Alcohol dehydrogenase (ADH) of acetic acid bacteria functions as the primary dehydrogenase of the ethanol oxidase respiratory chain, where it donates electrons to ubiquinone. ADH is a membrane-bound quinohemoprotein-cytochrome c complex which consists of subunits I (78 kDa), II (48 kDa), and III (14 kDa) and contains several hemes c as well as pyrroloquinoline quinone as prosthetic groups. To understand the role of the heme c moieties in the intramolecular electron transport and the ubiquinone reduction, the ADH complex of Gluconobacter suboxydans was separated into a subunit I/III complex and subunit II, then reconstituted into the complex. The subunit I/III complex, probably subunit I, contained 1 mol each of pyrroloquinoline quinone and heme c and exhibited significant ferricyanide reductase, but no Q1 reductase activities. Subunit II was a triheme cytochrome c and had no enzyme activity, but it enabled the subunit I/III complex to reproduce the Q1 and ferricyanide reductase activities. Hybrid ADH consisting of the subunit I/III complex of G. suboxydans ADH and subunit II of Acetobacter aceti ADH was constructed and it had showed a significant Q1 reductase activity, indicating that subunit II has a ubiquinone-binding site. Inactive ADH from G. suboxydans exhibiting only 10% of the Q1 and ferricyanide reductase activities of the active enzyme has been isolated separately from active ADH (Matsushita, K., Yakushi, T., Takaki, Y., Toyama, H., and Adachi, O (1995) J. Bacteriol. 177, 6552-6559). Using these active and inactive ADHs and also isolated subunit I/III complex, we performed kinetic studies which suggested that ADH contains four ferricyanide-reacting sites, one of which was detected in subunit I and the others in subunit II. One of the three ferricyanide-reacting sites in subunit II was defective in inactive ADH. The ferricyanide-reacting site remained inactive even after alkali treatment of inactive ADH and also after reconstituting the ADH complex from the subunits, in contrast to the restoration of Q1 reductase activity and the other ferricyanide reductase activities. Thus, the data suggested that the heme c in subunit I and two of the three heme c moieties in subunit II are involved in the intramolecular electron transport of ADH into ubiquinone, where one of the two heme c sites may work at, or close to, the ubiquinone-reacting site and another between that and the heme c site in subunit I. The remaining heme c moiety in subunit II may have a function other than the electron transfer from ethanol to ubiquinone in ADH.