In contrast to the complexities of protein synthesis, described in Chapter 5, protein catabolism requires only a few enzymes, termed proteases, to break the peptide bonds between amino acids. Some of these enzymes split off one amino acid at a time from the ends of the protein chain, whereas others break peptide bonds between specific amino acids within the chain, forming peptides rather than free amino acids.
Amino acids can be catabolized to provide energy for ATP synthesis, and they can also provide intermediates for the synthesis of a number of molecules other than proteins. Since there are 20 different amino acids, a large number of intermediates can be formed, and there are many pathways for processing them. A few basic types of reactions common to most of these pathways can provide an overview of amino acid catabo-lism.
Unlike most carbohydrates and fats, amino acids contain nitrogen atoms (in their amino groups) in addition to carbon, hydrogen, and oxygen atoms. Once the nitrogen-containing amino group is removed, the remainder of most amino acids can be metabolized to intermediates capable of entering either the glycolytic pathway or the Krebs cycle.
The two types of reactions by which the amino group is removed are illustrated in Figure 4-28. In the first reaction, oxidative deamination, the amino group gives rise to a molecule of ammonia (NH3) and is replaced by an oxygen atom derived from water to form
Keto acid Ammonia
Amino acid 1 Keto acid 2
Keto acid 1 Amino acid 2
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