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JOHN ESSIGMANN: Let's take
a look at storyboard 14,

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00:00:23,520 --> 00:00:25,500
where I discuss the Q cycle.

10
00:00:25,500 --> 00:00:28,320
If you look back at my previous
lecture in which I introduced

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00:00:28,320 --> 00:00:29,820
the respiratory
apparatus, you'll

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00:00:29,820 --> 00:00:32,430
see that there are three points
in the electron transport

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00:00:32,430 --> 00:00:34,620
chain, at which protons
are translocated

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00:00:34,620 --> 00:00:37,260
across the mitochondrial
inner membrane,

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00:00:37,260 --> 00:00:39,700
into the intermembrane space.

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00:00:39,700 --> 00:00:44,070
The three sites are complex one,
complex two, and complex four.

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00:00:44,070 --> 00:00:46,140
If your entry point
for the electrons

18
00:00:46,140 --> 00:00:48,810
is a pair of electrons
at NADH, you'll

19
00:00:48,810 --> 00:00:51,460
translocate about 10 protons.

20
00:00:51,460 --> 00:00:53,670
This generates the
electrochemical gradient

21
00:00:53,670 --> 00:00:58,530
that will be used in complex
five, the F naught f1 synthase.

22
00:00:58,530 --> 00:01:01,650
And the energy released in
proton translocation back

23
00:01:01,650 --> 00:01:06,187
into the cytoplasm, eventually
will be used to synthesize ATP.

24
00:01:06,187 --> 00:01:07,770
At this point, I'd
like to take a look

25
00:01:07,770 --> 00:01:10,170
at the hypothetical
mechanism by which proton

26
00:01:10,170 --> 00:01:12,720
translocation into the
intermembrane space

27
00:01:12,720 --> 00:01:15,150
happens in complex three.

28
00:01:15,150 --> 00:01:17,790
At the outset, let me say
that the mechanism here

29
00:01:17,790 --> 00:01:20,070
is different from the
mechanism at complex one

30
00:01:20,070 --> 00:01:21,150
and complex four.

31
00:01:21,150 --> 00:01:24,900
And I'll also mention that other
organisms have found other ways

32
00:01:24,900 --> 00:01:27,900
to solve this problem
of translocating protons

33
00:01:27,900 --> 00:01:30,730
into the intermembrane space.

34
00:01:30,730 --> 00:01:33,900
Let's look at panel
A of storyboard 14.

35
00:01:33,900 --> 00:01:37,470
At the upper left, we see
coenzyme Q in its quinone form.

36
00:01:37,470 --> 00:01:40,350
In this form, it's
a taxadiene dione.

37
00:01:40,350 --> 00:01:41,815
It's one of our co-factors.

38
00:01:41,815 --> 00:01:44,760
And we've also referred to it
in the past as a mobile electron

39
00:01:44,760 --> 00:01:45,840
carrier.

40
00:01:45,840 --> 00:01:48,720
In that regard, it falls into
the same grouping of molecules,

41
00:01:48,720 --> 00:01:51,860
such as the NAD plus, NADH pair.

42
00:01:51,860 --> 00:01:53,460
The squiggly line
at the lower right

43
00:01:53,460 --> 00:01:56,730
corner of the quinone molecule
depicts isoprene units.

44
00:01:56,730 --> 00:01:59,040
Actually, there's a
series of about six to 10

45
00:01:59,040 --> 00:02:01,650
of these isoprenes
in the Q molecule.

46
00:02:01,650 --> 00:02:04,380
These probably
facilitate association

47
00:02:04,380 --> 00:02:07,890
with hydrophobic regions,
such as regions in membranes.

48
00:02:07,890 --> 00:02:10,350
The structure of
coenzyme Q allows

49
00:02:10,350 --> 00:02:13,690
it to pick up electrons one at
a time from an electron donor.

50
00:02:13,690 --> 00:02:15,601
And deliver them one
at a time to a place

51
00:02:15,601 --> 00:02:17,100
where the electrons
are going to go.

52
00:02:17,100 --> 00:02:19,620
That is an electron recipient.

53
00:02:19,620 --> 00:02:22,140
This panel shows the
step-by-step mechanism

54
00:02:22,140 --> 00:02:24,240
by which electrons,
one at a time,

55
00:02:24,240 --> 00:02:29,160
can reduce the oxidized form
of coenzyme Q. Called Q here,

56
00:02:29,160 --> 00:02:34,230
or the quinone, to its fully
reduced form QH2, also known

57
00:02:34,230 --> 00:02:35,610
as the hydroquinone.

58
00:02:35,610 --> 00:02:38,520
These electrons come from
complex one, or complex two,

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00:02:38,520 --> 00:02:41,550
or another membrane bound
entry point for electrons

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00:02:41,550 --> 00:02:43,500
into respiration.

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00:02:43,500 --> 00:02:47,640
The first electron converts the
quinone Q to the semiquinone

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00:02:47,640 --> 00:02:48,690
radical.

