ENZYMES AS REAGENTS 



acid composition, molecular weight, DPN binding, turnover 

 number, and other catalytic properties is exhibited by prepa- 

 rations of glyceraldehyde-3-phosphate dehydrogenase from 

 yeast and from rabbit muscle (50). An unidentified crystalline 

 protein from papaya latex was "diagnosed" as lysozyme because 

 of a resemblance in amino acid composition to the egg white 

 enzyme (42). It is difficult to conceive that these similarities are 

 coincidental. Although some differences in the biological and 

 chemical properties of these related proteins exist, these seem 

 minor compared to the great similarities, particularly in view of 

 the fact that even enzyme preparations from a single source 

 reveal microheterogeneity on rigid examination (6). For 

 example, TDH from yeast was fractionated into proteins of 

 different electrophoretic behavior with apparently identical 

 biological activity (22). Such minor variations actually serve 

 to make us more fully aware of the remarkable precision of the 

 cellular machinery responsible for the production of these 

 complex polymers. It seems quite logical therefore to assume 

 that the specific assembly of amino acids takes place with tlie 

 aid of a structural guide. 



There are, on the other hand, enzymes from different 

 sources, for example, alcohol dehydrogenase from yeast and liver, 

 which have vastly different properties although they catalyze 

 the same reaction. It would be interesting to learn whether a 

 discrepancy in catalytic efficiency (alcohol dehydrogenase from 

 yeast is over 50 times as active as the liver enzyme) is mirrored 

 in pronounced differences of amino acid composition or sequence. 



The relation between protein structure and biological 

 activity of enzymes has been studied mainly by (7) chemical 

 alterations of the protein; (2) partial proteolytic degradation 

 of enzymes; (3) construction of enzyme models; and (4) 

 investigations of the active centers of the enzymes. 



MODIFICATION OF PROTEINS BY CHEMICAL ALTERATIONS 



Essentiality or nonessentiality of certain groups for enzy- 

 matic activity has been deduced from chemical alterations, such^ 



217 ' ■ ^ 



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