Quinohemoprotein alcohol dehydrogenases: structure, function, and physiology

Quino(hemo)protein alcohol dehydrogenases (ADH) that have pyrroloquinoline quinone (PQQ) as the prosthetic group are classified into 3 groups, types 1, 11, and III. Type I ADH is a simple quinoprotein having PQQ as the only prosthetic group, while type 11 and type III ADHs are quinohemoprotein having heme c as well as PQQ in the catalytic polypeptide. Type 11 ADH is a soluble periplasmic enzyme and is widely distributed in Proteobacteria such as Pseudomonas, Ralstonia, Comamonas, etc. In contrast, type III ADH is a membrane-bound enzyme working on the periplasmic surface solely in acetic acid bacteria. It consists of three subunits that comprise a quinohemoprotein catalytic subunit, a triheme cytochrome c subunit, and a third subunit of unknown function. The catalytic subunits of all the quino(hemo)protein ADHs have a common structural motif, a quinoprotein-specific superbarrel domain, where PQQ is deeply embedded in the center. In addition, in the type 11 and type III ADHs this subunit contains a unique heme c domain. Various type 11 ADHs each have a unique substrate specificity, accepting a wide variety of alcohols, as is discussed on the basis of recent X-ray crystallographic analyses. Electron transfer within both type 11 and Ill ADHs is discussed in terms of the intramolecular reaction from PQQ to heme c and also from heme to heme, and in terms of the intermolecular reaction with azurin and ubiquinone, respectively. Unique physiological functions of both types of quinohemoprotein ADHs are also discussed. (C) 2004 Elsevier Inc. All rights reserved.