ABSTRACT
Pyrroloquinoline quinone (PQQ) was first proposed in the 1960s as the third major prosthetic group (along with pyridine nucleotides and flavins) for redox enzymes (1). After about two decades, the structure of PQQ (Fig. 1) was determined by two groups (2, 3). PQQ is the ortho-quinone at the C4 and C5 positions of the quinone ring. The C5 carbonyl group in the oxidized form is very reactive towards nucleophiles such as alcohols, sugars, amines, ammonia, cyanide, and amino acids. Knowledge about PQQ in the view of biology, biochemistry, and electrochemistry has been studied and summarized in several reviews (4-12). Until now, many PQQ-harboring proteins or PQQ and heme-harboring proteins have been discovered but only in Gramnegative bacteria (Table 1). Most of the PQQ-harboring enzymes belonged to dehydrogenases (4-31): PQQ methanol dehydrogenases (PQQMDH), PQQ ethanol dehydrogenases (PQQEDH), and PQQ glucose dehydrogenases (PQQGDH).
