STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 169 



formation of peptide bonds, including the preliminary activation of the 

 amino acids. An increasing number of workers have begun to explore the 

 problem of the geochemical origin of amino acids, peptides, and proteins 

 (Oparin, 1957; Fox, 1956) and the evolution of biological polymers, but we 

 shall not consider these questions here. 



Protein synthesis may be considered to involve at least three major 

 problems: (1) the mechanism of formation of the peptide bond; (2) the 

 determmation of specificity in the order of amnio acids along the peptide 

 chain; and (3) the determination of the folding and interrelations of peptide 

 chains. Very httle is kno^vii about the last question; the second question, that 

 of amino acid selection, possibly on a template, will be considered only 

 briefly in a later section. 



1. Transpeptidation 



A reaction between the amnio group of an amino acid and the carboxyl of 

 another does not occur to a significant extent in solution. In the presence 

 of a proteolytic enzyme, which only increases the rate at which equihbrium 

 is reached, the equihbrium point is far over to hydrolysis rather than toward 

 synthesis. However, conditions may occasionally be set to favor synthesis, 

 as in the reaction 



chymotrypsiii 

 benzoyl L-tyrosine -\- glyciueanilide :f= ^ benzoyl-L-tyrosylglycineanilide -|- HgO 



In this case, the insolubility of the peptide and its removal from solution 

 provides the driving force for the reaction. It can therefore be asked 

 if such reactions are not biologically important in the deposition 

 of amino acids and peptides on insoluble particles. In other instances 

 it is conceivable that the removal of the reaction products by other 

 reactions may also assist in shifting the direction of the reaction towards 

 synthesis. 



A number of transpeptidation reactions are known in which new peptides 

 may be accumulated (Fruton, 1954). Of particular interest in this comiection 

 have been reactions of glutathione, which participates in transpeptidations 

 of the glutamyl portion of the molecule, as in the reaction represented in 

 formula (XXIV). 



y-glutamylcysteinylglycine + valine ^ y-glutamylvaline + cysteinylglyciiie 

 (glutathione) 



