Resonance Raman spectroscopy of the integral quinol oxidase complex of Sulfolobus acidocaldarius

The integral quinol oxidase complex of Sulfolobus acidocaldarius (DSM 639) was investigated by resonance Raman spectroscopy. The complex includes four heme a groups which constitute two functional entities, a587 and aa3, containing two low-spin hemes and a low-spin as well as a high-spin heme, respectively. RR spectra were obtained from the fully oxidized and fully reduced states of the complex using different excitation wavelengths in the Soret band region in order to disentangle the contributions from the four heme groups. For the oxidized state, this approach allowed for the identification of two spectrally different types of heme a which were assigned to the bishistidine ligated hemes a of aa3 and a587 (type II) and to the additional heme a of a587 which is ligated by a histidine and methionine (type I). The spectra of both heme a types differ substantially from that of beef heart cytochrome c oxidase. In particular, the formyl stretching modes of types II and I are upshifted by 8 and 15 cm-1, respectively, implying a largely hydrophobic environment of the formyl groups in the quinol oxidase of Sulfolobus. Furthermore, the RR spectra of the oxidized state reveal the characteristic marker bands of a five-coordinated and a six-coordinated high-spin state, indicating that heme a3 exists in a coordination equilibrium, which is in sharp contrast to the purely six-coordinated high-spin configuration of heme a3 in any (quinol or cytochrome) oxidases studied so far. Also the formyl stretching mode of heme a3 appears to be unusual as its frequency is substantially lower than in beef heart oxidase. In the fully reduced state, no heterogeneity of heme a3 is observed and also the spectra of the various hemes a are nearly indistinguishable. Moreover, the formyl stretching vibrations of all hemes a and a3 apparently coincide to one prominent peak at 1658 cm-1 characteristic for a non-hydrogen-bonded carbonyl group. This finding is unique compared to other aa3 oxidases in which the formyl stretchings give rise to widely separated bands at approximately 1610 and approximately 1665 cm-1 for heme a and a3, respectively. In both the oxidized and the reduced states, the spectra of the aa3 entity in the integral complex differ significantly from those of the isolated aa3 entity studied previously [Heibel, G., Anzenbacher, P., Hildebrandt, P., & Schäfer, G. (1993a) Biochemistry 32, 10878-10884], indicating substantial interactions between the various subunits of the integral complex.