CHAPTER 21The Proton-Motive Force

Figure 21.1: Chemiosmotic hypothesis: electron transport viathe respiratory chain pump protons into intermembrane space.

These protons can be used to do work in making ATP as theyare 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 thesynthesis 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 mitochondrialmatrix is coupled to ATP formation

- Estimated passage of 3 protons (H+) per ATP synthesized

Figure 21.3 Knob-and-stalk structure of ATP synthaseA molecular machine

Matrix

Intermembrane space

- 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 synthaseA molecular machine

Matrix

Intermembrane space

- 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

H+ exit into the matrix space

N C C

CH2O

C

O

O

H

H

H

The pH islower onthe intermem.side vs. matrix

Figure 21.9

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 oneb 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)

Starthere

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

10 H+ per O reduced

3 H+ per ATPsynth.

1 H+ translocated

Regulation of Oxidative phosphorylation

- Overall rate of oxidative phosphorylation depends uponsubstrate 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

=

- Translocation of 3 H+ are required by ATP synthase for eachATP 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

#H+ translocated/2e- 4 4 2

Complex I III IV

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 shuttleOne 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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