CHAPTER 21
The ProtonMotive Force
Figure 21.1: Chemiosmotic hypothesis: electron transport via
the respiratory chain pump protons into intermembrane space.
These protons can be used to do work in making ATP as they
are transported back into the matrix through ATPase.
It takes 3 H+ transported into the matrix to make one ATP molecule
Complex V: ATP Synthase
- F0F1 ATP Synthase uses the proton gradient energy for the
synthesis of ATP
- Structure is composed of a “knob-and-stalk” structure
- F1 (knob) contains the catalytic subunits for ATP production
- F0 (stalk) has a proton channel which spans the membrane
- Passage of protons through the F0 (stalk) into the mitochondrial
matrix is coupled to ATP formation
- Estimated passage of
3 protons (H+) per
ATP synthesized
Figure 21.3 Knob-and-stalk structure of ATP synthase
A molecular machine
Intermembrane
space
Matrix
-
H+ enter from the Intermembrane
space into subunit a.
-
Binding of this H+ induces a
clockwise rotation of the c ring
and the g subunit.
-
rotation of the g subunit also
induces conformational changes
in the a3b3 hexamer
-
A second H+ waiting in the lower
channel of subunit a is released into
the matrix.
Figure 21.3 Knob-and-stalk structure of ATP synthase
A molecular machine
Intermembrane
space
Matrix
-
The binding site for the ADP/ATP lies
in the clefts between adjacent a and b
subunits in the a3b3 cylinder.
-
The active site for ATP synthesis is
mostly part of the b subunits
-
One 120o rotation of the c-g subunits
requires the translocation of one proton.
Figure 21.7: A closer look at subunits c and a.
H+ enters from intermembrane space
Figure 21.9
The pH is
lower on
the intermem.
side vs. matrix
H+ exit into the matrix space
Figure 21.8: Proton movement across the membrane
Figure 21.8: Proton movement across the membrane
Negative charged
Asp
Figure 21.8: Proton movement across the membrane
Neutral Asp
The mechanism of ATP synthesis from ADP and Pi
The binding Change Mechanism
1. Open: new ATP is released
and ADP and Pi bind
2.
Loose: bound ADP and Pi
cannot be released
3. Tight: condensation of ADP
and Pi is favored to form ATP.
The ATP formed is very tightly
bound.
Figure 21.4
The mechanism of ATP synthesis from ADP and Pi
Figure 21.5 The binding Change Mechanism
Follow the yellow region of one
b subunit.
Figure 21.16 Transport of ATP, ADP and Pi across the
Inner mitochondrial membrane
- Once ATP is made, it must be sent out to the cytosol and more ADP
and Pi must be transported into the matrix.
- Adenine nucleotide translocase: unidirectional exchange of ATP
for ADP (antiport)
Start
here
Figure 21.16 Transport of ATP, ADP and Pi across the
Inner mitochondrial membrane
- Antiport of Pi and OH- occurs resulting in an electrical neutral
translocation. (or symport with a H+)
Figure 21.16: The Energy change regulates the use of fuels
1 H+ translocated
3 H+ per ATP
synth.
10 H+ per O reduced
Regulation of Oxidative phosphorylation
- Overall rate of oxidative phosphorylation depends upon
substrate availability and cellular energy demand
- Important substrates: NADH, FADH2, ADP
Resting state:
- ADP levels low
- NADH and FADH2 are
not oxidized via elec. transport
- CAC slows down
- Oxidative Phosph. Slows down
Active state:
- ADP levels rise
- NADH and FADH2 begin being
oxidized via elec. transport
- CAC is more active
- Oxidative Phosph. increases
Electrons do not flow to O2 unless ATP is in demand
The relationship between oxygen consumption (respiration)
and ATP synthesis (phosphorylation)
The P:O Ratio
P: O Ratio =
molecules of ADP phosphorylated
atoms of oxygen reduced
- Translocation of 3 H+ are required by ATP synthase for each
ATP produced
- 1 H+ is needed for transport of Pi, ADP and ATP
- NET: 4 H+ transported for each ATP synthesized
and transported
Calculation of the P:O ratio
Complex
#H+ translocated/2e-
I
4
III
4
IV
2
Recall that two species supplied 2 e- each for proton translocation:
NADH and succinate (FADH2)
For NADH: 10 H+ translocated/O (2 e-)
P/O = (10 H+/4 H+) = 2.5 ATP/O
For succinate: (FADH2  QH2) substrate = 6 H+/O (2 e-)
P/O = (6 H+/4 H+) = 1.5 ATP/O
Glycerol 3-phosphate shuttle
One method electrons from cytoplasmic NADH produce ATP
Malate – Aspartate shuttle
Another method electrons from cytoplasmic NADH produce ATP
Assignment
Read Chapter 21
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