A pyrroloquinoline quinine-dependent membrane-bound d-sorbitol dehydrogenase from Gluconobacter oxydans exhibits an ordered Bi Bi reaction mechanism

A membrane-bound pyrroloquinoline quinine (PQQ)-dependent d-sorbitol dehydrogenase (mSLDH) in Gluconobacter oxydans participates in the oxidation of d-sorbitol to l-sorbose by transferring electrons to ubiquinone which links to the respiratory chain. To elucidate the kinetic mechanism, the enzyme purified was subjected to two-substrate steady-state kinetic analysis, product and substrate inhibition studies. These kinetic data indicate that the catalytic reaction follows an ordered Bi Bi mechanism, where the substrates bind to the enzyme in a defined order (first ubiquinone followed by d-sorbitol), while products are released in sequence (first l-sorbose followed by ubiquinol). From these findings, we proposed that the native mSLDH bears two different substrate-binding sites, one for ubiquinone and the other for d-sorbitol, in addition to PQQ-binding and Mg2+-binding sites in the catalytic center.