The transmembrane electron carrier system, HmcABCDEF (Pires et al. 2006). Probably transfers electrons from the periplasm to the cytoplasm; involved in H₂ oxidation (periplasm) and sulfate reduction (cytoplasm). HmcA of 545 aas and 1 TMS; similar to proteins in TC#s 9.B.11 and 9.B.5. HmcB of 370 aas and 2 TMSs; similar to proteins in TC#s 5.A.3 and 5.B.12.  HmcC of 388 aas; similar to proteins in TC# 5.A.3.  HmcD of 47 aas and 1 TMS. TmcE of 226 aas and 5 or 6 TMSs; similar to proteins in TC#s 5.B.11 and 3.D.7, and possibly to proteins in 5.B.3.  TmcF of 461 aas and 0 TMSs; similar to proteins in TC#s 5.B.11, 5.B.12 and 3.D.7.