28 GENERAL CONCEPTS 



H H O H H O 



\ 1 ^ \ I ^ 



N— C— C b. a. N— C— C b. 



/ I \ / 1 \ 



H H OH H 1 OH 



CH3 



glycine alanine 



I 



+ H2O 



CH3 OH 



linkage 

 glycylalanine 



Figure 2.6. Structural formulas of the amino acids glycine and alanine, showing, a, 

 the amino group and, b, the acid (carboxyl) group. These are joined in a peptide linkage 

 to form glycylalanine by the removal of water. 



Protein molecules are made of simpler components, the amino acids, 

 some thirty or more of which are known. Since each type of protein con- 

 tains hundreds of amino acids, present in a certain proportion and in a 

 particular order, an almost infinite variety of different proteins is 

 possible. In recent years, powerful analytical methods have been de- 

 veloped which permit one to determine the arrangement of the amino 

 acids in a given protein molecule. This is an extremely difficult and 

 tedious task. Insulin, the hormone secreted by the pancreas and used in 

 the treatment of diabetes, was the first protein whose structure was 

 elucidated. Work culminating in 1957 revealed the structure of the 

 enzyme ribonuclease. 



Each cell contains hundreds of different proteins and each kind of 

 cell contains some proteins which are unique to it. There is evidence 

 that each species of animal and plant has certam proteins which are 

 different from those of all other species. The degree of similarity of the 

 proteins of two species is a measure of their evolutionary relationship. 

 The theory of species specificity states that the protoplasm of each 

 species has a characteristic pattern of its constituent proteins and that 

 this pattern differs at least slightly from that of related species and more 

 markedly from those of more distantly related species. Because of the 

 interactions of unlike proteins, grafts of tissue removed from one animal 

 will usually not grow when implanted on a host of a different species. 



Amino acids, the unit building blocks of proteins, differ in the 

 number and arrangement of their constituent atoms, but all contain an 

 amino group (NHo) and an acid group (COOH), whence their name. 

 The amino group enables the amino acid to act as a base and combine 

 with acids; the acid group enables it to combine with bases. For this 

 reason, amino acids and proteins are important biological "buffers" and 

 resist changes in acidity or alkalinity, thus protecting protoplasm. Pro- 

 tein molecules are built up by linkages (called peptide bonds) between 

 the amino group of one amino acid and the acid group of the adjacent 

 one (Fig. 2.6). Pure amino acids have a rather sweet taste. The proteins 

 eaten by an animal are not incorporated directly into the protoplasm 



