   1.17.5.3 | formate + menaquinone |
Fdh-N and dissimilatory nitrate reductase (Nar) can form a redox loop where proton motive force generation is best described as the sum of the following two effects. 1. Two protons, which are taken up from the cytoplasm at the Fdh-N menaquinone reduction site, are translocated across the membrane and released to the periplasm from the menaquinol oxidation site in Nar. 2. Two electrons are transferred from the formate oxidation site in periplasm to the NO3- reduction site in cytoplasm. This is not accompanied by an actual proton translocation across the membrane but generates a membrane potential, which is equivalent to 2 H+ translocation across the membrane. The result is consistent with the measured ratio of proton translocation to electron transfer in this system. In the catalytic site, the Mo directly takes up electrons from the bound substrate. These electrons are transferred to the beta subunit though the [4Fe4S] cluster (FeS-0) in the alpha subunit. The four [4Fe-4S] clusters in the beta subunit, which are aligned in the order of FeS-1, FeS-4, FeS-2, and FeS-3, connect the alpha and gamma subunits like an electric wire. From FeS-3 of the beta subunit, electrons are transferred to heme bP (P for periplasm) in the gamma subunit and then across the membrane to heme bC (C for cytoplasm). Menaquinone binds to a histidine ligand (Hisg169) of heme bC and can directly accept electrons through this residue. The electron transfer from formate (standard redox potential, 2420 mV) to menaquinone (275 mV) is a highly exergonic reaction, allowing the electron transfer against the membrane potential |
