Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Oxidative Degradation of Amino Acids. The Urea Cycle
There are 20 different pathways for the degradation of amino acid carbon skeletons

Proteins are composed of 20 standard Amino Acids, which differ in their carbon skeletons. Accordingly, there are 20 distinct Catabolic pathways for their degradation. Of the total energy consumed by the Organism, all these pathways combined account for no more than 10%. This means that THE CONTRIBUTION OF each individual amino acid to overall METABOLISM averages around 0.5%. It is thus clear that The Significance of these Amino Acid Pathways, taken separately, cannot compare with that of Glycolysis or The Citric Acid Cycle. Therefore, we will not examine them in detail. The reason is that the 20 diverse catabolic pathways responsible for Amino Acid Breakdown ultimately converge to yield just five products, which then enter The Citric Acid cycle and are fully oxidized there to CO₂ and H₂O (Fig. 19-4).

As can be seen in Fig. 19-4, the carbon skeletons of Ten amino acids are degraded to yield acetyl-CoA. Five amino acids are converted into α-ketoglutarate, three into succinyl-CoA, two into oxaloacetate, and two into fumarate. Here, we group the individual pathways for the 20 amino acids into schemes where each path leads to a specific product capable of entering the citric acid cycle. (Carbon atoms destined to enter the citric acid cycle are highlighted in red in the schemes.) We will discuss separately certain enzymatic steps of these pathways that are of special interest, either due to an unusual reaction mechanism or because of their medical importance.