THE REFINED STRUCTURE OF THE QUINOPROTEIN METHANOL DEHYDROGENASE FROM METHYLOBACTERIUM-EXTORQUENS AT 1.94 ANGSTROM

Background: Methanol dehydrogenase (MDH) is a bacterial periplasmic quinoprotein; it has pyrrolo-quinoline quinone (PQQ) as its prosthetic group, requires Ca2+ for activity and uses cytochrome c(L) as its electron acceptor. Low-resolution structures of MDH have already been determined. Results: The structure of the alpha(2) beta(2) tetramer of MDH from Methylobacterium extorquens has now been determined at 1.94 Angstrom with an R-factor of 19.85%. Conclusions: The alpha-subunit of MDH has an eight-fold radial symmetry, with its eight beta-sheets stabilized by a novel tryptophan docking motif The PQQ in the active site is held in place by a coplanar tryptophan and by a novel disulphide ring formed between adjacent cysteines which are bonded by an unusual non-planar trans peptide bond. One of the carbonyl oxygens of PQQ is bonded to the Ca2+, probably facilitating attack on the substrate, and the other carbonyl oxygen is out of the plane of the ring, confirming the presence of the predicted free-radical semiquinone form of the prosthetic group.