kOBERT A. ALBERTY 



dehydrogenases it has been possible to exclude all mechanisms 

 which require or allow H in reduced diphosphopyridine to 

 exchange with the medium during the oxidation-reduction 

 reaction (Vennesland and Westheimer (12)). 



It will also not be possible to discuss the theoretical con- 

 siderations of enzymatic mechanisms which are based on the 

 knowledge of physical organic chemistry. This approach to 

 mechanism is illustrated by the work of Westheimer (12) and of 

 Koshland (12). Enzymatic mechanisms and the mechanisms of 

 other catalytic processes must of necessity have many features in 

 common. Thus acid-catalyzed reactions have their enzymatic 

 counterparts. The enzyme is apparently able to provide a proton 

 at the required position in neutral solution so that it is un- 

 necessary to have a high concentration of hydrogen ions. In 

 addition the enzyme controls the specificity and stereospeci- 

 ficity of the reaction — a feature which is lacking in the non- 

 enzymatic reaction. One advantage of theoretical speculations 

 going hand in hand with experimental work is that important 

 new types of experiment are suggested. 



The most important concept which underlies the inter- 

 pretation of enzyme kinetic data is that one or more inter- 

 mediates are formed by reaction of the substrate with the 

 enzyme and that the product is produced by the decomposition 

 of such an intermediate. In 1913 Michaelis and Menten (13) 

 derived the rate equation for a simple mechanism based on the 

 concept of an enzyme-substrate complex and showed that this 

 hypothesis made possible a quantitative interpretation of the data 

 for the enzymatic inversion of sucrose. Although the Michaelis- 

 Menten equation was found to be widely applicable to enzymatic 

 reactions, the evidence that such an intermediate was actually 

 formed by a second-order reaction was only indirect until 

 Chance (3), some thirty years later, obtained direct evidence. 

 Chance showed that an intermediate was involved in the catal- 

 ysis by horse-radish peroxidase of the reaction between hydrogen 

 peroxide and malachite green and that the rate of appearance of 

 product was directly proportional to the concentration of this 



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