GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

12. METABOLIC ACTIVITY OF AEROBIC HETEROTROPHS

12.4. PROTEIN AND AMINO ACID CATABOLISM

Like other high-molecular-weight compounds, Proteins are initially degraded by extracellular proteases into fragments capable of easily penetrating The Cell — Peptides, oligopeptides, and partially Amino Acids (Fig. 12.7). Peptides enter the cell and are hydrolyzed by intracellular peptidases into amino acids. Amino acids are either utilized by the cell for Protein Synthesis or undergo transformations resulting in their deamination, after which they enter Intermediary METABOLISM.

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Fig. 12.7. Protein degradation and possible subsequent Transformations of Amino acids

Protein degradation in soil is accompanied by The formation of ammonia. This process involves the mineralization of nitrogen, or ammonification. Both Fungi and Bacteria participate in protein breakdown.

The initial reaction of Amino Acid Catabolism is decarboxylation, deamination, or Transamination. As a result of deamination and transamination, amino acids are converted into the corresponding oxo acids:

Decarboxylation occurs predominantly in an acidic environment. Protein AMINO ACID DECARBOXYLATION yields CO₂ and primary amines (also referred to as "biogenic" amines). The most notable among these are cadaverine, putrescine, and agmatine (formerly known as ptomaines or corpse poisons); they are produced from Lysine, Ornithine, and Arginine, respectively. Primary amines are detected during conventional putrefactive processes in the intestine and other anaerobic protein degradation pathways.

Deamination refers to the Cleavage of ammonia from an amino acid. Depending on The Fate of the carbon Skeleton of the amino acid, Oxidative Deamination, hydrolytic deamination, and deamination accompanied by the Formation of Unsaturated compounds are distinguished.

Oxidative deamination is the most common type of Amino Acid Breakdown. It proceeds via NAD(P)-dependent dehydrogenases:

Deamination accompanied by the formation of an unsaturated compound:

Hydrolytic deamination proceeds with the participation of hydratases:

The resulting 2-oxobutyrate is further oxidized by a multienzyme complex similar to the Pyruvate dehydrogenase complex, yielding propionyl-CoA.

During transamination, the amino group of an amino acid is transferred to a 2-oxo acid. Transamination is catalyzed by transaminases. Pyruvate and 2-oxoglutarate, for example, serve as amino group acceptors: