Respiratory Chain

This coupling of proton-motive force and ATP synthesis is called the chemiosmotic mechanism, or chemiosmosis.

the chemiosmotic mechanism for atp synthesis. The chemiosmotic mechanism uses ATP synthase to couple proton diffusion to ATP synthesis. This mechanism has three parts:

1. The flow of electrons from one electron carrier to another in the respiratory chain is a series of exergonic reactions that occurs in the inner mitochondrial membrane.

2. These exergonic reactions drive the endergonic pumping of H+ out of the mitochondrial matrix and across the

7.12 A Chemiosmotic Mechanism Produces ATP

! As electrons pass through the series of protein complexes in the respiratory chain, protons are pumped from the mitochondrial matrix into the intermembrane space. As the protons return to the matrix through ATP synthase, ATP is formed.

Mitochondrion

Mitochondrion

Nadh Respiratory ChainInner Knob Matrix
A highly magnified view of the inner mitochondrial membrane. "Lollipops" project into the mitochondrial matrix; these knobs catalyze the synthesis of ATP.

Outside of cell

Outside of cell

Mitochondria With Proton Pump
I Electrons (carried by NADH and FADH2) from glycolysis and the citric acid cycle "feed" the electron carriers of the inner mitochondrial membrane, which pump protons (H+) out of the matrix to the intermembrane space.

inner membrane into the intermembrane space. This pumping establishes and maintains a H+ gradient. 3. The potential energy of the H+ gradient, or protonmotive force, is harnessed by ATP synthase. This protein has two roles: It acts as a channel allowing the H+ to diffuse back into the matrix, and it uses the energy of that diffusion to make ATP from ADP and Pi.

ATP synthesis is a reversible reaction, and ATP synthase can also act as an ATPase, hydrolyzing ATP to ADP and P;:

If the reaction goes to the right, free energy is released, and that energy is used to pump H+ out of the mitochondrial matrix. If the reaction goes to the left, it uses free energy from H+ diffusion into the matrix to make ATP. What makes it prefer ATP synthesis? There are two answers to this question.

► ATP leaves the mitochondrial matrix for use elsewhere in the cell as soon as it is made, keeping the ATP concentration in the matrix low and driving the reaction toward the left. A person hydrolyzes about 1025 ATP molecules per day, and clearly the vast majority are recycled using the free energy from the oxidation of glucose.

► The H+ gradient is maintained by electron transport and proton pumping. (The electrons, you will recall, come from the oxidation of NADH and FADH2, which are themselves reduced by the oxidations of glycolysis and the citric acid cycle. So, one reason you eat is to replenish the H+ gradient!)

ATP synthase is a large multi-protein machine, containing 16 different polypeptides in mammals. It has two functional components. One of these components is the membrane channel for H+. The other component sticks out into the mitochondrial matrix like a lollipop (see Figure 7.12) and contains the active site for ATP synthesis. The actual mechanism of transferring energy from the H+ gradient to the phospho-rylation of ADP involves the physical rotation of the core of the enzyme, with this rotational energy transferred to ATP.

experiments demonstrate chemiosmosis. Two key experiments demonstrated (1) that a proton (H+) gradient across a membrane can drive ATP synthesis; and (2) that the enzyme ATP synthase is the catalyst for this reaction.

Experiment 1 tested the hypothesis that ATP synthesis is driven by the H+ gradient across an inner mitochondrial membrane (Figure 7.13). In this experiment, mitochondria without a food source were "fooled" into making ATP when researchers raised the H+ concentration in their environment. A sample of isolated mitochondria that had been exposed to a low H+ concentration was suddenly put in a medium with a high concentration of H+. The outer mitochondrial mem brane, unlike the inner one, is freely permeable to H+, so H+ rapidly diffused into the intermembrane space. This created an artificial gradient across the inner membrane, which the mitochondria used to make ATP from ADP and Pi. This result supported the hypothesis and provided strong evidence for chemiosmosis.

Experiment 2 tested the hypothesis that the enzyme ATPase couples a proton gradient to ATP synthesis. In this experiment, a proton pump isolated from a bacterium was added to artificial membrane vesicles. When an appropriate energy source was provided, H+ was pumped into the vesicles, creating a gradient. If mammalian ATP synthase was then inserted into the membranes of these vesicles and the energy source removed, the vesicles made ATP even in the absence of the usual electron carriers. Again, the result supported the hypothesis, showing that the enzyme ATP synthase is the coupling factor in the membrane.

uncoupling proton diffusion from atp production. For the chemiosmotic mechanism to work, the diffusion of H+ and the formation of ATP must be tightly coupled; that is, the protons must pass only through the ATP synthase channel in order to move into the mitochondrial matrix. If a second type of H+ diffusion channel (not ATP synthase) is inserted into the mitochondrial membrane, the energy of the H+ gradient is released as heat, rather than being coupled to the synthesis of ATP. Such uncoupling molecules are deliberately used by some organisms to generate heat instead of ATP. For example, the natural uncoupling protein thermogenin plays an important role in regulating the temperature of some mammals, especially newborn human infants, who lack the hair to keep warm, and of hibernating animals. We will describe this process in more detail in Chapter 41.

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Responses

  • jonas
    What allows protons to diffuse in and out of the inner mitochondrial membrane?
    8 years ago
  • Geronima
    What two gradients drive the H back across the membrane through the ATP synthase?
    8 years ago
  • valtteri m
    How the protons accumulates in mitochondrial matrix?
    7 years ago
  • jason
    How protondiffiuse in mitochonderion intermembrane space?
    7 years ago

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