HMWC (high-molecular-weight cytochrome c), ORF2, ORF3, ORF4, ORF5 and 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. Monomer. Binds 16 heme c groups per subunit. High-spin heme 15 has single axial histidine ligand and the other hemes are low-spin bis-histidinyl coordinated.