| Abstract: | A histidine auxotroph of Saccharomyces cerevisiae has been used to metabolically incorporate 1,3-15N2] histidine into yeast cytochrome c oxidase. Electron nuclear double resonance (ENDOR) spectroscopy of cytochrome a in the 15N]histidine-substituted enzyme reveals an ENDOR signal which can be assigned to hyperfine coupling of a histidine 15N with the low-spin heme, thereby unambiguously identifying histidine as an axial ligand to this cytochrome. Comparison of this result with similar ENDOR data obtained on two 15N-substituted bisimidazole model compounds, metmyoglobin-15N]imidazole and bis15N]imidazole tetraphenyl porphyrin, provides strong evidence for bisimidazole coordination in cytochrome a. |
