Variation of Substrate Structure

1 Variation of Substrate Structure. Whereas linear free energy relationships play a central role in the determination of non-enzymic mechanisms, they are much less important for enzymes, for two reasons. First, enzymes have evolved to bind their natural substrates, and substituents introduced in an attempt merely to alter electron demand at the transition state may have many other interactions with the enzyme protein. The result is very noisy Hammett and Bronsted plots. Whereas conclusions can be drawn from non-enzymic rates varying over a factor of 3, with enzyme reactions, to see any trend above the noise it is usually necessary to have rates ranging over several orders of magnitude. 

The second problem arises from the multi-step nature of enzyme reactions by the time electronic demand has been altered sufficiently to see a trend, the rate-determining step may have been changed. 

Nonetheless, values for aryl glycosides hydrolysed by glycosidases continue to be measured the first such, for aryl p-glucosides hydrolysed by almond emulsin (which measured the effect of leaving group acidity from tabulated Hammett a constants, rather than directly from the leaving group pK ) dates 

The closer to zero the true /lig value is, the more Bronsted plots become difhcult to differentiate from random scatter caused by nonchemical rate-limiting events. 

Because of the strong and specific interactions made with the glycone by glycosyl-transferring enzymes, successful attempts to manipulate electronic demand by substitution in the glycone have been few. A 1980 study on the A-acetylhexosaminidase of Aspergillus niger (probably GH 20) recorded (F/A) —0.2 and of —1.4 for the effect of successive fluorination in 

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