Using the traditional methods of the organic chemist, it can be shown that protein yield a mixture of amino acids when hydrolyzed by acid, alkali or enzymes. Most protein yield some 19 different amino acids. They are all alpha amino acids, and with the exception of glycine, (which contains no asymetric atom), they all belong to the L-series with reference to the alpha-carbon atom.
R-C(alpha)H-COOH-NH3
The structure of typical amono acid is given above, R represent some group which determine the particular kind of amino acid in question.
In the original protein molecule these amino acids are joined together by bonds between the carboxyl group of one amino acid and the amino group of another, with the splitting off of water.
The bond between two amino acides, -CO-NH- is called the peptide bond. When just two amino acids are involved, the resulting compound is called a dipeptide. The availability of an amino and a corboxyl group in the dipeptide makes further combinations with other (or the same) amino acids possible, so that we can build, tripeptide, tetrapeptides, and so on; that is to say, we build polypeptides. The large synthetic straight chain polypeptides begin to resemble protein in both their chemical and physical properties.
This polypeptide theory of the structure of proteins, due in large measure to Emil Fischer - is, however, only a partial explanation. The evidence which has accumulated more recently is for the existence, within the protein molecule, of large ring systems made up of many amino acids.
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