Resonance Raman study of cytochrome aa₃ from Sulfolobus acidocaldarius

The single subunit terminal oxidase of Sulfolobus acidocaldarius, cytochrome aa₃, was studied by resonance Raman spectroscopy. Results on the fully oxidized, the fully reduced, and the reduced carbon monoxide complex are reported and compared with those of sucaryotic cytochrome oxidase. It is shown that in both redox states the hemex a and a₃ are in the six-coordinated low-spin and six-coordinated high-spin configuration, respectively. The resonance Raman spectra reveal far-reaching similarities of this archaebacterial with mammalian or plant enzymes except for the reduced form of heme a. The formyl substituent of this heme appears above 1 640 cm^-1, ruling out significant hydrogen bonding interactions which is in sharp contrast to beef heart cytochrome oxidase. In addition, frequency upshifts of the marker bands ν₄ and ν₂ are noted indicating differences in the electron density distribution within the molecular orbitals of the porphyrin.