Active-site structure of the soluble quinoprotein glucose dehydrogenase complexed with methylhydrazine: A covalent cofactor-inhibitor complex

Soluble glucose dehydrogenase (s-GDH) from the bacterium Acinetobacter calc oaceticus is a classical quinoprotein. It requires the cofactor pyrroloquin oline quinone (PQQ) to catalyze the oxidation of glucose to gluconolactone, The precise catalytic role of PQQ in s-GDH and several other PQQ-dependent enzymes has remained controversial because of the absence of comprehensive structural data. We have determined the crystal structure of a ternary com plex of s-GDH with PQQ and methylhydrazine, a competitive inhibitor of the enzyme. This complex, refined at 1.5-Angstrom resolution to an R factor of 16.7%, affords a detailed view of a cofactor-binding site of s-GDH. Moreove r, it presents the first direct observation of covalent PQQ adduct in the a ctive-site of a PQQ-dependent enzyme, thereby confirming previous evidence that the C5 carbonyl group of the cofactor is the most reactive moiety of P QQ.