Open AccessRole of protonatable groups of bovine heart bc 1 complex in ubiquinol binding and oxidation
Author(s) -
Covián Raúl,
MorenoSánchez Rafael
Publication year - 2001
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1046/j.0014-2956.2001.02521.x
Subject(s) - protonation , chemistry , deprotonation , ubiquinol , catalysis , medicinal chemistry , stereochemistry , reaction rate constant , cytochrome c , coenzyme q – cytochrome c reductase , kinetics , organic chemistry , biochemistry , ion , physics , quantum mechanics , mitochondrion
The pH dependence of the initial reaction rate catalyzed by the isolated bovine heart ubiquinol‐cytochrome c reductase ( bc 1 complex) varying decylbenzoquinol (DBH) and decylbenzoquinone (DB) concentrations was determined. The affinity for DBH was increased threefold by the protonation of a group with p K a = 5.7 ± 0.2, while the inhibition constant ( K i ) for DB decreased 22 and 2.8 times when groups with p K 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 p K a ≈ 5.7 group blocked catalysis, indicating its role in proton transfer. The kinetic model assumed that the deprotonation of two groups (p K a values of 7.5 ± 0.03 and ≈ 9.2) decreases the catalytic rate by diminishing the redox potential of the iron–sulfur (Fe–S) cluster. The protonation of the p K a ≈ 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 p K 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 p K a 5.2–5.7 and p K a 7.5–7.7 groups with Glu272 of cytochrome b and His161 of the Fe–S protein is discussed.