5. Arsenic poisoning: As previously described (see p. 101), pentavalent arsenic (arsenate) can interfere with glycolysis at the glyceraldehyde 3phosphate step, thereby decreasing ATP production. However, arsenic poisoning is due primarily to inhibition of enzyme complexes that require lipoic acid as a coenzyme, including PDH, α-ketoglutarate dehydrogenase (see E. below), and branched-chain α-keto acid dehydrogenase (see p. 266). Arsenite (the trivalent form of arsenic) forms a stable complex with the thiol (−SH) groups of lipoic acid, making that compound unavailable to serve as a coenzyme. When it binds to lipoic acid in the PDHC, pyruvate (and, consequently, lactate) accumulates. As with PDHC deficiency, this particularly affects the brain, causing neurologic disturbances and death.
B. Citrate synthesis
The irreversible condensation of acetyl CoA and OAA to form citrate (a tricarboxylic acid) is catalyzed by citrate synthase, the initiating enzyme of the TCA cycle (Fig. 9.4). This aldol condensation has a highly negative change in standard free energy ([∆G0] see p. 70), which strongly favors citrate formation. The enzyme is inhibited by citrate (product inhibition). Substrate availability is another means of regulation for citrate synthase. The binding of OAA greatly increases the enzyme’s affinity for acetyl CoA. [Note: Citrate, in addition to being an intermediate in the TCA cycle, is a source of acetyl CoA for the cytosolic synthesis of fatty acids (see p. 183) and cholesterol (see p. 220). Citrate also inhibits phosphofructokinase-1 (PFK-1), the rate-limiting enzyme of glycolysis (see p. 99), and activates acetyl CoA carboxylase (the rate-limiting enzyme of fatty acid synthesis, see p. 183).]
C. Citrate isomerization
Citrate is isomerized to isocitrate through hydroxyl group migration catalyzed by aconitase (aconitate hydratase), an iron-sulfur protein (see Fig. 9.4). [Note: Aconitase is inhibited by fluoroacetate, a plant toxin that is used as a pesticide. Fluoroacetate is converted to fluoroacetyl CoA that condenses with OAA to form fluorocitrate, a potent inhibitor of aconitase.]
D. Oxidative decarboxylation of isocitrate
Isocitrate dehydrogenase catalyzes the irreversible oxidative decarboxylation of isocitrate to α-ketoglutarate, yielding the first of three NADH molecules produced by the cycle and the first release of CO2 (see
Fig. 9.4). This is one of the rate-limiting steps of the TCA cycle. The enzyme is allosterically activated by ADP (a low-energy signal) and Ca2+ and is inhibited by ATP and NADH, levels of which are elevated when the cell has abundant energy stores.
E. Oxidative decarboxylation of α-ketoglutarate