PDF Ebook The Role of Proteins In Metabolism
Proteins occupy a unique position in metabolism. Not only may they serve as sources of energy for the organism, but they constitute the most important raw materials out of which the complex structures of the body are built. While carbohydrates and fats are employed chiefly as fuels for the "protoplasmic fires," proteins, in the well balanced diet, serve mainly for purposes of synthesis.
Modern investigations have revealed the fact that proteins are composed of at least twenty amino acids. The amounts and methods of union of these acids vary widely in proteins of different sources. Indeed, no two proteins are exactly alike. Inasmuch as body proteins have their origin in those of the food, the latter must undergo complete disintegration before they can be utilized for synthetic purposes. This preparatory process is accomplished in the alimentary tract. At the completion of digestion the free amino acids pass unchanged into the portal circulation. Thence they are distributed throughout the organism, and serve as the substrates for the many synthetic transformations involved in the production of tissue constituents.
Very little information is available concerning the mechanism of protein anabolism. Each tissue protein is believed to be synthesized in the locality in which it is to exist. But the dominant factors which enable the cells to combine the amino acids with each other in the correct order, and in exactly the right proportions, are still beyond the realm of our comprehension. It may be shown mathematically that the number of possible isomeric proteins containing a single molecule of each of the twenty amino acids, united by the so-called peptid linkage, amounts to the incredible figure of 60 X 1020. When one recalls that proteins are not made up of single molecules of the individual amino acids, but contain many molecules of each kind; and that several methods of union, instead of one,may be employed in their synthesis, the number of possibilities is seen to be infinite. Thus the unerring accuracy with which specific proteins are synthesized out of the varying mixture of amino acids circulating in the blood is one of the most astounding attributes of living things.
But proteins are not the sole tissue components which originate in amino acids. Numerous non-protein nitrogenous materials arise in a like fashion. Thus, many of the hormones, and certain of the so-called "extractives" are manufactured from amino acids. Of the hormones, epinephrine and thyroxine are the best known chemically. Their structures indicate that they are probably derived from tyrosine or phenylalanine.
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