And this happens in athletic

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00:39:13,000 --> 00:39:17,000
events where you exercise really,
really hard, you know, like sprints

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00:39:17,000 --> 00:39:22,000
or speed skating or something like
that.  Because what happens is

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00:39:22,000 --> 00:39:26,000
you're exercising so hard that you
use up the oxygen in your muscles

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00:39:26,000 --> 00:39:31,000
faster than your bloodstream
can bring you more.

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00:39:31,000 --> 00:39:34,000
So what you're doing is you're
making your muscles go anaerobic.

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00:39:34,000 --> 00:39:38,000
It's like you're going back way,
way in evolution when there's was no

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00:39:38,000 --> 00:39:41,000
oxygen around.
And your muscles have to keep

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00:39:41,000 --> 00:39:45,000
working, so what they do,
since there's no oxygen around,

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00:39:45,000 --> 00:39:49,000
they stick it on pyruvate and you
get lactic acid in your muscles.

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00:39:49,000 --> 00:39:52,000
So if you go out for the track team
in the spring and you haven't

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00:39:52,000 --> 00:39:56,000
exercised and you run a whole lot of
sprints and, God,

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00:39:56,000 --> 00:40:00,000
your muscles are so sore,
they're all full of lactic acid.

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00:40:00,000 --> 00:40:04,000
So you don't have to worry about it
accumulating in your muscles from

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00:40:04,000 --> 00:40:08,000
eating yogurt,
but it does show up in this kind of

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00:40:08,000 --> 00:40:12,000
way.  And the other thing then that
the way nature has found to recycle

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00:40:12,000 --> 00:40:16,000
these NADHs is to it this way,
to carry out a transformation where

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00:40:16,000 --> 00:40:20,000
you get two molecules of carbon
dioxide and two molecules

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00:40:20,000 --> 00:40:30,000
of acetaldehyde.

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00:40:30,000 --> 00:40:37,000
And this can be converted to the two
molecules of carbon dioxide and two

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00:40:37,000 --> 00:40:44,000
molecules of ethanol.
So this is the fermentation that we

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00:40:44,000 --> 00:40:51,000
talked about.  And so when those
yeasts that we saw growing the other

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00:40:51,000 --> 00:40:58,000
day are busy metabolizing sugar into
ethanol and carbon dioxide,

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00:40:58,000 --> 00:41:05,000
the reason they're doing it is they
need to get energy to carry out all

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00:41:05,000 --> 00:41:13,000
the biosynthetic reactions that they
need to make more biomaterial.

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00:41:13,000 --> 00:41:16,000
But what's happening to the whole
system is that you're generating

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00:41:16,000 --> 00:41:19,000
carbon dioxide and making stuff into
ethanol.  So it doesn't matter if

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00:41:19,000 --> 00:41:22,000
people are making wine or beer or
something they're going to distil to

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00:41:22,000 --> 00:41:26,000
make whiskey or brandy or something.
It's all the basic thing.  The

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00:41:26,000 --> 00:41:29,000
yeast take the sugars,
make it into carbon dioxide and to

478
00:41:29,000 --> 00:41:33,000
ethanol.
But when you're making bread you're

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00:41:33,000 --> 00:41:37,000
only really interested in the carbon
dioxide because those little bubbles

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00:41:37,000 --> 00:41:41,000
then expand when you heat it up and
that's what makes bread rise.

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00:41:41,000 --> 00:41:46,000
And that was an open fermentation,
as you can guess, like in making

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00:41:46,000 --> 00:41:50,000
wine.  People like to have a closed
system so that,

483
00:41:50,000 --> 00:41:54,000
for example, a lactic acid bacteria
doesn't get in and turn your whole

484
00:41:54,000 --> 00:41:59,000
set of grapes into something that
would be sour.  Sour wine.

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00:41:59,000 --> 00:42:03,000
So that's where we'll stop today,
the most ancient of these

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00:42:03,000 --> 00:42:08,000
energy-producing things.
Again, you don't have to memorize

487
00:42:08,000 --> 00:42:12,000
all this, but I think,
hopefully if you think about,

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00:42:12,000 --> 00:42:17,000
you'll see some really, really
important concepts that are critical

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00:42:17,000 --> 00:42:21,000
to understanding how life works.
OK?  See you on Wednesday.  Happy

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00:42:21,000 --> 00:42:24,000
Valentine's Day.