A compensatory double mutation of the alanine-86 to leucine mutant located in the hinge region of the iron–sulfur protein of the yeast cytochrome bc1 complex

Mutations in the hinge region connecting the membrane anchor to the extra-membranous head-group of the iron–sulfur protein can impede proper assembly and function of the cytochrome bc1 complex. Mutating the conserved alanines, residues 86, 90, and 92, located in the hinge region resulted in a 30–50% decrease in enzymatic activity without loss of the iron–sulfur protein [J. Bioenerg. Biomembr. 31 (1999) 215]. The lowered enzymatic activity in the A86L mutant was shown to result from steric interference between the side chains of Leu-86 and Leu-89 [Biochemistry 40 (2001) 327]. The compensatory double mutant A86L/L89A restored activity to wild type levels and relieved the steric hindrance; however, the L89A mutant did not assemble properly into the bc1 complex. Molecular modeling studies of these mutants compared to the wild type have suggested that the hydrophobic residues located in the hinge region are critical to the motion of the head group of the iron–sulfur protein during catalysis.