Regulatory Enzymes of the Citric Acid Cycle

The citric acid cycle is primarily known as the final stage of carbohydrate metabolism, but is also the final pathway in fatty acid catabolism and many amino acids. It is a oxidative metabolic pathway converting carbon atoms to CO2 and drives ATP synthesis. The citric acid cycle takes place in the cytosol of prokaryotes and in the mitochondria of eukaryotes. In prokaryotes and eukaryotes, the cycle takes place in eight steps. The cycle always starts with carbon atoms in the form of acetyl groups. In the case of carbohydrate metabolism, pyruvate enters the citric acid cycle by utilizing pyruvate dehydrogenase to transfer coenzyme A to an acetyl group resulting in acetyl-CoA. The net equation of the citric acid cycle is: Acetyl-CoA + GDP + Pi + 3NAD+ + Q --> 2CO2 + CoA + GTP + 3NADH + QH2.

Citrate Synthase and Aconitase

• The first reaction of the citric acid cycle consists of acetyl-CoA condensing with oxaloacetate via citrate synthase to produce citrate. This particular step is exergonic and is one of the few enzymes capable of synthesizing a carbon-carbon bond without a metal ion cofactor.

  The second reaction in the citric acid cycle is a reversible isomerization reaction. Citrate is catalyzed to isocitrate via an intermediate molecule named aconitate and the enzyme aconitase.

Isocitrate Dehydrogenase and Alpha-Ketoglutarate Dehydrogenase

• The third reaction involves isocitrate undergoing oxidative decarboxylation via isocitrate dehydrogenase forming alpha-ketoglutarate. This reaction also reduces NAD+ to NADH and releases a CO2.

  The fourth step in the citric acid cycle is another oxidative decarboxylation reaction, releases another CO2 molecule, and reduces another NAD+ to NADH. In this reaction, alpha-ketoglutarate forms Succinyl-CoA via alpha-ketoglutarate dehydrogenase.

Succinyl-CoA Synthetase and Succinate Dehydrogenase

• The fifth step of the citric acid cycle utilizes succinyl-CoA synthetase to cleave succinyl-CoA into succinate. During this reaction a phosphate group replaces CoA on succinyl-CoA to produce a succinyl-phosphate. Succinyl-phosphate then donates the phosphate group to a His residue which produces the product succinate. The phosphate group is transferred from the His residue to a GDP molecule to also release a GTP molecule.

  The last three reactions will convert succinate to the starting substrate oxaloacetate.

  The sixth reaction is a reversible dehydrogenation reaction that converts succinate to fumarate via succinate dehydrogenase. This reaction also coverts and FAD into FADH2.

Fumarase and Malate Dehydrogenase

• The seventh reaction utilizes fumarase to catalyze a reversible hydration of fumarate to malate. This reaction also utilizes a water molecule.

  The final step of the citric acid cycle regenerates an oxaloacetate via malate dehydrogenase from malate. This final reaction returns the cycle to its original state and releases another NADH from NAD+.

Insight

• Understanding the unique characteristics of the citric acid cycle will help clarify the purpose and process of the cycle. The citric acid cycle has three irreversible steps that serve as regulation points. The three irreversible reactions are reaction one, three and four. These three steps help regulate the frequency of the cycle.

  The citric acid cycle also has intermediates that are precursors to other metabolic molecules and functions which is why the citric acid cycle cannot be solely categorized as catabolic or anabolic. Citrate is used for fatty acid and cholesterol synthesis; alpha-ketoglutarate is used for amino acid and nucleotide synthesis; succinyl-CoA is used for heme synthesis; malate is used for pyruvate synthesis; oxaloacetate is used for glucose synthesis.

  Another important component is how the six NADH and two QH2 molecules are reoxidized. The six NADH molecules lead to 18 ATP molecules and the two QH2 molecules lead to four ATP molecules. This accounting system is based off one glucose molecule, which causes two citric acid cycles.