Learning from hydrogenases: location of a proton pump and of a second FMN in bovine NADH--ubiquinone oxidoreductase (Complex I).

Hydrogenases have clear evolutionary links to the much more complex NADH-ubiquinone oxidoreductases (Complex I). Certain membrane-bound [NiFe]-hydrogenases presumably pump protons. From a detailed comparison of hydrogenases and Complex I, it is concluded here that the TYKY subunit in these enzymes is a special 2[4Fe-4S] ferredoxin, which functions as the electrical driving unit for a proton pump. The comparison further revealed that the flavodoxin fold from [NiFe]-hydrogenases is presumably conserved in the PSST subunit of Complex I. It is proposed that bovine Complex I and the soluble NAD(+)-reducing hydrogenase from Ralstonia eutropha each contain a second FMN group.