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Chapter 22: Aerobic Respiration and Energy Production

Terms in this set (54)

Just like glycolysis, the citric acid cycle is responsive to the energy needs of the cell. The pathway speeds up when there is a greater demand for ATP, and it slows down when ATP is in excess. Four enzymes or enzyme complexes involved in the complete oxidation of pyruvate are allosteric enzymes. Here are the enzymatic steps that keep the pathway regulated:

1. Conversion of pyruvate to acetyl CoA. The pyruvate dehydrogenase complex is inhibited by high concentrations of ATP, acetyl CoA, and NADH. Of course, the presence of these compounds in abundance signals that the cell has an adequate supply of energy, and thus energy metabolism is slowed.

2. Synthesis of citrate from oxaloacetate and acetyl CoA. The enzyme citrate synthase is an allosteric enzyme. In this case, the negative effector is ATP. Again, this is logical because an excess of ATP indicates that the cell has an abundance of energy.

3. Oxidation and decarboxylation of isocitrate to alpha-ketoglutarate. Isocitrate dehydrogenase is also an allosteric enzyme; however, the enzyme is controlled by the positive allosteric effector, ADP. ADP is a signal that the levels of ATP must be low, and therefore the rate of the citric acid cycle should be increased. Interestingly, isocitrate dehydrogenase is also inhibited by high levels of NADH and ATP.

4. Conversion of alpha-ketoglutarate to succinyl CoA. The alpha-ketoglutarate dehydrogenase complex is inhibited by high levels of products of the reactions that catalyzes, namely NADH and succinyl CoA. It is further inhibited by high concentrations of ATP.