The sensitivity of an enzyme to its environment has provoked much interest both for its immediate relevance to biochemistry and for the use of enzymes in chemical synthesis. The intercellular or extracellular environment in which an enzyme naturally operates is crowded with macromolecular, small-molecule, and ionic solutes and hence is markedly different from the dilute aqueous buffer solutions commonly cited for comparisons of biochemical processes. We report the results of a kinetic study into the effects of such a crowded solution on the rate of an enzyme-mediated process-the trypsin-catalyzed hydrolysis of a nonnatural substrate ester. The catalytic rate constant decreases linearly with solvent polarity, but substrate binding is independent of the concentration of added crowding agent up to 395 g/L.

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The ready elimination of phenol/phenoxide from the O-phenyl oxime 10E derived from 8-dimethylamino-1-naphthaldehyde, necessarily involving proton transfer from carbon, is catalysed by the neighbouring NMe 2 group at pH > 9. However, reaction is faster, rather than slower, at lower pH. It is shown that the step involving proton transfer is not cleanly rate determining at any pH: the preferred route involves syn/anti isomerization to form the more reactive Z-isomer. The rate constant for the anti elimination cannot be extracted from the available data, so no reliable estimate of effective molarity (EM) is possible.

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