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00:02:48,690 --> 00:02:51,870
The semiquinone will then pick
up another proton and electron

64
00:02:51,870 --> 00:02:55,050
to form the fully reduced
species, the hydroquinone,

65
00:02:55,050 --> 00:02:57,020
or QH2.

66
00:02:57,020 --> 00:03:00,920
The hydroquinone QH2 is
fully loaded with electrons.

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00:03:00,920 --> 00:03:03,740
The fully reduced hydroquinone
will next deliver its two

68
00:03:03,740 --> 00:03:06,260
electrons to complex three.

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00:03:06,260 --> 00:03:08,000
The technical name
of complex three

70
00:03:08,000 --> 00:03:12,500
is coenzyme Q, cytochrome
c oxidoreductase.

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00:03:12,500 --> 00:03:14,960
This name gives a hint
that the electrons

72
00:03:14,960 --> 00:03:18,680
are going to pass from
coenzyme Q in its reduced form,

73
00:03:18,680 --> 00:03:21,890
to an oxidized form
of cytochrome c, which

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00:03:21,890 --> 00:03:26,360
is also non-covalently
associated with complex three.

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00:03:26,360 --> 00:03:30,140
Let's look at panel B. A
single molecule of QH2,

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00:03:30,140 --> 00:03:32,720
the hydroquinone, contains
two electrons that

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00:03:32,720 --> 00:03:35,480
are going to be transferred
to complex three.

78
00:03:35,480 --> 00:03:38,920
One electron is going to
reduce iron three to iron two,

79
00:03:38,920 --> 00:03:41,559
in a first molecule
of cytochrome C.

80
00:03:41,559 --> 00:03:43,100
And then the second
electron is going

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00:03:43,100 --> 00:03:47,240
to be used to reduce a second
molecule of cytochrome c.

82
00:03:47,240 --> 00:03:49,730
It's convenient to break
this overall reaction

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00:03:49,730 --> 00:03:50,880
scheme into two parts.

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00:03:50,880 --> 00:03:53,020
I'll call them cycle
one and cycle two.

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00:03:53,020 --> 00:03:55,940
In cycle one, the first
molecule of cytochrome C

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00:03:55,940 --> 00:03:57,380
is going to be reduced.

87
00:03:57,380 --> 00:03:59,720
And then in cycle two,
the second molecule

88
00:03:59,720 --> 00:04:01,900
will be reduced.

89
00:04:01,900 --> 00:04:06,130
I've further broken down
the cycle into eight steps.

90
00:04:06,130 --> 00:04:08,140
At the upper left
of complex three,

91
00:04:08,140 --> 00:04:10,480
you'll see a site that
I've marked with an x.

92
00:04:10,480 --> 00:04:12,910
This is an iron
cell for protein.

93
00:04:12,910 --> 00:04:17,410
This is going to be that docking
site for QH2, the semiquinone.

94
00:04:17,410 --> 00:04:19,959
Sometimes it is called
the Q delta site.

95
00:04:19,959 --> 00:04:24,910
The reduced quinone, QH2, gives
up two protons and one electron

96
00:04:24,910 --> 00:04:28,450
to form the semiquinone
radical, Q.minus.

97
00:04:28,450 --> 00:04:30,610
The electron in
step three goes over

98
00:04:30,610 --> 00:04:33,430
to reduce ferric ion
to ferrous iron, that

99
00:04:33,430 --> 00:04:37,180
is iron three to iron
two in cytochrome c.

100
00:04:37,180 --> 00:04:39,910
As I mentioned
earlier, cytochrome c

101
00:04:39,910 --> 00:04:43,060
is another one of our
mobile electron carriers.

102
00:04:43,060 --> 00:04:46,660
It's going to float away
and go off to complex four.

103
00:04:46,660 --> 00:04:49,900
At this point in step four
B, the semiquinone radical

104
00:04:49,900 --> 00:04:52,000
is going to give up
its second electron

105
00:04:52,000 --> 00:04:56,110
to cytochrome bL,
which is a constituent

106
00:04:56,110 --> 00:04:58,300
of the complex three.

107
00:04:58,300 --> 00:05:01,180
From the standpoint of
coenzyme Q, at this point

108
00:05:01,180 --> 00:05:03,610
it's lost both of its electrons.

109
00:05:03,610 --> 00:05:06,190
It has been converted to
the fully oxidized form

110
00:05:06,190 --> 00:05:10,360
Q, as depicted in step
four A. The electron

111
00:05:10,360 --> 00:05:13,740
at cytochrome bL flows
into cytochrome bH.

112
00:05:13,740 --> 00:05:15,310
And from there, it
will subsequently

113
00:05:15,310 --> 00:05:20,770
flow into a molecule of the
parental diquinone Q. The one

114
00:05:20,770 --> 00:05:23,920
electron reduction of
the diquinone Q, results

115
00:05:23,920 --> 00:05:26,440
in the formation of the
semiquinone radical,

116
00:05:26,440 --> 00:05:28,230
once again.

117
00:05:28,230 --> 00:05:30,090
This is at position Y--

118
00:05:30,090 --> 00:05:34,980
the way I've drawn the complex
three in panel B. Position Y

119
00:05:34,980 --> 00:05:38,140
is also called the Qi site.

