Another recent coupling model is by Stuchebrukhov and colleagues who have suggested that it is the high pKa of QH -(anionic quinol) that drives proton pumping . Anionic quinol (QH -) has indeed been considered a key catalytic intermediate earlier . However, further experimental verification is needed to confirm if anionic quinol forms at a sufficient high occupancy during complex I turnover to be considered as a true catalytic intermediate (see section 6).