ENZYMES AS REAGENTS 



center as well as about the role of the activating groups in the 

 native protein. This knowledge may also lead to the synthesis 

 of the enzyme models not too far removed from biological reality. 



ENZYME MODELS 



From the earliest organic enzyme model recognized by 

 Liebig (30), to the recent models of specific esterase activity 

 exhibited by SH compounds (33) and tlie polyfunctional 

 catalysts of mutarotation (47), a very large number of enzyme 

 models have been studied {cf. 27). Some of them, e.g., the 

 carboxylase models of Langenbeck, show remarkable similarity 

 to enzymes in regard to certain kinetic properties, product 

 inhibition, and inactivation of the catalyst. The polyfunctional 

 catalysts of Swain and Brown exhibit a high substrate specificity 

 and form substrate complexes resembling those formed by 

 enzymes. 



Study of model reactions has often clarified our thinking 

 about the active center of enzymes and their mechanism of 

 action, but a word of caution should be added concerning the 

 indiscriminate use of model reactions. For example, the 

 chemical oxidation of aldehydes in the presence of phosphates 

 has been used as a model of enzymatic dehydrogenation of 

 aldehydes (52), in spite of the fact that the course and products of 

 the chemical and enzymatic oxidation are quite different. 



Enzymes as Catalysts 



The basic concepts of Michaelis concerning enzyme-sub- 

 strate interactions have stood the test of time, although the simple 

 original formulation has undergone some essential alterations. 

 Among the more important factors which required consideration 

 were: the presence of more than one active center, multiple 

 points of substrate attachment, the occurrence of more than one 

 enzyme-substrate intermediate, the participation of multiple 

 substrates, including cofactors and water, and the effect of 

 specific buffers and of pYi on the active center as well as on 

 activating groups. 



221 



