RESONANCE RAMAN-SPECTRA AND OPTICAL-PROPERTIES OF OXIDIZED CYTOCHROME-OXIDASE

Raman spectra of oxidized cytochrome oxidase and its inhibitor complexes with cyanide and formate have been recorded by using 441.6-nm HeCd laser excitation. Photo-reduction effects were avoided by flowing the protein samples through the scattering volume. As an aid in the interpretation of the protein data, Raman spectra of low- and high-spin ferric heme a complexes dissolved in water or in non-hydrogen-bonding organic solvents were recorded. The model compound data demonstrate that heme a vibrational bands in the 1540-1660-cnr1 region are sensitive to iron spin state and indicate that the Raman spectrum of oxidized cytochrome oxidase obtained with 441.6-nm excitation is due primarily to vibrations of low-spin cytochrome a3+. The spectra of the inhibitor complexes of the enzyme are consistent with this interpretation. The selective enhancement of cytochrome a3+ vibrational modes under these conditions is rationalized by using simple considerations of the Raman excitation profile and the optical spectra of cytochromes a and a3 deduced by W. H. Vanneste [(1966) Biochemistry 5, 838-848]. In contrast to the Raman spectrum of reduced cytochrome a3, those of oxidized and reduced cytochrome a do not show a well-defined heme a formyl vibration in the 1670-cm-1 region. The model compound data indicate that either hydrogen bonding or lack of conjugation of the formyl π electrons with the porphyrin π system can account for this observation. The implications which this may have for heme-heme interaction in the protein are discussed. © 1979, American Chemical Society. All rights reserved.