Intermonomer electron transfer in the bc1 complex dimer is controlled by the energized state and by impaired electron transfer between low and high potential hemes

The cytochrome bc1 complex (commonly called Complex III) is the central enzyme of respiratory and photosynthetic electron transfer chains. X-ray structures have revealed the bc1 complex to be a dimer, and show that the distance between low potential (bL) and high potential (bH) hemes, is similar to the distance between low potential hemes in different monomers. This suggests that electron transfer between monomers should occur at the level of the bL hemes. Here, we show that although the rate constant for bL → bL electron transfer is substantial, it is slow compared to the forward rate from bL to bH, and the intermonomer transfer only occurs after equilibration within the first monomer. The effective rate of intermonomer transfer is about 2-orders of magnitude slower than the direct intermonomer electron transfer.