RL: We know by now that glycolysis takes place in the cytoplasm, and leads to the formation of two molecules of pyruvate per molecule of glucose. These pyruvate molecules are converted into acetyl coenzyme A, also known as acetyl-CoA, and this happens inside the mitochondrial matrix. Therefore, acetyl-CoA is the key metabolic intermediate that links glycolysis and the citric acid cycle. For clarity, we will not show the coenzyme portion of acetyl-CoA, and instead are simply representing it as the two-carbon molecule acetate. So how does the pyruvate get into the mitochondrial matrix? This depends on a carrier protein that transports pyruvate into the mitochondrial matrix, where there it is oxidized to form acetyl-CoA by the enzyme pyruvate dehydrogenase. In this reaction, a molecule of carbon dioxide is generated as pyruvate is oxidized to acetyl-CoA, and a molecule of NAD+ is reduced to NADH. This last reaction is not shown for clarity. The citric acid cycle consists of the following eight steps, and also harvests chemical energy, producing a total of three NADH molecules. In the first step of the citric acid cycle, the enzyme citrate synthase combines the two-carbon acetyl group from acetyl-CoA with the four-carbon molecule oxaloacetate. This generates the six-carbon molecule citrate. In the second step, citrate is isomerized into isocitrate by aconitase. In the third step, isocitrate is oxidized by isocitrate dehydrogenase, generating the five-carbon molecule alpha-ketoglutarate. In the process, one carbon dioxide molecule is released, and one NAD+ molecule is reduced to NADH. In the fourth step, alpha-ketoglutarate is decarboxylated and oxidized to produce another molecule of carbon dioxide by alpha-ketoglutarate dehydrogenase. The resulting molecule is combined with coenzyme A, forming succinyl-CoA. In addition, another molecule of NAD+ is reduced to form NADH. Note that, in this diagram, we are showing the product of this reaction as succinate, and are omitting the coenzyme A for clarity. In the fifth step, succinyl coenzyme A is converted into succinate by succinyl-CoA synthetase, releasing coenzyme A. This reaction also produces one molecule of GTP, another high-energy molecule, which is omitted from this diagram for clarity. In the sixth step, succinate is oxidized to fumarate by succinate dehydrogenase. Unlike all the other enzymes in the citric acid cycle, succinate dehydrogenase is embedded in the inner mitochondrial membrane, and is also a component of the electron transport chain, in which it is also called Complex II. As Complex II oxidizes succinate, it receives two electrons that it will ultimately pass on to coenzyme Q, which in turn will deliver its electrons to Complex III. Another interesting aspect of Complex II is that it uses FAD rather than NAD+ to accept electrons. In the seventh step, fumarate is converted to malate by the enzyme fumarase. And finally, in the eighth step, the enzyme malate dehydrogenase oxidizes malate to re-form oxaloacetate, and reduces a molecule of NAD+ to NADH in the process. This regenerated oxaloacetate can then combine with another acetyl-CoA in the next cycle.
