HMWC (high-molecular-weight cytochrome c precursor), ORF2, ORF3, ORF4, ORF5, ORF6 in the HMC operon form a transmembrane protein complex that allows electron flow from the periplasmic hydrogenase to the cytoplasmic enzymes that catalyze reduction of sulfates. ORF2 is a transmembrane redox protein.