The chemical constitution of the proteins / by R.H.A. Plimmer.

  • Plimmer, Robert Henry Aders, 1877-1955.
Date:
1917-
Catalogue details

Licence: Public Domain Mark

Credit: The chemical constitution of the proteins / by R.H.A. Plimmer. Source: Wellcome Collection.

    20/196 (page 4)
    17. Tryptophan, C,,H,,.N.O., or B-indole-a-aminopropionic acid. C—CH,.CH(NH,). COOH \ E. Ammonia. These seventeen amino acids, together with ammonia, which is present in the form of acid amide groups, are the basis of the com- position of the protein molecule. They are generally referred to as ‘ Bau-steine,” the bricks, or foundation stones; but as the English translation of the German word is not entirely expressive of its mean- ing, it is preferable to use the term unit or element for these compounds in their relation to the proteins. Most proteins contain all these amino acids or units in various proportions, but some proteins, such as gelatin, contain only fourteen or fifteen, and some, such as salmine, are built up of three or four units, these units being di-amino acids and histidine. Full data of the pro- portions, so far as is known, are given in the tables on pp. II1I-130, and further reference to details is made on pp. 63-84. Our purely chemical knowledge of the proteins has led to a greater knowledge of the digestion of proteins. Proteins are completely con- verted into their constituent units in the alimentary canal of animals; in this form they reach the bloodstream and circulate to the various organs of the body. Each organ rebuilds its tissue from the circulat- ing amino acids, as well as giving rise to them during its catabolism. In plants, too, protein is transferred from one region to another in the form of amino acids, We think of and work with proteins in terms of amino acids. All considerations and deductions as to the dietetic value of proteins can no longer be based upon their total nitrogen content, but must be based upon the amounts of the various amino acids in their molecule. Nitrogen equivalence is not amino acid equi- valence. Proteins devoid of certain units are of no value for the maintenance of life; proteins deficient in certain amino acids are of little or of no value for ensuring the growth of young animals. These particulars are given in Cathcart’s Physzology of Protein Metabolism, second edition. The analysis of proteins with special reference to those units of metabolic value is therefore of the greatest importance. The long list of amino acids is sufficient evidence of the complexity of the protein molecule; and, as yet, it seems to be incomplete, for several other products have been described :—