Useful Notes on the Breakdown of Protein

Pathways of protein and amino acid degradation are different from synthetic pathways, and various types of reactions are involved:

The breakdown of protein molecules is initiated by division into two or more smaller molecules that may still be of a high molecular weight. The molecular weight of protein molecules is in the range of 15,000 to 5 million.

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Proteinases responsible for the initial breakdown by hydrolysis of internal peptide bonds of large molecules are themselves proteins. Bacterial enzymes act poorly on some purified proteins, and growth is usually poor or absent when pure protein is the only nitrogen source available.

Vigorous growth may be essential for the production of protein-splitting enzymes. Collagen and some other proteins, however, are readily attacked by both aerobes and anaerobes under favourable growth conditions.

Because collagen is the type of protein found in bones, connective tissue, and skin, it is readily attacked in vivo by pathogenic CI. Welchii. Elastin, the fibrous protein found in ligaments, is attached by strains of Vibrio comma and Pseudomonas.

Keratin the protein of hair and nails is not readily attacked by bacteria until after breakage of the disulfide (S—S) linkage. Casein and gelatin are readily attacked by a large number of microorganisms.

Proteoses, peptones, and peptides, which are partially digested protein, are readily attached by a broad spectrum of microorganisms. Proteinases of microorganisms are similar in some respects to those found in animals, although there are extremely wide variations of enzyme types among microorganisms.

Bacterial proteinases are generally exocellular and break down proteins that cannot pass through the cytoplasmic barrier. Since passage through the cytoplasmic barrier itself is controlled by proteinaceous enzymes, termed permeases, the picture of bacterial breakdown of protein becomes deeply involved.

Peptidases are usually intracellular, and the breakdown of peptides, therefore, proceeds inside cells. Why do enzymes not break down the protein of cells that produce them?

The answer to this question is poorly understood, but it is presumed that enzymes of a cell are not specific for its own proteins or that homologous cell proteins have to be altered in some manner before enzymes of containing cells will attack them.

Final protein hydrolysis yields amino acids, but these are further degraded by microbial action:

Final products of protein hydrolysis are amino acids. This is not the end of enzyme action, however, as each amino acid is ultimately broken down, changed, or resynthesized into protein.

This process, which is one of paramount importance, is catalyzed by the enzyme transaminase and requires the presence of the coenzyme pyridoxal (amine) phosphate. When amino acids are broken down, ammonia is given off.

Ammonia may be reconverted into nitrites and finally into nitrates by Nitrosomonas and Nitrobacter, respectively. This process is described in the nitrogen cycle. These coenzyme forms are built around vitamin B₆ (pyridoxine).

One common type of amino acid breakdown is oxidative deamination, which in microbial metabolism seldom results in formation of the corresponding keto-acid as a final product. The keto-acid is usually changed rapidly by secondary reactions.

The process may be illustrated by the conversion of alanine to pyruvate, as just shown. Keto-acids that result from amino acid degradation can be isolated under favourable conditions but are usually rapidly degraded. The most prevalent reactions on keto-acids are reduction and decarboxylation. For example, ketobutyrate may be changed to propionate as follows:

Decarboxylation of amino acids, which does not result in the production of ammonia, is carried out by some bacteria, and resulting amines were termed ptomaines by workers. Ptomaines (amines) possessed foul odors and were considered to be highly toxic.

They were once thought to be the cause of ptomaine poisons from the ingestion of spoiled meats but have been shown to exert low, if any, toxicity when ingested, although they may cause disturbances if injected parenterally. The following are some amino acids and their related amines:

Biological properties of some of these amines are of interest. Cadaverine obtained its name from the odors or corpses, or cadavers. The amine probably arises by bacterial action on the cadaver and is responsible for a large portion of the accompanying odor. Putrescine characterizes putrefying materials.

Although probably not produced by microbial action on tryptophan, serotonin (S-hydroxytryptamine) is important because of the neurological action on animals. Histamine causes the contraction of uterine muscles and plays an important role in allergic reactions in animals. It also causes a lowering of blood pressure.

Amines formed by decarboxylation of amino acids are themselves attacked by microorganisms, among which are a number of potential and active pathogens. Mycobacteria act on cadaverine, histamine, and putresu i.e., Putrescine and cadaverine are oxidized by Pseudomonas pyocyanea and Proteus morganii, and Staphylococcus aureus will oxidize cadaverine. The usual breakdown products of amino acids, or amines derived from them, by aerobic metabolism are ammonia, carbon dioxide, and water.