METABOLISM OF PROTEINS

INTRODUCTION: The main role of amino acids is in the synthesis of structural and functional proteins. Unlike carbohydrates and fats, there is no storage form of proteins in the body. A 70 kg man has an average protein turnover rate of 400 g per day. This is almost the same amount synthesized, and same amount broken down. The non-essential amino acids are either derived from the diet or synthesized in the body, and the essential amino acids are obtained from the diet. Even if one essential amino acid is deficient, protein synthesis cannot take place.

THE AMINO ACID POOL: The body amino acid pool is always in a dynamic steady state. In an adult, the rate of synthesis of proteins balances the rate of degradation, so that nitrogen balance is maintained.

DIGESTION OF PROTEINS: The dietary proteins are denatured on cooking and therefore more easily digested. All these enzymes are hydrolases (class 3 enzymes) in nature. Proteolytic enzymes are secreted as inactive zymogens which are converted to their active form in the intestinal lumen. This would prevent autodigestion of the secretory acini.

The proteolytic enzymes include: 1) Endopeptidases: These act on peptide bonds inside the protein molecule, so that the protein becomes successively smaller and smaller units. This group includes Pepsin, Trypsin, Chymotrypsin, and Elastase.

2) Exopeptidases, which act at the peptide bond only at the end region of the chain. This group includes: 2-A. Carboxypeptidase acts on the peptide bond only at the carboxy terminal end on the chain. 2-B. Aminopeptidase, which acts on the peptide bond only at the amino terminal end on the chain.

The digestion of protein is effected by enzymes in: A) Stomach B) Pancreas and C) Intestinal cells

GASTRIC DIGESTION OF PROTEINS: In the stomach, hydrochloric acid is secreted. It makes the pH optimum for the action of pepsin and also activates pepsin. The acid also denatures the proteins.

1. Rennin: Rennin otherwise called Chymosin, is active in infants and is involved in the curdling of milk.

2. Pepsin: It is secreted by the chief cells of stomach as inactive pepsinogen. The conversion of pepsinogen to pepsin is brought about by removal of 44 amino acids from the N-terminal end, by the hydrochloric acid. The optimum pH for activity of pepsin is around 2. Pepsin is an endopeptidase. Pepsin catalyses hydrolysis of the bonds formed by carboxyl groups of Phe, Tyr, Trp and Met. By the action of pepsin, proteins are broken into proteoses and peptones.

PANCREATIC DIGESTION OF PROTEINS: The optimum pH for the activity of pancreatic enzymes about pH 8 is provided by the alkaline bile and pancreatic juice. The secretion of pancreatic juice is stimulated by the peptide hormones, Cholecystokinin and Pancreozymin. Pancreatic juice contains the important endopeptidases, namely Trypsin, Chymotrypsin, Elastase and Carboxypeptidase. These enzymes are also secreted as zymogens that is trypsinogen, chymotrypsinogen and pro-elastase, so that the pancreatic acinar cells are not autolysed. All the three are serine proteases, that is to say, the active centers of these enzymes contain serine residues.

RELATED;

1.  DIGESTION OF CARBOHYDRATES

2.  METABOLISM AND METABOLIC DISORDERS

3.  BIOCHEMISTRY