Four pairs of electrons are transferred during one turn of the TCA cycle: three pairs reducing three NAD+ to NADH and one pair reducing FAD to FADH2. Oxidation of one NADH by the ETC leads to formation of three ATP, whereas oxidation of FADH2 produces two ATP (see p. 77). The total yield of ATP from the oxidation of one acetyl CoA is shown in Figure 9.7. Figure 9.8 summarizes the reactions of the TCA cycle. [Note: The cycle does not involve the net consumption or production of intermediates. Two carbons entering as acetyl CoA are balanced by two CO2 exiting.] dinucleotides; GDP and GTP = guanosine di-and triphosphates; Pi = inorganic phosphate.
IV. CYCLE REGULATION
In contrast to glycolysis, which is regulated primarily by PFK-1, the TCA cycle is controlled by the regulation of several enzymes (see Fig. 9.8). The most important of these regulated enzymes are those that catalyze reactions with highly negative ∆G0: citrate synthase, isocitrate dehydrogenase, and the αketoglutarate dehydrogenase complex. Reducing equivalents needed for oxidative phosphorylation are generated by the PDHC and the TCA cycle, and both processes are upregulated in response to a decrease in the ATP/ADP ratio.
V. CHAPTER SUMMARY