Fourier transform infrared (FTIR) and step-scan time-resolved FTIR spectroscopies reveal a unique active site in cytochrome caa₃ oxidase from Thermus thermophilus

Fourier transform infrared (FTIR) and step-scan time-resolved FTIR difference spectra are reported for the [carbonmonoxy]cytochrome caa₃ from Thermus thermophilus. A major C-O mode of heme a₃ at 1958 cm^-1 and two minor modes at 1967 and 1975 cm^-1 (7:1:1) have been identified at room temperature and remained unchanged in H₂O/D₂O exchange. The observed C-O frequencies are 10 cm^-1 higher than those obtained previously at 21 K (Einarsdóttir, O., Killough, P. M., Fee, J. A., and Woodruff, W. H. (1989) J. Biol. Chem. 264, 2405-2408). The time-resolved FTIR data indicate that the transient Cu_B¹⁺-CO complex is formed at room temperature as revealed by the CO stretching mode at 2062 cm^-1. Therefore, the caa₃ enzyme is the only documented member of the heme-copper superfamily whose binuclear center consists of an a₃-type heme of a β-form and a Cu_B atom of an α-form. These results illustrate that the properties of the binuclear center in other oxidases resulting in the α-form are not required for enzymatic activity. Dissociation of the transient Cu_B¹⁺-CO complex is biphasic. The rate of decay is 2.3 x 10⁴ s^-1 (fast phase, 35%) and 36.3 s^-1 (slow phase, 65%). The observed rate of rebinding to heme a₃ is 34.1 s^-1. The implications of these results with respect to the molecular motions that are general to the photodynamics of the binuclear center in heme-copper oxidases are discussed.