Conformation of coenzyme pyrroloquinoline quinone and role of Ca(2+) in the catalytic mechanism of quinoprotein methanol dehydrogenase

The ab initio structures of 2,7,9-tricarboxypyrroloquinoline quinone (PQQ), semiquinone (PQQH), and dihydroquinone (PQQH2) have been determined and compared with ab initio structures of the (PQQ)Ca(2+), (PQQH)Ca(2+), and (PQQH2)Ca(2+) complexes as well as the x-ray structure of (PQQ)Ca(2+) bound at the active site of the methanol dehydrogenase (MDH) of methyltropic bacteria. Plausible mechanisms for the MDH oxidation of methanol involving the (PQQ)Ca(2+) complex are explored via ab initio computations and discussed. Considering the reaction of methanol with PQQ in the absence of Ca(2+), nucleophilic addition of methanol to the PQQ C-5 carbonyl followed by a retro-ene elimination is deemed unlikely due to large energy barrier. A much more favorable disposition of the methanol C-5 adduct to provide formaldehyde involves proton ionization of the intermediate followed by elimination of methoxide concerted with hydride transfer to the oxygen of the C-4 carbonyl. Much the same transition state is reached if one searches for the transition state beginning with Asp-303–CO(2)(−)general-base removal of the methanol proton of the (PQQ)Ca(2+)O(H)CH(3) complex concerted with hydride transfer to the oxygen at C-4. For such a mechanism the role of the Ca(2+) moiety would be to (i) contribute to the formation of the ES complex (ii) provide a modest decrease in the pK(a) of methanol substrate,; and (iii) polarize the oxygen at C-5.