The kinetic mechanism of xanthine dehydrogenase and related enzymes.

Xanthine dehydrogenase and related enzymes contain multicomponent internal electron transfer chains. The topographical arrangement of this chain in such that the oxidation of substrate and the reduction of the electron acceptor occur at separate non-overlapping sites that communicate by intramolecular electron transfer. The steady-state kinetic behaviour of these enzymes is thus ideally suited to a two-site ping-pong mechanism, with random addition of substrates and products at the two sites. The formal mechanism of these enzymes is presented here as a mixture of rapid equilibrium random segments connected by a steady-state segment, i.e. a rapid-equilibrium random (two-site) hybrid ping-pong mechanism. Such a mechanism is likely to be operative in a variety of oxidative enzymes containing multiple redox-active prosthetic groups.