Electron and proton transfer in the M. smegmatis III2IV2 supercomplex

The M. smegmatis respiratory III2IV2 supercomplex consists of a complex III (CIII) dimer flanked on each side by a complex IV (CIV) monomer, electronically connected by a di-heme cyt. cc subunit of CIII. The supercomplex displays a quinol oxidation-oxygen reduction activity of -90 e-/s. In the current work we have investigated the kinetics of electron and proton transfer upon reaction of the reduced supercomplex with molecular oxygen. The data show that, as with canonical CIV, oxidation of reduced CIV at pH 7 occurs in three resolved components with time constants -30 mu s, 100 mu s and 4 ms, associated with the formation of the so-called peroxy (P), ferryl (F) and oxidized (O) intermediates, respectively. Electron transfer from cyt. cc to the primary electron acceptor of CIV, CuA, displays a time constant of <= 100 mu s, while re-reduction of cyt. cc by heme b occurs with a time constant of -4 ms. In contrast to canonical CIV, neither the P -> F nor the F -> O reactions are pH dependent, but the P -> F reaction displays a H/D kinetic isotope effect of -3. Proton uptake through the D pathway in CIV displays a single time constant of -4 ms, i.e. a factor of -40 slower than with canonical CIV. The slowed proton uptake kinetics and absence of pH dependence are attributed to binding of a loop from the QcrB subunit of CIII at the D proton pathway of CIV. Hence, the data suggest that function of CIV is modulated by way of supramolecular interactions with CIII.