THE STRUCTURE AND FUNCTION OF PQQ-CONTAINING QUINOPROTEINS

About 30 years ago, it was shown that bacterial methanol and glucose d ehydrogenases contained a completely novel type of prosthetic group wh ich was subsequently identified as pyrrolo-quinoline quinone (PQQ). Qu inoproteins were originally proteins containing PQQ but this definitio n has since been broadened to include those proteins containing other types of quinone-containing prosthetic groups, and the X-ray structure s of representatives of each type of quinoprotein have recently been p ublished. This review is mainly concerned with the structure and funct ion of the PQQ-containing methanol dehydrogenase and related proteins. The basic structure is a 'propeller' fold superbarrel structure made up of 8 beta-sheet 'propeller blades) which are held together by novel tryptophan-docking motifs. The PQQ in the active site is coordinated to a Ca2+ ion and is maintained in position by a stacked tryptophan an d a novel 8-membered ring structure made up of a disulphide bridge bet ween adjacent cysteine residues. This review describes these features and discusses these in relation to previously proposed mechanisms for this enzyme.