Certain methanogenic archaea produce their „proton motive force“ (pmf) with the aid of the heterodisulfide reductase reaction. The CoM-CoS-heterodisulfide acts as electron acceptor, the reductase itself is reduced by cytochrome b which gains electrons by oxidizing methanophenazine (MPH). MPH transports electrons from factor F420 and it translocates protons during its redox cycle.

The parts of the coenzymes involved in electron transport and proton translocation from F420 in the cytoplasm to MPH outside the cell are shown in the figure.
Mark the oxidation state numbers of the S-atoms in the heterodisulfide and the two monosulfide coenzymes in the drop down lists (1-4).