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in a CO-Sensing transcriptional S.Kumazaki,H.Nakajima,T.Sakaguchi,E.Nakagawa,H.Shinohara,K.Yoshihara,S.Aono CooA from Rhodospirillum rubrum is a transcriptional activator in which a heme prosthetic group acts as a CO sensor and regulates the activity of the protein. In this study, the electronic relaxation of the heme, and the concurrent recombination between ligands and the heme at  280 K were examined in
an effort to understand the environment around the heme and the dynamics
of the ligands.
Upon photoexcitation of the reduced CooA at 400 nm, electronic relaxation
of the heme occurred with
time constants of 0.8 and 1.7 ps. The ligand rebinding was substantially
completed with a time
constant of 6.5 ps, followed by a slow relaxation process with a time
constant of 173 ps. In the case of
CO-bound CooA, relaxation of the excited heme occurred with two time constants,
1.1 and 2.4 ps, which were largely similar to those with reduced CooA.
The subsequent CO recombination process was remarkably fast compared with
that of other CO-bound heme proteins. It was well described as a
biphasic geminate recombination process with time constants of 78 ps (60%)
and 386 ps (30%).
About 10% of the excited heme remained unligated at 1.9 ns. The dynamics
of rebinding of CO thus will
help us to understand how the physiologically relevant diatomic molecule
approaches the heme binding site in CooA with picosecond resolution.
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