The respiratory cytochrome bc(1) complex functions as a protonmotive ubiquinol:cytochrome c oxidoreductase. Lysine 228 (K228) located within the quinol reduction (Q(i)) site of the bc(1) complex, has been reported as a key residue for proton transfer during the redox chemistry cycle to substrate quinone at Q(i). In yeast, while single mutations had no effect, the combination of K228L and F225L resulted in a severe respiratory growth defect and inhibition of O-2 consumption in intact cells. The inhibition was overcome by uncoupling the mitochondrial membrane or by suppressor mutations in the region of K228L-F225L. We propose that the K228L mutation introduces energetic (and kinetic) barriers into normal electronand proton transfer chemistry at Q(i), which are relieved by dissipation of the opposing protonmotive force or through the restoration of favourable intraprotein proton transfer networks via suppressor mutation. (C) 2020 Elsevier Inc. All rights reserved.