The overproduction of energy in a cell is wasteful and its underproduction is deadly. A cell must regulate the activities of key enzymes to maintain stable energy levels through the stable production of metabolic intermediates. The TCA cycle has four points of regulation, starting with the production of acetyl-CoA, which feeds into the cycle.

The production of acetyl-CoA from pyruvate is regulated by allosteric and covalent mechanisms. Let’s use what you know about enzyme regulation to identify the allosteric effectors on the screen as inhibitors or activators. These effectors regulate the enzyme complex pyruvate dehydrogenase, which catalyzes the conversion of pyruvate to acetyl-CoA.

Drag the effector molecules to their correct location in the table. Then click Submit to see if you’re correct.

Yes, that’s correct.

No, that’s incorrect.

Acetyl-CoA and NADH are products of this reaction. When their levels are high, they act to inhibit the enzyme complex that makes them. CoA and NAD+ are substrates of the reaction. When their levels are high, they activate the enzyme complex to consume them. The energy released in this oxidation reaction is ultimately used to synthesize ATP. Thus, high levels of ATP inhibit the reaction.

AMP levels are tied to ADP levels. When ADP is consumed in ATP synthesis, both ADP and AMP levels decrease. Thus, AMP is an activator because its concentration is tied to the concentration of the substrate, ADP.

The other three points of regulation are related to the three exergonic steps—those steps with large, negative free energy changes. Steps 1, 3, and 4 are catalyzed by allosterically regulated enzymes. The activities of these enzymes are either activated or inhibited in response to the binding of effectors or to covalent modification.

The rates of glycolysis and the TCA cycle are integrated. For example, recall that citrate, an intermediate of the TCA cycle, inhibits phosphofructokinase, the enzyme that catalyzes the third reaction of glycolysis. The TCA cycle is the major pathway of carbohydrate oxidation in aerobic cells. Some cycle intermediates are used as precursors in the biosynthesis of amino acids. Other metabolic reactions replenish TCA cycle intermediates. All of these pathways are regulated by and integrated with each other.

Now that we’ve identified the points at which the TCA cycle is regulated, let’s look at why regulation is important.

Aerobic metabolism produces more energy than metabolism in the absence of oxygen. This is because carbon is fully oxidized to carbon dioxide in aerobic metabolism. In the TCA cycle, some of the oxidation energy is used for making one ATP equivalent in reaction 5, the reaction that produces GTP. The rest of the energy coming from carbon oxidation is captured by the coenzymes NAD and FAD, which become reduced. Three NADH and one FADH2 are made for every turn of the TCA cycle. Every NADH drives the synthesis of 2.5 to 3 ATP molecules, and every FADH2 drives the synthesis of 1.5 to 2 molecules of ATP. When we add in the GTP from reaction 5, this means that one turn of the TCA cycle generates between 10 and 12 molecules of ATP!

SECTION_005: Summary

Two carbon atoms enter the TCA cycle in the form of acetyl-CoA and two carbon atoms leave as carbon dioxide. There are eight reactions in the TCA cycle, half of which are oxidation-reduction reactions. The oxidation of carbon is coupled to the reduction of three molecules of NAD and one molecule of FAD. These reduced coenzymes feed electrons into the electron transport chain. As the electrons are transferred through the transport chain, the released potential energy is captured and used to synthesize ATP. Additional released energy drives the synthesis of one molecule of GTP, which is energetically equivalent to ATP.

Because the TCA cycle is a cyclic metabolic pathway, the starting molecule, oxaloacetate, is regenerated in the last step. Two carbon atoms enter the cycle and two carbon atoms must leave in order for oxaloacetate to be regenerated. However, the carbon atoms that are released as carbon dioxide in one turn of the cycle are not the same as the carbon atoms that enter as part of acetyl-CoA. Experiments with radioactively labeled molecules showed that the atoms are shuffled as they travel through the cycle.

As we’ve seen in glycolysis, metabolic pathways are strictly regulated. The conversion of pyruvate to acetyl-CoA is controlled, as are the activities of the other enzymes that catalyze reactions with large, negative free energy changes. In addition, the flow of metabolites through the TCA cycle is coordinated with many other metabolic pathways, including glycolysis.

The TCA cycle is the major pathway of carbohydrate oxidation in aerobic cells. It generates three NADH and one FADH2 molecules. The energy stored in these reduced coenzymes is released in the electron transport chain and drives the synthesis of additional ATP molecules by oxidative phosphorylation.

Recall that two molecules of pyruvate enter the TCA cycle per glucose molecule and that one NAD+ is reduced for every pyruvate converted into acetyl-CoA. Two ATP and two NADH are generated in glycolysis.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education