Citric Acid Cycle (Krebs Cycle)

The citric acid cycle represents the central pathway for catabolic conversion of fatty acids, monosaccharides such as glucose, fructose etc., and amino acids hydrolysed from proteins. The enzymes associated with this pathway are found in the matrix of the mitochrondrian. This link illustrates the organelle.

One source of acetyl-coenzyme-A produced in the mitochrondrian is linked to the production of pyruvate from aldohexoses in the cytoplasmic process called glycolysis. Pyruvate interactes with a multisub-unit enzyme complex known as pyruvate dehydrogenase. The basic conversion is illustrated below:

The overall course of  this reaction is quite complex: coenzymes associated with the conversion, in addition to Coenzyme A, are thiamine pyrophosphate, lipoic acid and FAD. This conversion is illustrated by clicking on the following link.

The citric acid cycle (Krebs or tricarboxylic acid) represents the central pathway by which carbons derived from carbohydrates, triacylglycerols and proteins are elevated to the oxidation state of carbon dioxide. The reactions associated with this process are detailed within the following link.

This cycle has an alternative pathway that is found mainly in plants and is called the gyloxalate shunt. This sequence is depicted by clicking on the following link. This pathway represents our initial introduction to the problems related to the movement of molecules from the mitochrondrial matrix into the cytoplasm and back into the matrix. Here, the critical molecule oxaloacetate has no transporter for movement into the cytoplasm. However the corresponding amino acid, asparate; derived from oxaloacetate via a transamination reaction does. This conversion and its reversal in the glyoxasome affords a pathway fro such movement. This pathway is very important in plants because it provides a molecular link between fatty acids derived from triacylglycerols and carbohydrates associated with the oxaloacetate structure when exported as discussed above.

The transamination mechanism is important and is illustrated in the following link.

Finally, This process is not a closed process as suggested by the transamination-type reactions. Also, it is not in a strict sense just catabolic. It contributes in a variety of ways to anabolic events eg, AA synthesis; therefore we will describe the cycle as amphibolic in nature. This link illustrates this nature. In addition, the cycle is highly regulated both by specific intermediates as well as the NAD+/NADH ratio. This link illustrates the significant regulators of the citric acid cycle