Membrane-bound glucose dehydrogenase (mGDH) in Escherichia coli is one of the pivotal pyrroloquinoline quinone (PQQ)-containing quinoproteins coupled with the respiratory chain in the periplasmic oxidation of alcohols and sugars in Gram-negative bacteria. We compared mGDH with other PQQ-dependent quinoproteins in molecular structure and attempted to trace their evolutionary process. We also review the role of residues crucial for the catalytic reaction or for interacting with PQQ and discuss the functions of two distinct domains, radical formation in PQQ, and the presumed existence of bound quinone in mGDH.