There is a consensus that yeast and mammalian respiratory complexes undergo an Active (A)-Deactive (D) state transition, which is of physiological relevance . In mammals, the low energy D state is presumably separated from the A state by a high activation energy barrier. The D state likely prevents excessive ROS production in mammalian complex I, which in turn has important implications in ischaemic-reperfusion injury . At the biochemical level, the nature of A and D states is well established . However, the D state is not observed in complex I from several species, such as Chaetomium thermophilum and Drosophila melanogaster as well as in bacterial complexes . Interestingly, the Drosophila complex I, shows a catalytic lag phase, while Chaetomium complex lacks even this. The underlying reasons behind these variances remain obscure. Conversion of A to D state is reached in the absence of electron donors/acceptors when the enzyme is idling. To convert it back to the A state, several cycles of NADH oxidation and Q reduction are needed .