INTRAMOLECULAR ELECTRON-TRANSPORT IN QUINOPROTEIN ALCOHOL-DEHYDROGENASE OF ACETOBACTER-METHANOLICUS - A REDOX-TITRATION STUDY

Quinohemoprotein-cytochrome c complex alcohol dehydrogenase (ADH) of a cetic acid bacteria consists of three subunits, of which subunit I con tains pyrroloquinoline quinone (pQQ) and heme c, and subunit II contai ns three heme c components. The PQQ and heme c components are believed to be involved in the intramolecular electron transfer from ethanol t o ubiquinone. To study the intramolecular electron transfer in ADH of Acetobacter methanolicus, the redox potentials of heme c components we re determined with ADH complex and the isolated subunits I and II of A . methanolicus, as well as hybrid ADH consisting of the subunit I/III complex of Gluconobacter suboxydans ADH and subunit II of A. methanoli cus ADH. The redox potentials of hemes c in ADH complex were -130, 49, 188, and 188 mV at pH 7.0 and 24, 187, 190, and 255 mV at pH 4.5. In hybrid ADH, one of these heme c components was largely changed in the redox potential. Reduced ADH was fully oxidized with potassium ferricy anide, while ubiquinone oxidized the enzyme partially. The results ind icate that electrons extracted from ethanol at PQQ site are transferre d to ubiquinone via heme c in subunit I and two of the three hemes c i n subunit II. (C) 1998 Elsevier Science B.V.