120
00:05:38,140 --> 00:05:40,360
At this point, the
semiquinone radical

121
00:05:40,360 --> 00:05:42,610
will just stay where it
is for a few minutes.

122
00:05:42,610 --> 00:05:46,540
We'll pick it up again in the
second half of the Q Cycle.

123
00:05:46,540 --> 00:05:49,570
To summarize what happened in
the first half of the Q Cycle,

124
00:05:49,570 --> 00:05:52,810
two protons have been
translocated from the matrix

125
00:05:52,810 --> 00:05:54,910
into the intermembrane space.

126
00:05:54,910 --> 00:05:56,500
And one electron
has been transferred

127
00:05:56,500 --> 00:06:00,250
to cytochrome c, which
then translocates

128
00:06:00,250 --> 00:06:03,580
across the outer surface of the
mitochondrial inner membrane

129
00:06:03,580 --> 00:06:05,980
to complex four.

130
00:06:05,980 --> 00:06:08,470
At this point, please
look at panel C.

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00:06:08,470 --> 00:06:11,070
Now we're going to take a look
at the second half of the Q

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00:06:11,070 --> 00:06:12,040
Cycle.

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00:06:12,040 --> 00:06:14,200
At the bottom of
complex three in step 1,

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00:06:14,200 --> 00:06:16,600
you'll see the orphaned
semiquinone radical

135
00:06:16,600 --> 00:06:17,710
that we just created.

136
00:06:17,710 --> 00:06:20,200
Keep in mind that we
are going to come back

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00:06:20,200 --> 00:06:22,070
and use that radical
in a couple of minutes.

138
00:06:22,070 --> 00:06:23,290
So bear with me.

139
00:06:23,290 --> 00:06:25,780
At step two, we see
a second molecule

140
00:06:25,780 --> 00:06:28,850
of the hydroquinone, QH2,
inside the mitochondrial inner

141
00:06:28,850 --> 00:06:29,860
membrane.

142
00:06:29,860 --> 00:06:32,830
We're going to borrow this
molecule for a short time.

143
00:06:32,830 --> 00:06:34,480
And then we're
going to restore it.

144
00:06:34,480 --> 00:06:37,070
So this QH2 is,
essentially, catalytic

145
00:06:37,070 --> 00:06:39,280
in the overall reaction scheme.

146
00:06:39,280 --> 00:06:42,340
In step three, we see that
this borrowed molecule

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00:06:42,340 --> 00:06:47,080
of the hydroquinone, QH2, gives
up two protons and one electron

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00:06:47,080 --> 00:06:50,960
just as we had seen in the
first half of the cycle.

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00:06:50,960 --> 00:06:52,450
You will note that
we are reducing

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00:06:52,450 --> 00:06:56,200
a second molecule of cytochrome
c, which is then going

151
00:06:56,200 --> 00:06:58,020
to go off to complex four.

152
00:06:58,020 --> 00:07:00,550
And we're also going
to be producing

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00:07:00,550 --> 00:07:03,850
a second molecule of
coenzyme Q, the fully

154
00:07:03,850 --> 00:07:06,840
oxidized form of the co-factor.

155
00:07:06,840 --> 00:07:09,970
In step four B, the
semiquinone radical, just as

156
00:07:09,970 --> 00:07:12,090
happened in cycle
one, will give up

157
00:07:12,090 --> 00:07:14,650
its electron to cytochrome bL.

158
00:07:14,650 --> 00:07:17,350
The electron will then
go to cytochrome bH,

159
00:07:17,350 --> 00:07:20,650
and then will flow into
the semiquinone radical

160
00:07:20,650 --> 00:07:23,740
that we had left over
from the first cycle.

161
00:07:23,740 --> 00:07:27,310
This radical exists at site
Y, or as I called it before,

162
00:07:27,310 --> 00:07:30,190
the Qi site of complex three.

163
00:07:30,190 --> 00:07:32,800
The reduction of the
semiquinone radical at step six

164
00:07:32,800 --> 00:07:36,160
is concomitant in step
seven with the acquisition

165
00:07:36,160 --> 00:07:38,680
of two protons from the matrix.

166
00:07:38,680 --> 00:07:41,020
This series of
reactions, ultimately,

167
00:07:41,020 --> 00:07:45,030
results in restoration
of QH2, the semiquinone

168
00:07:45,030 --> 00:07:46,960
in the mitochondrial
inner membrane.

169
00:07:46,960 --> 00:07:49,630
At this point, we have
restored the molecule of QH2,

170
00:07:49,630 --> 00:07:51,850
at step eight, that we
had borrowed, initially,

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00:07:51,850 --> 00:07:53,260
at step two.

172
00:07:53,260 --> 00:07:55,210
The second cycle
has also produced

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00:07:55,210 --> 00:07:58,870
net Q, the oxidized
form of the co-factor,

174
00:07:58,870 --> 00:08:02,430
which is now free to go on
to complexes one and two,

175
00:08:02,430 --> 00:08:04,990
and to pick up more
reducing equivalents.

