Structure and mechanism of soluble quinoprotein glucose dehydrogenase

Soluble glucose dehydrogenase (s-GDH; EC 1.1.99.17) is a classical quinopro tein which requires the cofactor pyrroloquinoline quinone (PQQ) to oxidize glucose to gluconolactone. The reaction mechanism of PQQ-dependent enzymes has remained controversial due to the absence of comprehensive structural d ata. We have determined the X-ray structure of s-GDH with the cofactor at 2 .2 Angstrom resolution, and of a complex with reduced PQQ and glucose at 1. 9 Angstrom resolution. These structures reveal the active site of s-GDH, an d show for the first time how a functionally bound substrate interacts with the cofactor in a PQQ-dependent enzyme. Twenty years after the discovery o f PQQ, our results finally provide conclusive evidence for a reaction mecha nism comprising general base-catalyzed hydride transfer, rather than the ge nerally accepted covalent addition-elimination mechanism. Thus, PQQ-depende nt enzymes use a mechanism similar to that of nicotinamide- and flavin-depe ndent oxidoreductases.