Cyanide binding and active site structure in heme-copper oxidases: Normal coordinate analysis of iron-cyanide vibrations of CN− complexes of cytochromes ba3 and aa3

The cyanide isotope-sensitive low-frequency vibrations of ferrous cyano complexes of cytochrome a₃ are studied for cytochrome ba₃ from Thermus thermophilus and cytochrome aa₃ from bovine heart. Cyanide complexes of ba₃ display three isotope sensitive frequencies at 512, 485, and 473 cm⁻¹. The first is primarily an Fe—C stretching motion, whereas the lower wavenumber modes are bending motions. These iron-cyanide vibrations are independent of the redox levels of the other metal centers in the protein. On the other hand, the fully reduced bovine derivative complexed with cyanide gives rise to a bending vibration at 503 cm⁻¹ and a stretching vibration at 469 cm⁻¹. That is, the ordering of the stretching and bending frequencies is reversed from that of the bacterial protein. These results are analyzed by normal coordinate calculations to obtain comparative models for the binuclear O₂ reducing site of the two proteins. We find that the observed frequencies are consistent with a linear Fe—C—N group and larger Fe—C stretching force constant (2.558 mdyn/Å) for ba₃ and a slightly bent Fe—C—N group (angle ∼ 170°) and a smaller Fe—C stretching force constant (2.335 mdyn/Å) for aa₃. Thus, there are significant differences in the interaction of cyanide with ferrous a₃ in the two proteins that are most likely caused by a weaker proximal histidine interaction and stronger peripheral heme electron withdrawing effects in ba₃. Possible sources of these protein-induced effects are discussed. Using the analysis developed here, comparison of the FeCN stretching and bending frequencies of the ferrous bovine a₃-CN complex to those obtained from the ferric a₃-CN complex suggests that upon conversion of the resting to the fully reduced protein, a conformational change occurs that constrains the ligand binding site. © 1998 John Wiley & Sons, Inc. Biospectroscopy 4: 1–15, 1998