REFINED CRYSTAL-STRUCTURE OF METHYLAMINE DEHYDROGENASE FROM PARACOCCUS-DENITRIFICANS AT 1.75 ANGSTROM RESOLUTION

The three-dimensional structure of the quinoprotein methylamine dehydr ogenase from Paracoccus denitrificans has been refined at 1.75 Angstro m resolution utilizing the DNA-based protein sequence. The final model incorporates 8034 atoms per molecule, including 552 molecules of solv ent, and gives an R-factor of 0.163. The molecule is an H2L2 hetero-te tramer containing a non-crystallographic 2-fold axis of symmetry. The 373-residue H subunit is folded into seven repeats of a four-stranded antiparallel beta-sheet motif, arranged in a propeller-like pattern ab out a pseudo-7-fold rotational axis of symmetry. Each L subunit contai ns 131 residues folded in a tight structure composed of five beta-stra nds in two sheets and crosslinked by six disulfide bonds. In addition there is an intrasubunit covalent Linkage between two tryptophan side- chains that form the unique redox center, tryptophan tryptophylquinone (TTQ). The active site contains the O-6 carbonyl of TTQ, the side-cha ins of Asp32L, Asp76L, Tyr119L and Thr122L, and two solvent molecules. A potential ''gate'' (Phe55H) separates the closed active-site cavity from a channel containing a group of highly ordered water molecules t o bulk solvent. Phe55H and Tyr119L, and a number of neighboring oxygen atoms, may also provide a binding site for monovalent cations that ar e known to affect the reactivity and spectral properties of TTQ as wel l as the oxidative half reaction. The overall reaction has been dissec ted into a number of discrete steps that may require participation by several individual amino acid residues in the active site acting as ge neral acids and bases. (C) 1998 Academic Press Limited.