Fig. 7.

Possible functions of 2M complexes. a Extra proton pumping subunit may be used to extrude a fifth proton per reaction in order to conserve energy under conditions of lowered transmembrane proton motive force, such as in alkaliphilic environments or potentially as an adaptation to slow growth. b The extra proton may be used for extra driving force to conduct reverse electron transport onto low potential electron acceptors like ferredoxin (Fd) for use in carbon or nitrogen fixation