Further studies on the substrate specificity and inhibition of the stereospecific CS2 secondary alkylsulphohydrolase of Comamonas terrigena

Publisher Website
Google Scholar
Abstract
A series of D-alkan-2-yl sulfate esters (C7-C14) were prepared by sulfation of the resolved parent alcohols by a method that entails complete retention of configuration. These sulfate esters were tested as substrates for the stereospecific CS2 secondary alkylsulfohydrolase of C. terrigena. Vmax reached a maximum with the C9 compound, whereas log Km decreased linearly as the alkyl-chain length was increased from C7 to C14. A parallel series of L-alkan-2-yl sulfates was also prepared, and these esters, together with homologous series of primary alkyl sulfates and primary alkanesulfonates, were competitive inhibitors of the CS2 enzyme. For each series of compounds, log Ki [Ki = inhibition constant] values decreased linearly with increasing alkyl-chain length. Plots of chain length against the standard free energy of binding (.DELTA.G0) of substrate and inhibitors to the CS2 enzyme showed that the standard free energy of association of a -CH2- group with the enzyme was 2.0-2.4 kJ/mol for all classes of compound studied, indicating an important contribution from hydrophobic interactions to the overall binding. Plots for D-alkan-2-yl sulfate substrates and primary alkyl sulfate inhibitors were nearly coincident, suggesting that the overall interaction between a primary ester and the enzyme is the same as that between the isomeric secondary substrate and the enzyme. Plots for L-alkan-2-yl sulfate and alkanesulfonate inhibitors were very similar to each other, but were displaced by 1.5-3.0 kJ/mol from that for substrate binding. This indicates that the binding of any one of these particular inhibitors involves 1 C atom fewer than the number involved in binding a substrate of the same chain length. These observations are discussed in terms of a 3-point attachment of substrate to the enzyme involving the alkyl chain, sulfate group and the C-1 methyl group.