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Magnetic circular dichroism study of cytochrome ba3 from Thermus thermophilus: Spectral contributions from cytochromes b and a-3 and nanosecond spectroscopy of carbon monoxide photodissociation intermediates


, : Magnetic circular dichroism study of cytochrome ba3 from Thermus thermophilus: Spectral contributions from cytochromes b and a-3 and nanosecond spectroscopy of carbon monoxide photodissociation intermediates. Biochemistry 31(39): 9376-9387

Near-UV-vis magnetic and natural circular dichroism (MCD and CD) spectra of oxidized reduced, and carbonmonoxy-complexed cytochrome ba-3, a terminal oxidase from the bacterium Thermus thermophilus, and nanosecond time-resolved MCD (TRMCD) and CD (TRCD) spectra of the unligated species formed after photodissociation of the CO complex are presented. The spectral contributions of individual cytochromes b and a-3 to the Soret region MCD are identified. TRMCD spectroscopy is used to follow the spin state change (S = 0 to S = 2) in cytochrome a-3-2+ following photodissociation of the CO complex. There is prompt appearance of the high-spin state after photolysis, as found previously in mammalian cytochrome oxidase (Goldbeck, R. A., Dawes, T. D., Einarsdottir, O., Woodruff, W. H., and Kliger, D. S. (1991) Biophys. J. 60, 125-134). Peak shifts of 1-10 nm appear in the TRMCD, TRCD, and time-resolved UV-vis absorption spectra of the photolyzed enzyme throughout its observable lifetime, indicating that the photolyzed enzyme does not relax to its equilibrium deliganded form before recombination with CO occurs hundreds of milliseconds later. Direct heme-heme interaction is not found in cytochrome ba-3 but red-shifts in the MCD and absorption spectra of both cytochromes b and (photolyzed) a-3 are correlated with a CO-liganded form of the protein. The long time (tau gtorsim 1 s) needed for relaxation of the cytochrome b and a-3 peaks to their static positions suggests that CO binding to a-3 induces a global conformational change in the protein that weakly perturbs the MCD and absorption spectra of b and photolyzed a-3. Fe-a-3 binds CO more weakly in cytochrome ba-3 than in cytochrome aa-3. The MCD spectrum of reduced enzyme solution placed under 1 atm of CO contains a peak at 446 nm that shows apprx 30% of total cytochrome a-3 remains pentacoordinate, high-spin.

US$19.90

DOI: 10.1021/bi00154a008


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