The Stereoselective Inhibition of Functional -SH Groups of Dehydrogenases

Abstract

Most of the NAD⁺ dependent dehydrogenases are inactivated by SH reagents e. g. iodoacetic acid or its amide. In many cases the active SH group seems to be involved in NAD⁺ binding. The reactivity of these SH groups is highly different. In the three dimensional protein structure other functional groups may be close enough to cause activation or masking in the substitution reaction. With at last two such groups differently influencing the reactivity a center of chirality with respect to the chemical reactions of the SH group is formed. Therefore the antipodes of asymmetric reagents should show different reaction rates with such SH groups of dissymmetric functionality. This proved to be the case with several enzymes (1, 2, 5, 6). When reacting the dehydrogenases with the antipodes of α-iodopropionic acid or α-ioclopropionamide some of these show stereoselective inactivation. By monitoring the stereoselectivity of the alkylation reactions one may gain some information on the structure of active sites:

• The steric arrangement of groups in the neighbourhood of SH groups may be investigated (1, 2).

• One can try to find correlations between stereoselective reactivity and stereospecific enzymatic catalysis.

• One may follow conformational transitions which change the steric surroundings of the SH groups.