INHIBITION PATTERNS OF A MODEL COMPLEX MIMICKING THE REDUCTIVE HALF-REACTION OF SULFITE OXIDASE

Different inhibition types of the saturation kinetics involving a synt hesized model complex, [Bu(4)N](2)[(MoO2)-O-VI(mnt)(2)] (E) (where mnt (2-) = 1,2-dicyanoethylenedithiolate), and HSO3- as the substrate (S) by structurally similar anions SO42-, H2PO4- and H,PO,- have been show n for the first time in relevance to the reductive half reaction of th e native enzyme sulphite oxidase. SO42- acts as a competitive inhibito r. The mixed-type noncompetitive inhibition by H2PO4- and the sigmoida l-type inhibition by H2PO3- are explained by a diamond-configuration r andom-order model. This involves a random binding sequence of the subs trate and the inhibitor, and forms, in addition to two binary complexe s [enzyme-substrate (ES) and enzyme-inhibitor (EI)], one enzyme-substr ate-inhibitor-type ternary complex (ESI) by participation of at least one more binding site in addition to the catalytic site. This is possi ble in the present case only by co-ordination enhancement of molybdenu m 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 experimen tal data suggests that the inhibition by H2PO4- is due to the mechanis m following either equilibrium conditions or a combination of steady-s tate 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- o r H2PO3- is productive, but at a lower rate than that of the enzyme-su bstrate binary complex. Mixed-type inhibition with H2PO4- is actually of the type called 'partially mixed competitive and non-competitive' a s the inhibitor binds both to the catalytic site and to the allosteric site. The sigmoidal-type inhibition by H2PO3- is similar to heterotro pic allosteric effect of mixed V,K type with the distinction, however, that the significance of co-operativity in this case is of kinetic im portance only.