THE ACTIVE-SITE STRUCTURE OF THE CALCIUM-CONTAINING QUINOPROTEIN METHANOL DEHYDROGENASE

Pyrroloquinoline quinone (PQQ), widely found in nature, serves as the redox cofactor in bacterial methanol dehydrogenase (MEDH), a heterotet rameric enzyme that oxidizes methanol to formaldehyde. The refined str ucture of MEDH at 2.4-angstrom resolution, based on recently obtained amino acid sequence data, reveals that the PQQ, located in a central c hannel of the disk-shaped protein, is sandwiched between a Trp side ch ain and a very unusual vicinal disulfide. A Ca2+ ion forms a bridge be tween PQQ and the protein molecule, very close to a putative substrate binding pocket. The vicinal disulfide may form during PQQ incorporati on and possibly act to hold the latter in place.