期刊
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
卷 107, 期 49, 页码 21010-21015出版社
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1008603107
关键词
double-laser technique; T. thermophilus ba(3); oxygen reduction; slow-fast kinetics; O-2 channel
资金
- National Institutes of Health [GM53788, GM 35342]
- Science Foundation Ireland [BICF685]
Kinetic studies of heme-copper terminal oxidases using the CO flow-flash method are potentially compromised by the fate of the photodissociated CO. In this time-resolved optical absorption study, we compared the kinetics of dioxygen reduction by ba(3) cytochrome c oxidase from Thermus thermophilus in the absence and presence of CO using a photolabile O-2-carrier. A novel double-laser excitation is introduced in which dioxygen is generated by photolyzing the O-2-carrier with a 355 nm laser pulse and the fully reduced CO-bound ba(3) simultaneously with a second 532-nm laser pulse. A kinetic analysis reveals a sequential mechanism in which O-2 binding to heme a(3) at 90 mu MO2 occurs with lifetimes of 9.3 and 110 mu s in the absence and presence of CO, respectively, followed by a faster cleavage of the dioxygen bond (4.8 mu s), which generates the P intermediate with the concomitant oxidation of heme b. The second-order rate constant of 1 x 109 M-1 s(-1) for O-2 binding to ba(3) in the absence of CO is 10 times greater than observed in the presence of CO as well as for the bovine heart enzyme. The O-2 bond cleavage in ba(3) of 4.8 mu s is also approximately 10 times faster than in the bovine enzyme. These results suggest important structural differences between the accessibility of O-2 to the active site in ba(3) and the bovine enzyme, and they demonstrate that the photodissociated CO impedes access of dioxygen to the heme a(3) site in ba(3), making the CO flow-flash method inapplicable.
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