REACTION OF CYANIDE WITH CYTOCHROME-BA3 FROM THERMUS-THERMOPHILUS - SPECTROSCOPIC CHARACTERIZATION OF THE FE(II)A3-CN-BULLET-CU(II)B-CN COMPLEX SUGGESTS 4 N-14 ATOMS ARE COORDINATED TO CU(B)

Cytochrome ba3 from Thermus thermophilus reacts slowly with excess HCN at pH 7.4 to create a form of the enzyme in which Cu(A), cytochrome b, and Cu(B) remain oxidized, while cytochrome a3 is reduced by one electron, presumably with the formation of cyanogen. We have examined this form of the enzyme by UV-visible, resonance Raman, EPR, and electron nuclear double resonance spectroscopies in conjunction with permutations of C-13- and N-15-labeled cyanide. The results support a model in which one CN- binds through the carbon atom to ferrous a3, supporting a low-spin (S = 0) configuration on the Fe; bridging by this cyanide to the Cu(B) is weak or absent. Four N-14 atoms, presumably donated by histidine residues of the protein. provide a strong equatorial ligand field about Cu(B); a second CN- is coordinated through the carbon atom to Cu(B) in an axial position.