116 KERATIN AND KERATINIZATION 



enzymes associated with the oxidative degradation of sugars, etc., are 

 located here. The internal organization of the mitochondrion (Fig. 22a, 

 p. 45) seems admirably designed as a site for the organized array of these 

 enzymes which seem necessary to effect the sequence of reactions envisaged 

 in multi-enzyme reactions. 



The mitochondrion is also a seat for the synthesis of low molecular 

 weight key substances such as adenosine triphosphate (ATP) which, 

 because of the so-called " high energy bonds " they contain, are able to 

 effect many biochemical reactions demanding an expenditure of energy. 

 Probably among these reactions is the formation of the peptide bonds 

 linking amino acids in the polypeptide chains in proteins. 



Fischer's original suggestion that proteins were polypeptides containing 

 peptide bonds formed by the reaction : 



R lt CH(NH 2 ).COOH + R 2 .CH(NH 2 )COOH 



= R^HfNH^.CO.NHCHfR^COOH + H 2 



has been amply confirmed both by degradative and synthetic methods. 

 The free energy necessary for the synthesis of a peptide bond has been 

 determined from thermodynamic data on reactants yielding peptides or 

 compounds containing peptide links. It lies in the range of 2000-4000 cal/ 

 mole. From the corresponding equilibrium constant, it may be calculated 

 that this value requires in equilibrium 99% of the material on the side of 

 hydrolysis. Obviously energy must be introduced into the system for 

 synthesis to approach completion. 



A method of moving the equilibrium in the direction of synthesis by 

 selecting reactants which would yield insoluble products was devised by 

 Bergmann and Fraenkel-Conrat (1937). In the presence of a hydrolytic 

 enzyme such reactions will move in the direction of synthesis as the 

 product is removed from solution. At the present time a synthesis 

 catalysed by proteolytic enzymes is not thought probable. Nevertheless, 

 in the formation of an insoluble protein such as keratin, the reaction could 

 conceivably be assisted by the removal of the product in the form of 

 insoluble fibrils, etc. That peptide-bond synthesis requires energy has 

 been repeatedly emphasized on biochemical grounds (Borsook, 1955). 

 Siekevitz (1952) showed that the uptake of " tagged " amino acids in 

 homogenates is more closely linked to phosphorylation than to direct 

 oxidation. Zamecnik and Keller (1954) showed that incorporation of 

 amino acids into microsomes proceeds only in the presence of an ATP- 

 generating source. In the cell ATP generation is a function of the mito- 

 chondria and they are thus shown to be indirectly involved in peptide 

 formation. Borsook (1955) showed that for each peptide bond formed one 

 molecule of ATP is broken down. 



Nevertheless the amount of energy involved is not large in comparison 



