MCAT Biochemistry Review
Chapter 10: Carbohydrate Metabolism II: Aerobic Respiration
10.4 Oxidative Phosphorylation
We have arrived at the payout site of aerobic respiration: ATP synthesis. Knowing the nuances of ATP synthesis is an absolute must by Test Day. The link between electron transport and ATP synthesis starts with a protein complex called ATP synthase, which spans the entire inner mitochondrial membrane and protrudes into the matrix.
A small fraction—only 13 of the approximately 100 polypeptides necessary for oxidative phosphorylation—are encoded by mitochondrial DNA. The significance of this fact is that mitochondrial DNA has a mutation rate nearly ten times higher than that of nuclear DNA.
The proton-motive force interacts with the portion of ATP synthase that spans the membrane, which is called the F0 portion. F0 functions as an ion channel, so protons travel through F0 along their gradient back into the matrix. As this happens, a process called chemiosmotic coupling allows the chemical energy of the gradient to be harnessed as a means of phosphorylating ADP, thus forming ATP. In other words, the ETC generates a high concentration of protons in the intermembrane space, which then flows through the F0 ion channel of ATP synthase back into the matrix. As this happens, the other portion of ATP synthase, which is called the F1 portion, utilizes the energy released from this electrochemical gradient to phosphorylate ADP to ATP, as demonstrated in Figure 10.16. The specific mechanism by which ADP is actually phosphorylated is still a matter of debate.
Figure 10.16. ATP Synthase Reaction ATP synthase generates ATP from ADP and inorganic phosphate by allowing high-energy protons to move down the concentration gradient created by the electron transport chain.
Chemiosmotic coupling describes a direct relationship between the proton gradient and ATP synthesis. It is the predominant mechanism accepted in the scientific community when describing oxidative phosphorylation. However, another mechanism called conformational couplingsuggests that the relationship between the proton gradient and ATP synthesis is indirect. Instead, ATP is released by the synthase as a result of conformational change caused by the gradient. In this mechanism, the F1 portion of ATP synthase is reminiscent of a turbine, spinning within a stationary compartment to facilitate the harnessing of gradient energy for chemical bonding.
When tackling complex mechanisms such as chemiosmotic coupling on Test Day, it's easy to make mistakes such as interpreting a pH drop to be a [H+] drop instead of a rise in proton concentration. Always read actively in order to avoid such mistakes.
So we now know how we generate ATP, but how much energy was required to do so? When the proton-motive force is dissipated through the F0 portion of ATP synthase, the free energy change of the reaction, ΔG°′, is a highly exergonic reaction. This makes sense because phosphorylating ADP to form ATP is an endergonic process. So, by coupling these reactions, the energy harnessed from one reaction can drive another.
Uncouplers are compounds that prevent ATP synthesis without affecting the ETC, thus greatly decreasing the efficiency of the ETC/oxidative phosphorylation pathway. Because ADP builds up and ATP synthesis decreases, the body responds to this perceived lack of energy by increasing O2 consumption and NADH oxidation. The energy produced from the transport of electrons is released as heat. An example would be the fever experienced with toxic levels of salicylates, including aspirin.
Because the citric acid cycle provides the electron-rich molecules that feed into the ETC, it should come as no surprise that the rates of oxidative phosphorylation and the citric acid cycle are closely coordinated. Always think of O2 and ADP as the key regulators of oxidative phosphorylation. If O2 is limited, the rate of oxidative phosphorylation decreases, and the concentrations of NADH and FADH2 increase. The accumulation of NADH, in turn, inhibits the citric acid cycle. The coordinated regulation of these pathways is known as respiratory control. In the presence of adequate O2, the rate of oxidative phosphorylation is dependent on the availability of ADP. The concentrations of ADP and ATP are reciprocally related; an accumulation of ADP is accompanied by a decrease in ATP and the amount of energy available to the cell. Therefore, ADP accumulation signals the need for ATP synthesis. ADP allosterically activates isocitrate dehydrogenase, thereby increasing the rate of the citric acid cycle and the production of NADH and FADH2. The elevated levels of these reduced coenzymes, in turn, increase the rate of electron transport and ATP synthesis.
MCAT Concept Check 10.4:
Before you move on, assess your understanding of the material with these questions.
1. What is the difference between the ETC and oxidative phosphorylation? What links the two?
2. The ΔG° of NADH reducing oxygen directly is significantly greater than any individual step along the electron transport chain. If this is the case, why does transferring electrons along the ETC generate more ATP than direct reduction of oxygen by NADH?