At the structural level, it remains debated if conformational variations observed by cryoEM are associated with the A/D transition or represent different catalytic intermediates . Based on structural data, several propositions have been made that show how the enzyme may convert from D to A (or A to D) form (see also ). For instance, loops of subunits NDUFS7, NDUFS2 and ND1 facing the Q tunnel may undergo restructuring upon conversion of D state to A state. Similarly, a section of the TMH3 of ND6 subunit rotates from 𝜋-helical conformation in D state to 𝛼-helical in A state.