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

Background: Methanol dehydrogenase (MDH) is a bacterial periplasmic qu inoprotein; it has pyrrolo-quinoline quinone (PQQ) as its prosthetic g roup, requires Ca2+ for activity and uses cytochrome c(L) as its elect ron acceptor. Low-resolution structures of MDH have already been deter mined. Results: The structure of the alpha(2) beta(2) tetramer of MDH from Methylobacterium extorquens has now been determined at 1.94 Angst rom with an R-factor of 19.85%. Conclusions: The alpha-subunit of MDH has an eight-fold radial symmetry, with its eight beta-sheets stabiliz ed by a novel tryptophan docking motif The PQQ in the active site is h eld in place by a coplanar tryptophan and by a novel disulphide ring f ormed between adjacent cysteines which are bonded by an unusual non-pl anar trans peptide bond. One of the carbonyl oxygens of PQQ is bonded to the Ca2+, probably facilitating attack on the substrate, and the ot her carbonyl oxygen is out of the plane of the ring, confirming the pr esence of the predicted free-radical semiquinone form of the prostheti c group.