Role of protonatable groups of bovine heart bc1complex in ubiquinol binding and oxidation

The pH dependence of the initial reaction rate catalyzed by the isolated bovine heart ubiquinol-cytochrome c reductase (be, complex) varying decylbenzoquinol (DBH) and decylbenzoquinone (DB) concentrations was determined. The affinity for DBH was increased threefold by the protonation of a group with pK(a) = 5.7 +/- 0.2, while the inhibition constant (K-i) for DB decreased 22 and 2.8 times when groups with pK(a) = 5.2 +/- 0.6 and 7.7 +/- 0.2, respectively, were protonated. This suggests stabilization of the protonated form of the acidic group by DBH binding. Initial rates were best fitted to a kinetic model involving three protonatable groups. The protonation of the pK(a) approximate to 5.7 group blocked catalysis, indicating its role in proton transfer. The kinetic model assumed that the deprotonation of two groups (pK(a) values of 7.5 +/- 0.03 and approximate to9.2) decreases the catalytic rate by diminishing the redox potential of the iron-sulfur (Fe-S) cluster. The protonation of the pK(a) approximate to 7.5 group also decreased the reaction rate by 80-86%, suggesting its role as acceptor of a proton from ubiquinol. The lack of effect on the K-m for DBH when the pK(a) 7.5-7.7 group is deprotonated suggests that hydrogen bonding to this residue is not the main factor that determines substrate binding to the Q(o) site. The possible relationship of the pK(a) 5.2-5.7 and pK(a) 7.5-7.7 groups with Glu272 of cytochrome b and His161 of the Fe-S protein is discussed.