176
00:08:04,990 --> 00:08:07,990
Just looking at the second half
of the Q Cycle, what we see

177
00:08:07,990 --> 00:08:10,870
is that we've translocated
another two protons

178
00:08:10,870 --> 00:08:12,850
into the intermembrane space.

179
00:08:12,850 --> 00:08:16,900
And we have transferred a
second electron to cytochrome c.

180
00:08:16,900 --> 00:08:20,500
So cycle one and
cycle two, each,

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00:08:20,500 --> 00:08:22,990
result in the translocation
of two protons

182
00:08:22,990 --> 00:08:25,240
from the matrix to the
intermembrane space.

183
00:08:25,240 --> 00:08:28,090
And each results in the
transfer of one electron

184
00:08:28,090 --> 00:08:29,770
to cytochrome c.

185
00:08:29,770 --> 00:08:33,130
Looking at the whole Q Cycle,
that is cycle one and cycle two

186
00:08:33,130 --> 00:08:37,539
together, we get a pair of
electrons that enter from QH2.

187
00:08:37,539 --> 00:08:41,320
Four protons are translocated
into the intermembrane space.

188
00:08:41,320 --> 00:08:44,140
And two reduced molecules
of cytochrome c,

189
00:08:44,140 --> 00:08:46,510
which then migrate
to complex four,

190
00:08:46,510 --> 00:08:49,420
where they'll be later oxidized.

191
00:08:49,420 --> 00:08:54,580
Let's turn to storyboard
15 and panel A. In panel A,

192
00:08:54,580 --> 00:08:58,050
I'm showing another way
to look at the Q Cycle.

193
00:08:58,050 --> 00:09:00,340
I'll emphasize that this
is not as chemically

194
00:09:00,340 --> 00:09:04,290
accurate as the two step process
that I showed you previously.

195
00:09:04,290 --> 00:09:06,670
And I'm not going to
discuss it here further.

196
00:09:06,670 --> 00:09:08,740
Nevertheless, I think
that this presentation

197
00:09:08,740 --> 00:09:10,810
makes it relatively
easy for you to see

198
00:09:10,810 --> 00:09:14,850
the overall stoichiometry
of the Q Cycle.

199
00:09:14,850 --> 00:09:18,450
Let's now turn to
storyboard 15, panel B.

200
00:09:18,450 --> 00:09:22,000
In the way of a high-level
review of electron transport

201
00:09:22,000 --> 00:09:25,650
so far, electrons from
nutrient oxidation

202
00:09:25,650 --> 00:09:29,670
have been deposited into the
electron transport chain.

203
00:09:29,670 --> 00:09:32,190
The transfer of
electrons to oxygen

204
00:09:32,190 --> 00:09:35,490
resulted in energy that
is used to power pumps--

205
00:09:35,490 --> 00:09:39,480
pumps that pump protons into
the intermembrane space.

206
00:09:39,480 --> 00:09:42,480
We've looked at the Q Cycle
as one example, of several,

207
00:09:42,480 --> 00:09:44,970
of how protons are pumped.

208
00:09:44,970 --> 00:09:47,760
It took energy to create
this proton gradient.

209
00:09:47,760 --> 00:09:49,410
When we release
the proton gradient

210
00:09:49,410 --> 00:09:53,870
by allowing the protons to
flow through the ATP synthase,

211
00:09:53,870 --> 00:09:56,040
we're going to be able to
use that energy in order

212
00:09:56,040 --> 00:09:58,740
to accomplish the
otherwise energetically

213
00:09:58,740 --> 00:10:03,030
uphill phosphorylation
of ADP into ATP.

214
00:10:03,030 --> 00:10:06,720
Now let's look at storyboard
15, panel C. This panel

215
00:10:06,720 --> 00:10:11,190
shows the F naught F1 proton
translocating ATP synthase,

216
00:10:11,190 --> 00:10:13,770
also known as the ATP synthase.

217
00:10:13,770 --> 00:10:16,020
To put things in
perspective, at the top

218
00:10:16,020 --> 00:10:18,210
is the intermembrane
space, which

219
00:10:18,210 --> 00:10:20,910
is where the protons
have been translocated.

220
00:10:20,910 --> 00:10:23,610
In the middle is the
mitochondrial inner membrane.

221
00:10:23,610 --> 00:10:26,460
And at the bottom is the
mitochondrial matrix.

222
00:10:26,460 --> 00:10:30,380
Because we've pumped protons
into the intermembrane space,

223
00:10:30,380 --> 00:10:33,030
its pH is about three
quarters of a pH unit

224
00:10:33,030 --> 00:10:36,810
lower than the pH of the
matrix of the mitochondria.

225
00:10:36,810 --> 00:10:39,510
Proton flow is going to
be regulated in response

226
00:10:39,510 --> 00:10:41,730
to physiologic needs.

227
00:10:41,730 --> 00:10:45,610
I'll talk about that regulation
and how it occurs later.

228
00:10:45,610 --> 00:10:49,200
For now, however, let's look at
protons flowing through the F

229
00:10:49,200 --> 00:10:50,850
naught F1 complex.

