THE SYNTHESIS OF PROTEINS 443 



The Synthesis of Proteins. — The theory that proteins are formed by 

 the condensation of numerous amino acid molecules was first suggested by 

 Emil Fischer. He succeeded in linking together eighteen amino acid molecules 

 (fifteen of glycine, three of leucine) and thus producing a synthetic poly- 

 peptide in which the amino acids were bound together by peptide linkages. 

 A peptide linkage is one in which the amino group of one amino acid molecule 

 is united with the carboxyl group of another amino acid molecule, water being 

 split off in the process. The simplest dipeptide is that produced by condensa- 

 tion of two molecules of glycine : 



CH2-COOH 



CH2- COOH -f Hh-N CHo-COOH 



I L__ J I , 



NH2 H > CH^CO— NH 



NH2 ' 



The dipeptide formed by the condensation of the two amino acid molecules 

 possesses an amino group and a carboxyl group to which other amino acids 

 can be linked. The addition of amino acid molecules by peptide linkages 

 to either or both of these groups still leaves amino and carboxyl groups 

 present in the resulting molecule. Polypeptides, peptones, proteoses and 

 finally proteins are probably formed by the condensation of more and more 

 amino acid molecules in this way. According to this view a protein mole- 

 cule is a long chain-like structure composed of hundreds of amino acid residues 

 welded together through peptide linkages. 



Some properties of proteins can not be satisfactorily explained in terms 

 of the peptide linkage theory which suggests that other linkages also occur 

 in the natural proteins. Some investigators consider that proteins are con- 

 densation products of complex cyclic compounds rather than of amino acids. 



Every species of plant or animal produces characteristic and specific pro- 

 teins which are not found in other species. Therefore a very large number 

 of kinds of proteins must exist. It has generally been considered that the 

 number of possible combinations of amino acids found in proteins is virtually 

 innumerable. Random combination of 19 different kinds of amino acid 

 molecules in the formation of a polypeptide chain of only 50 molecular units 

 could take place in 10*^ different ways. 



Recent investigations by Bergmann and Nieman (1938) seem to indicate, 

 however, that the structure of protein molecules is governed by general prin- 

 ciples which greatly limit the number of possible combinations of amino acids. 

 They find that the number of amino acid residues in many simple natural 

 proteins is either 288 or a whole number multiple thereof. 



