The role of SQ species in the proton pumping mechanism of complex I has remained unclear . The formation of SQ (neutral or anionic) is inevitable because electron transfer from the terminal FeS cluster N2 to Q occurs one at a time. However, several distinct experiments suggest that SQ species are not populated at a high enough occupancy to have a key role in the proton pumping mechanism of complex I . Furthermore, there is growing evidence from experiments, simulations and theoretical modeling that anionic quinol QH -may be a key species in the proton pumping mechanism of complex I (25-27). Below we explore the possibility how proton-injection driven proton pumping (see also section 2) can be achieved without SQ as a catalytic intermediate.