230
00:10:50,850 --> 00:10:52,740
Let's imagine that
the broken line

231
00:10:52,740 --> 00:10:56,286
indicates a channel, through
which protons will flow.

232
00:10:56,286 --> 00:10:57,660
Other structural
features include

233
00:10:57,660 --> 00:11:00,480
a shaft, which I've
indicated as gamma,

234
00:11:00,480 --> 00:11:03,990
and a ring, in which
the shaft is embedded,

235
00:11:03,990 --> 00:11:06,180
which I've indicated
as the C-ring.

236
00:11:06,180 --> 00:11:09,030
When protons flow, both
the shaft and the C-ring

237
00:11:09,030 --> 00:11:13,690
will move, as you'll see,
in a clockwise rotation.

238
00:11:13,690 --> 00:11:15,400
At the bottom of the
overall apparatus

239
00:11:15,400 --> 00:11:18,790
are three alpha and
three beta subunits.

240
00:11:18,790 --> 00:11:20,530
These are not going to rotate.

241
00:11:20,530 --> 00:11:22,270
That is, they're
going to stay fixed

242
00:11:22,270 --> 00:11:26,272
in position while the C-ring
and the gamma subunit rotate.

243
00:11:26,272 --> 00:11:29,050
The beta subunit
contains a site x,

244
00:11:29,050 --> 00:11:32,080
which is going to be the active
site for conversion of ADP

245
00:11:32,080 --> 00:11:34,960
plus inorganic phosphate to ATP.

246
00:11:34,960 --> 00:11:38,890
I've drawn a little elbow on
the bottom of the gamma subunit.

247
00:11:38,890 --> 00:11:41,080
Let's imagine that
protons are flowing.

248
00:11:41,080 --> 00:11:43,720
And the flow makes that
subunit spin around.

249
00:11:43,720 --> 00:11:45,670
And further, imagine
that the elbow is

250
00:11:45,670 --> 00:11:48,040
bumping into the active sites--

251
00:11:48,040 --> 00:11:49,420
that is, the x sites.

252
00:11:49,420 --> 00:11:51,370
There are three of
the x sites, one

253
00:11:51,370 --> 00:11:53,230
on each of the b
subunits at the bottom

254
00:11:53,230 --> 00:11:55,180
of the overall apparatus.

255
00:11:55,180 --> 00:11:59,500
Imagine that the energy involved
is translocated from the elbow

256
00:11:59,500 --> 00:12:01,150
to the active sites.

257
00:12:01,150 --> 00:12:05,050
And that's what's going to
help us align ADP and Pi,

258
00:12:05,050 --> 00:12:08,920
to make the overall
chemical reaction favorable.

259
00:12:08,920 --> 00:12:11,260
That is a hypothetical,
but not far fetched way

260
00:12:11,260 --> 00:12:15,010
to think about the
way that ATP is made.

261
00:12:15,010 --> 00:12:18,190
Let's now look at panel
D. The current view

262
00:12:18,190 --> 00:12:21,160
is that the active site
oscillates among three

263
00:12:21,160 --> 00:12:23,260
different conformations
during the time

264
00:12:23,260 --> 00:12:25,300
the protons are translocated.

265
00:12:25,300 --> 00:12:27,070
These three
conformational states

266
00:12:27,070 --> 00:12:31,630
are referred to as O for open,
L for loose, and T for tight.

267
00:12:31,630 --> 00:12:35,260
In the open state, nothing is
present at the active site x.

268
00:12:35,260 --> 00:12:37,240
The conversion of
open to loose is

269
00:12:37,240 --> 00:12:41,620
concomitant with the binding
of ADP and inorganic phosphate.

270
00:12:41,620 --> 00:12:44,200
As protons continue to
flow, the loose site

271
00:12:44,200 --> 00:12:46,840
is converted into
the tight state.

272
00:12:46,840 --> 00:12:50,020
The energy associated with this
conformational change results

273
00:12:50,020 --> 00:12:52,810
in the alignment of the
inorganic phosphate,

274
00:12:52,810 --> 00:12:54,850
such that the conditions
are now favorable

275
00:12:54,850 --> 00:12:58,300
for it to phosphorylate
ADP and form ATP.

276
00:12:58,300 --> 00:13:01,780
Further proton flow results in
the tight state being converted

277
00:13:01,780 --> 00:13:05,620
to the open state, which
ejects the formed ATP out

278
00:13:05,620 --> 00:13:08,090
into the mitochondrial matrix.

279
00:13:08,090 --> 00:13:12,790
So at each cycle, of
open to loose to tight,

280
00:13:12,790 --> 00:13:14,980
a molecule of ATP is made.

281
00:13:14,980 --> 00:13:16,780
And then it is ejected.

282
00:13:16,780 --> 00:13:19,520
This goes on again,
and again, and again.

283
00:13:19,520 --> 00:13:23,580
That's the overall mechanism
by which ATP is synthesized.

284
00:13:23,580 --> 00:13:26,970
At the high level, proton
flow results in the movement

285
00:13:26,970 --> 00:13:28,420
as the turning of a rotor.

