Infrared evidence of cyanide binding to iron and copper sites in bovine heart cytochrome c oxidase. Implications regarding oxygen reduction.
Cyanide binding to bovine heart cytochrome c oxidase at five redox levels has been investigated by use of infrared and visible-Soret spectra. A C-N stretch band permits identification of the metal ion to which the CN- is bound and the oxidation state of the metal. Non-intrinsic Cu, if present, is detected as a cyanide complex. Bands can be assigned to Cu+CN at 2093 cm-1, Cu2+CN at 2151 or 2165 cm-1, Fe3+CN at 2131 cm-1, and Fe2+CN at 2058 cm-1. Fe2+CN is found only when the enzyme is fully reduced whereas the reduced Cu+CN occurs in 2-, 3-, and 4-electron reduced species. A band for Fe3+CN is not found for the complex of fully oxidized enzyme but is for all partially reduced species. Cu2+CN occurs in both fully oxidized and 1-electron-reduced oxidase. CO displaces the CN- at Fe2+ to give a C-O band at 1963.5 cm-1 but does not displace the CN- at Cu+. Another metal site, noted by a band at 2042 cm-1, is accessible only in fully reduced enzyme and may represent Zn2+ or another Cu+. Binding of either CN- or CO may induce electron redistribution among metal centers. The extraordinary narrowness of ligand infrared bands indicates very little mobility of the components that line the O2 reduction site, a property of potential advantage for enzyme catalysis. The infrared evidence that CN- can bind to both Fe and Cu supports the possibility of an O2 reduction mechanism in which an intermediate with a mu-peroxo bridge between Fe and Cu is formed. On the other hand, the apparent independence of Fe and Cu ligand-binding sites makes a heme hydroperoxide (Fe-O-O-H) intermediate an attractive alternative to the formation an Fe-O-O-Cu linkage.
Yoshikawa
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Department of Biochemistry, Colorado State University, Fort Collins 80523.
Caughey
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WS
D000818:Animals; D001665:Binding Sites; D002417:Cattle; D003300:Copper / Q000378:metabolism*; D003486:Cyanides / Q000378:metabolism*; D003576:Electron Transport Complex IV / Q000378:metabolism*; D007501:Iron / Q000378:metabolism*; D009206:Myocardium / Q000201:enzymology*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D013053:Spectrophotometry; D013055:Spectrophotometry, Infrared
D003486:Cyanides; D003300:Copper; D007501:Iron; D003576:Electron Transport Complex IV; D010100:Oxygen