Kinetic treatment of coupled electron and proton transfer in flash-photolysis experiments on carbon monoxide-inhibited mixed-valence cytochrome c oxidase
Ai. Kotelnikov et al., Kinetic treatment of coupled electron and proton transfer in flash-photolysis experiments on carbon monoxide-inhibited mixed-valence cytochrome c oxidase, J PHYS CH B, 105(24), 2001, pp. 5789-5796
Coupled electron and proton transfer observed in flow-flash experiments on
GO-inhibited mixed-valence cytochrome c oxidase is discussed in terms of a
model proposed by Brzezinski and co-workers [J. Bioenerg. Biomembr, 1998, 3
0, 99-107], The model includes two redox states of the heme a/heme a(3) pai
r and two states, protonated and deprotonated, of a redox-linked group L, w
hich is in contact with bulk solution via a proton conducting channel. The
proton channel is represented by another protolytic group L ', which is in
equilibrium with bulk solution, but not with group L. The theory reproduces
the experimentally observed pH dependence of the slow kinetics of heme a r
eduction following dissociation of the enzyme-CO complex, and additionally
predicts a pH dependence of the fast kinetics due to varying proton equilib
rium between group L and bulk solution prior to dissociation. The rates of
internal proton transfer between L and L ' in the reduced and oxidized stat
es, and the bimolecular rate of protonation of L ' by bulk protons have bee
n evaluated from the present theory and experimental data. The protonation
rate of the group L in the reduced state of heme a(3) is k(on)(red) = 10(4)
s(-1) From the observed pH dependence of the rate constant for the slow ki
netic phase of backward electron transfer the rate of L ' protonation is es
timated to be kappa ' (on) = 5 x 10(11) M-1 s(-1).