286
00:13:28,420 --> 00:13:30,000
It is a lot like
a water wheel that

287
00:13:30,000 --> 00:13:33,030
might drive the rotation of a
millstone, that grinds grain

288
00:13:33,030 --> 00:13:34,530
into flour.

289
00:13:34,530 --> 00:13:36,840
The rotation of the
shaft provides energy

290
00:13:36,840 --> 00:13:38,970
to bump into the
x sites, resulting

291
00:13:38,970 --> 00:13:41,160
in the conversion
of a first subunit

292
00:13:41,160 --> 00:13:44,280
to the open conformation,
another subunit

293
00:13:44,280 --> 00:13:49,290
to the loose state, and
a third subunit to tight.

294
00:13:49,290 --> 00:13:50,970
Then the process
starts over again,

295
00:13:50,970 --> 00:13:55,050
with each cycle producing
one molecule of ATP.

296
00:13:55,050 --> 00:13:56,190
As one final point--

297
00:13:56,190 --> 00:13:59,880
a point that explains how
this process is regulated.

298
00:13:59,880 --> 00:14:03,300
Proton flow results from
the binding of ADP--

299
00:14:03,300 --> 00:14:05,310
that is, ADP.

300
00:14:05,310 --> 00:14:07,410
This will become
important in a few minutes

301
00:14:07,410 --> 00:14:11,130
when we talk about how the
whole system is regulated.

302
00:14:11,130 --> 00:14:13,620
Now let's look at storyboard 16.

303
00:14:13,620 --> 00:14:16,560
Let's look at all four
panels, panels A through D.

304
00:14:16,560 --> 00:14:18,070
In the last part
of this lecture,

305
00:14:18,070 --> 00:14:20,010
I'd like to talk
about how respiration

306
00:14:20,010 --> 00:14:23,280
is coupled to the TCA
cycle and glycolysis.

307
00:14:23,280 --> 00:14:25,314
This is really the
first time in 5.07

308
00:14:25,314 --> 00:14:26,730
that we're going
to see the beauty

309
00:14:26,730 --> 00:14:29,250
of coordinated
pathway regulation.

310
00:14:29,250 --> 00:14:32,190
I'm going to use a
physiological scenario in order,

311
00:14:32,190 --> 00:14:34,909
hopefully, to make
it all make sense.

312
00:14:34,909 --> 00:14:37,200
In this scenario, imagine
that you're being chased down

313
00:14:37,200 --> 00:14:39,120
the street by a dog.

314
00:14:39,120 --> 00:14:41,310
In order to start running
away from the dog,

315
00:14:41,310 --> 00:14:44,490
we're going to need some ATP
that's very quickly generated

316
00:14:44,490 --> 00:14:46,930
from glucose by glycolysis.

317
00:14:46,930 --> 00:14:50,040
The TCA cycle is also
going to be operative,

318
00:14:50,040 --> 00:14:52,330
along with a pathway
we haven't seen so far,

319
00:14:52,330 --> 00:14:54,090
fatty acid oxidation.

320
00:14:54,090 --> 00:14:57,930
In both cases, the TCA cycle
and fatty acid oxidation,

321
00:14:57,930 --> 00:15:00,580
reduced electron carriers
are going to be produced.

322
00:15:00,580 --> 00:15:03,150
They're going to give up their
electrons to the electron

323
00:15:03,150 --> 00:15:04,410
transport chain.

324
00:15:04,410 --> 00:15:08,910
Ultimately, to reduce oxygen
to water and with concomitant

325
00:15:08,910 --> 00:15:11,860
pumping of protons into
the intermembrane space,

326
00:15:11,860 --> 00:15:13,590
resulting in the
lowering of the pH--

327
00:15:13,590 --> 00:15:17,890
that is, acidification of
the intermembrane space.

328
00:15:17,890 --> 00:15:19,690
As I mentioned
earlier, that flow

329
00:15:19,690 --> 00:15:21,640
of protons through
the ATP synthase

330
00:15:21,640 --> 00:15:23,620
is triggered by
the binding of ADP

331
00:15:23,620 --> 00:15:27,587
to the x site on the beta
subunit of the enzyme complex.

332
00:15:27,587 --> 00:15:29,920
As your muscles are working
hard and you're running away

333
00:15:29,920 --> 00:15:32,200
from the dog, the
concentration of ADP

334
00:15:32,200 --> 00:15:35,170
in the mitochondrial matrix
is going to be increasing.

335
00:15:35,170 --> 00:15:38,650
Thus, proton flow is
going to be initiated.

336
00:15:38,650 --> 00:15:40,870
As you run more and
more, the protons

337
00:15:40,870 --> 00:15:43,900
are depleted from the
intermembrane space.

338
00:15:43,900 --> 00:15:47,500
It's not known exactly how
this reduction in proton

339
00:15:47,500 --> 00:15:49,960
concentration results
in accelerated

340
00:15:49,960 --> 00:15:53,320
movement of electrons through
the electron transport chain.

