The traditional teaching classifies different.proteins into three major groups,

The traditional teaching classifies different.proteins into three major groups, viz, simple, conjugated and derived proteins. Each group is further divided into several subgroups (table 7.11.1). Fibrous and globular proteins The three dimensional shape of most of the proteins (e.g. albumins, globulins) are globular and hence they are called globular proteins. A few proteins (e.g. collagen, found in fibrous tissue), have elongated structure and they are called fibrous protein. Denaturation of proteins Normally, the white of the egg is liquid and almost colorless. On boiling, the white of the egg becomes solid and white, yet the number of ammo acids and their sequence in the polypeptide chains remain same. This is an example of denaturation; the egg white has been denatured by the heat. A protein is denatured when its secondary or tertiary structure is disrupted. The term 'native protein' means a protein which has not been denatured, i.e., in the form as it occurs in the nature. Introduction The food proteins are digested in the digestive tract and are ultimately converted into amino acids Only the amino acids are absorbed, the proteins /proteose/peptone/ polypeptides are not. [The newborns are, however, exceptions. They can and do absorb, unbroken, whole protein molecules to some extent. Thus, in colostrum of the mother's breast, there are many immunoglobulm molecules which can be absorbed as such by the new borns and thus the new borns straightaway get the passive immunity from the mother]. Amino acid pool In our body, it may be imagined, that there is an 'amino acid pool' (pool = a pond or a tank). The pool includes such places like plasma and tissue fluid. Various tissues pick up their required amino acids from this pool and synthesize their characteristic proteins. At the same time, tissues, which are disintegrating, discharge the amino acids (obtained by breakdown of proteins of the disintegrating tissues) into the pool. The size of this pool remains practically constant. Excess aminoacids, when present, are treated in such a way that the NH2 group is removed and converted into urea and the ketoacid (i.e., the amino acid minus the NH2 portion) merges in the stream of carbohydrate or lipid metabolism; or, it is utilized to form a new amino acid. It should be known that the liver picks up the amino acids so extensively from the portal venous blood and anabolizes or catabolizes them that following meals the amino acid pool does not show appreciable increase. Figure 7.12.1 shows the avenues through which the pool loses or gams amino acid Dynamic equilibrium In the 1930s, Rudolf Schonheimer of pre war Germany (who was awarded a Nobel Prize for his works on biochemistry) experimentally showed that : When a dog is fed with isotopically labelled aminoacid (the nitrogen being tagged, 15N), the urea in the urine of the dog does not contain 15N in the initial few days. But if the dog is killed at this stage, his liver, muscles and other organs reveal the presence of 15N. Schonheimer concluded that the amino acids of the food protein replaced the aminoacids of the body proteins and thus were incorporated in the body of the dog whereas the aminoacids of the body proteins were metabolised and appeared as urinary urea in other words, the total amount of amino acids of the body remain same but the individual molecules of the amino acids are being continuously replaced by those of the food proteins. This is called 'dynamic equilibrium'. Anabolism of protein. Necessity of constant and regular supply of food proteins For replenishment of wear and tear, the various tissues pick up ammo acids from the 'amino acid pool' and synthesize the protein, characteristic of the tissue in question. In our body, large number of cells are dying daily and they must be replenished by the newly synthesized cells. In growing children, additional proteins (additional to the quantity required for replenishment of wear and tear) are required for the growth. Similar additional deposits occur in the convalescents and in persons undergoing physical exercise programme for muscular hypertrophy. .Some endocrine organs (pancreas, parathyroid, hypothalamus etc. ) pick up amino acids for synthesizing their specific hormones which are protein in nature. All enzymes are protein in nature. Cells secreting enzymes, therefore, pick up aminoacids from the pool to prepare the enzymes. Lactating women require extra protein for their milk proteins. CATABOLISM OF AMINOACIDS The fate of the aminoacids, which have become excess, is as follows: These ammo acids undergo catabolism. For this, the amino group is removed first. The NH2 group is utilized for urea synthesis and urea is excreted via the urine. The NH2 free part of the amino acid is a keto acid. The fate of this keto acid has been described subsequently. The removal of NH2 group from the amino acid can be achieved by various ways, e.g., (i) transamination, and(ii) deamination. Further there are more than one way to deaminate the amino acids (see below) :Transamination In this process, the NH2 in the a position of an amino acid (the 'donor' acid) is transferred to a keto acid (the 'recipient' acid). As a result, the donor acid becomes the corresponding a keto acid and the recipient becomes the corresponding amino acid. The general reaction is shown below. The reaction is catalyzed by transaminase (also called aminotransferase) group of enzymes. The 'recipient' amino acid is either a ketoglutaric acid (also called a oxoglutaric acid) or pyruvic acid or oxaloacetic acid. On amination, they