Different inhibition types of the saturation kinetics involving a synthesized model complex, [Bu4N]2[MoVIO2(mnt)2] (E) (where mnt2- = 1,2-dicyanoethylenedithiolate), and HSO3- as the substrate (S) by structurally similar anions SO42-, H2PO4- and H2PO3- have been shown for the first time in relevance to the reductive half reaction of the native enzyme sulphite oxidase. SO42- acts as a competitive inhibitor. The mixed-type non-competitive inhibition by H2PO4- and the sigmoidal-type inhibition by H2 PO3- are explained by a diamond-configuration random-order model. This involves a random binding sequence of the substrate and the inhibitor, and forms, in addition to two binary complexes [enzyme-substrate (ES) and enzyme-inhibitor (EI)], one enzyme-substrate-inhibitor-type ternary complex (ESI) by participation of at least one more binding site in addition to the catalytic site. This is possible in the present case only by co-ordination enhancement of molybdenum in E. This co-ordination expansion is brought about by nucleophilic attack of the substrate or the inhibitor at the molybdenum, forming a hepta-coordinated binary complex with the generation of an oxoanionic functional site, called the allosteric site. Analysis of the experimental data suggests that the inhibition by H2PO4- is due to the mechanism following either equilibrium conditions or a combination of steady-state and equilibrium conditions. With H2PO3-, the inhibition is due to the mechanism following the steady-state conditions. It is also shown that the ternary complex involving the enzyme, substrate and H2PO4- or H2 PO3- is productive, but at a lower rate than that of the enzyme-substrate binary complex. Mixed-type inhibition with H2PO4- is actually of the type called ‘partially mixed competitive and non-competitive’ as the inhibitor binds both to the catalytic site and to the allosteric site. The sigmoidal-type inhibition by H2PO3- is similar to heterotropic allosteric effect of mixed V,K type with the distinction, however, that the significance of co-operativity in this case is of kinetic importance only.
Received 3 January 1996/20 May 1996; accepted 25 June 1996