341
00:15:53,320 --> 00:15:56,050
It's tempting to
speculate that one or more

342
00:15:56,050 --> 00:15:59,170
of the reductase centers, within
the electron transfer chain,

343
00:15:59,170 --> 00:16:02,300
may be pH sensitive.

344
00:16:02,300 --> 00:16:04,420
Let's imagine that
that is the case.

345
00:16:04,420 --> 00:16:07,030
So let's imagine that
the raising of pH--

346
00:16:07,030 --> 00:16:10,300
that is, the decrease in
hydrogen ion concentration

347
00:16:10,300 --> 00:16:12,730
in the intermembrane
space, results

348
00:16:12,730 --> 00:16:16,180
in the facilitated flow of
electrons from complex one

349
00:16:16,180 --> 00:16:18,670
or complex two, to oxygen.

350
00:16:18,670 --> 00:16:23,500
Next, let's look to the far
left at the bottom of panel D.

351
00:16:23,500 --> 00:16:26,740
As you draw more electrons
into respiration,

352
00:16:26,740 --> 00:16:30,520
NADH is going to be
converted to NAD plus.

353
00:16:30,520 --> 00:16:34,600
It's important, now, to remember
that NADH feedback inhibits

354
00:16:34,600 --> 00:16:38,070
any step at which it's
produced in the TCA cycle.

355
00:16:38,070 --> 00:16:40,450
Thus, as you're running
away from the dog,

356
00:16:40,450 --> 00:16:43,396
the NADH concentration drops.

357
00:16:43,396 --> 00:16:44,770
And that means
that there's going

358
00:16:44,770 --> 00:16:48,110
to be less inhibition of
the steps at which NADH

359
00:16:48,110 --> 00:16:51,340
is produced in the TCA cycle.

360
00:16:51,340 --> 00:16:54,640
Thus, looking at the
big picture, the binding

361
00:16:54,640 --> 00:16:59,260
of increased concentrations
of ADP to the ATP synthase,

362
00:16:59,260 --> 00:17:02,840
over on the far right, results
in the activation of the TCA

363
00:17:02,840 --> 00:17:05,260
cycle and other
metabolic pathways that

364
00:17:05,260 --> 00:17:07,299
will result in the
delivery of more electrons

365
00:17:07,299 --> 00:17:09,040
through the electron
transport chain,

366
00:17:09,040 --> 00:17:10,900
in order to allow
you to continue

367
00:17:10,900 --> 00:17:12,750
to run away from the dog.

368
00:17:12,750 --> 00:17:16,720
Next, let's imagine that you've
been running for quite a while.

369
00:17:16,720 --> 00:17:20,200
And let's look at complex
four, where oxygen is bound.

370
00:17:20,200 --> 00:17:23,470
Sooner or later you're going
to be becoming oxygen limited.

371
00:17:23,470 --> 00:17:25,329
You're going to be
panting heavily.

372
00:17:25,329 --> 00:17:28,300
This means that electron
flow from a reduced electron

373
00:17:28,300 --> 00:17:31,450
carriers to oxygen is
going to slow down.

374
00:17:31,450 --> 00:17:35,590
As we become more hypoxic,
respiration starts to fail,

375
00:17:35,590 --> 00:17:38,470
but hypoxia
dramatically increases

376
00:17:38,470 --> 00:17:41,740
the levels or activities of
the enzymes of glycolysis.

377
00:17:41,740 --> 00:17:43,990
Thus, glycolysis
will switch over

378
00:17:43,990 --> 00:17:47,740
to becoming our principal
ATP generation machine.

379
00:17:47,740 --> 00:17:49,820
As we increase the
rate of glycolysis,

380
00:17:49,820 --> 00:17:52,360
the flux goes from
glucose to pyruvate,

381
00:17:52,360 --> 00:17:54,880
but the pyruvate
can't be oxidized

382
00:17:54,880 --> 00:17:56,650
because it can't
enter respiration,

383
00:17:56,650 --> 00:17:59,290
because we don't have enough
oxygen present in order

384
00:17:59,290 --> 00:18:01,490
to oxidize it.

385
00:18:01,490 --> 00:18:04,720
So what happens is we
activate lactate dehydrogenase

386
00:18:04,720 --> 00:18:07,300
to convert pyruvate to lactate.

387
00:18:07,300 --> 00:18:10,790
Conversion of pyruvate to
lactate has two consequences.

388
00:18:10,790 --> 00:18:13,480
The first is that conversion
results in the conversion

389
00:18:13,480 --> 00:18:15,310
of NADH to NAD plus.

390
00:18:15,310 --> 00:18:18,190
Remember, that this is
the lactate shuttle.

391
00:18:18,190 --> 00:18:21,000
And that NAD plus is
now available to allow

392
00:18:21,000 --> 00:18:23,260
glyceraldehyde
3-phosphate dehydrogenase

393
00:18:23,260 --> 00:18:28,030
to continue processing
molecules of glucose into ATP.

394
00:18:28,030 --> 00:18:29,830
A second consequence
of activation

395
00:18:29,830 --> 00:18:31,630
of lactate dehydrogenase
is the fact

396
00:18:31,630 --> 00:18:34,540
that lactate spills out
into the blood, where

397
00:18:34,540 --> 00:18:35,940
it acidifies the blood.

