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The catalytic properties of geranylgeranyl diphosphate (GGPP) synthase EC 2.5.1.29 purified from Methanobacterium thermoformicicum SF-4 were studied by kinetic procedures. The plots of 1/v versus 1/S and inhibition patterns by enzyme reaction products, PPi and GGPP, showed that the GGPP synthase reaction mechanism is an ordered-sequential Bi Bi one. Monovalent cations at low concentration (0.05 M) enhanced the enzyme activity, but at high concentration (0.4 M) they were inhibitory, except for K+. The K+ ion was found to be a modifier forming a parallel reaction pathway and accelerated the binding of substrates to the enzyme, especially the binding of isopentenyl diphosphate (IPP). When substrate concentrations are near the Km values, the rate-limiting step of the GGPP synthase reaction may be the substrate-binding step, probably the IPP-binding step, rather than the conversion step of the enzyme-farnesyl diphosphate-IPP complex to the enzyme-PPi-GGPP complex.
Tachibana et al. (Wed,) studied this question.