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Catabolism of Proteins and Amino Acids

©2000 Timothy Paustian, University of Wisconsin-Madison

Remember that proteins are polymers. They are large and just like lipids need to be broken down into smaller pieces before being transported into the cell. A class of extracellular enzymes called proteases break down proteins into peptides (short polymers of amino acids). There are many different proteases synthesized by cells and each of them have a different specificity. The ones used in catabolism tend to be nonspecific and attack many different peptide bonds between amino acids.

The small peptides produced can then be transported into the cell where they are further degraded into amino acids. Several of the amino acids are structurally so similar to important intermediates in the TCA cycle and other major metabolic pathways that it is a simple matter to convert them into "central metabolites". In most cases this involves removal of the amino group (deamination). Below is listed some deaminations and the products produced.

Whereas 2-oxoglutarate, oxaloacetate and pyruvate are central metabolites and can be easily metabolized, 2-oxoisovalerate and 2-oxoisocaproate are not and must be handled by specific catabolic pathways. Eventually these pathways lead into glycolysis or the TCA cycle.

The specific catabolic products produced depend upon the amino acid and it is not useful to study all twenty pathways. The general pattern to understand is that proteins are broken into amino acids by proteases. The amino acids are attacked by various pathways that then feed into the TCA cycle to generate energy.

Figure 1 - A general pathway for the catabolism of proteins and amino acids. Specific pathways for every amino acid are not shown The ability to grow on a certain amino acid as sole carbon and energy source is specific to the strain of bacteria and not every bacteria is going to be capable of growth on an amino acid..

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