398
00:18:35,940 --> 00:18:37,390
The pH drops.

399
00:18:37,390 --> 00:18:40,150
Now let's think about
what the consequences are

400
00:18:40,150 --> 00:18:42,610
when the pH of the blood drops.

401
00:18:42,610 --> 00:18:45,520
I want you to think back to the
lectures in which JoAnne talked

402
00:18:45,520 --> 00:18:48,880
about cooperatively, the
Bohr effect, and the affinity

403
00:18:48,880 --> 00:18:50,730
of oxygen for hemoglobin.

404
00:18:50,730 --> 00:18:54,910
JoAnne told us that protons
are heterotropic allosteric

405
00:18:54,910 --> 00:18:58,390
effectors that, effectively,
loosen the affinity of oxygen

406
00:18:58,390 --> 00:18:59,890
for hemoglobin.

407
00:18:59,890 --> 00:19:02,950
So as you're running
away from the dog,

408
00:19:02,950 --> 00:19:05,920
glycolysis becomes
the main pathway.

409
00:19:05,920 --> 00:19:09,040
You produce a lot of lactate
that goes into the blood.

410
00:19:09,040 --> 00:19:10,570
The pH drops.

411
00:19:10,570 --> 00:19:13,390
Oxygen lowers its
affinity for hemoglobin,

412
00:19:13,390 --> 00:19:17,800
and thus, becomes more available
for the pathway of respiration.

413
00:19:17,800 --> 00:19:22,150
Oxygen is now back in the system
and binds to complex four.

414
00:19:22,150 --> 00:19:25,930
So you kind of get what we
could call a second wind that

415
00:19:25,930 --> 00:19:28,690
allows you to continue
using respiration

416
00:19:28,690 --> 00:19:30,490
to run away from the dog.

417
00:19:30,490 --> 00:19:33,520
In other words, you've
adapted to the stressful state

418
00:19:33,520 --> 00:19:37,330
and are now able to
continue running.

419
00:19:37,330 --> 00:19:39,760
Now let's put it all together.

420
00:19:39,760 --> 00:19:41,260
A dog starts chasing you.

421
00:19:41,260 --> 00:19:44,140
Your readily available energy
reserves, and free glucose,

422
00:19:44,140 --> 00:19:45,640
as well as glycogen,
will produce

423
00:19:45,640 --> 00:19:49,360
rapid ATP that will get you
moving away from the dog.

424
00:19:49,360 --> 00:19:52,720
Additionally, respiration will
be contributing lots of ATP

425
00:19:52,720 --> 00:19:54,640
to allow you to escape the dog.

426
00:19:54,640 --> 00:19:57,640
And as you're running,
ADP concentrations

427
00:19:57,640 --> 00:19:59,980
will go up inside
your mitochondria.

428
00:19:59,980 --> 00:20:03,160
That will, basically,
release the proton gradient,

429
00:20:03,160 --> 00:20:07,120
allowing the ATP synthase to
work very quickly and very hard

430
00:20:07,120 --> 00:20:09,740
to make a lot more ATP.

431
00:20:09,740 --> 00:20:12,310
NADH concentrations
in the mitochondria

432
00:20:12,310 --> 00:20:14,710
will drop with
physical activities.

433
00:20:14,710 --> 00:20:18,250
This helps boot up the
dehydrogenases of the TCA

434
00:20:18,250 --> 00:20:18,820
cycle.

435
00:20:18,820 --> 00:20:20,830
They become less inhibited.

436
00:20:20,830 --> 00:20:23,050
Your TCA cycle will
accelerate, in order

437
00:20:23,050 --> 00:20:25,690
to produce more reducing
equivalents to power

438
00:20:25,690 --> 00:20:28,030
the electron transport chain.

439
00:20:28,030 --> 00:20:31,750
All of this will continue until
your oxygen becomes limiting,

440
00:20:31,750 --> 00:20:37,120
then respiration starts to fail,
but the hypoxic state turns up

441
00:20:37,120 --> 00:20:39,880
the activities of the
enzymes of glycolysis,

442
00:20:39,880 --> 00:20:44,230
thus glycolysis accelerates
and helps accommodate.

443
00:20:44,230 --> 00:20:46,810
But pyruvate, at the
end of glycolysis,

444
00:20:46,810 --> 00:20:49,570
cannot be further oxidized
because there's not enough

445
00:20:49,570 --> 00:20:53,060
oxygen. It has to be
converted to lactate.

446
00:20:53,060 --> 00:20:55,420
The lactate acidifies the blood.

447
00:20:55,420 --> 00:20:58,240
The Bohr effect causes
the release of more oxygen

448
00:20:58,240 --> 00:21:01,720
into the blood, making it more
available to complex four,

449
00:21:01,720 --> 00:21:04,840
in order to reboot the
electron transport chain.

450
00:21:04,840 --> 00:21:08,410
Overall, this is a marvelously
regulated physiological system

451
00:21:08,410 --> 00:21:12,240
that seems to have evolved in
order to promote our survival.