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Studies on monomeric cytochrome c oxidases
OXYGEN
DO48 STUDIES
ON MONOMERIC
315
C YTOCEROMEi C OXIDASES
A.E.Aitken, M.T.Wilson Department of Chemistry and Biological Chemistry, University of Essex, Wivenhoe Park, Colchester, Essex, U.K. Cytochrome c oxidase (E.C.1.9.3.1.) the terminal electron acceptor of the mitochondrion has been isolated from a number of eukaryote species. The enzyme from beef, sheep, horse and pig is isolated as dimers and higher aggregates of the functional unit (f.u.) but may be rendered monomeric by removal of the large hydrophobic subunit III. The hammerhead shark enzyme is isolated as the monomer while retaining subunit III. All monomers exhibit biphasic Eadie-Hofstee plots at low ionic strength (Km values for horse cyt c binding to the I1high affinity lVsite are :beef 0.19w; horse 0.04w; sheep 0.08w; pig 0.05m; shark O.l7@I) and, therefore, explanations of such kinetic behaviour in terms of models of the dimeric state of the enzyme, in which cytochrome c binding to monomer within a dimer shows negative cooperativity, may be eliminated, in agreement with Taha et al [l]. The electron transfer between reactions cytochrome c and either dimeric or monomeric oxidase(s) are essentially similar (both at the level of cytochrome a and Cu,) indicating that the electron entry site on the enzyme is unmodified by monomerisation [2]. Partial reduction of the monomer or dimer initiates a transition in the enzyme which permits rapid cyanide binding to the binuclear (cytochrome a3.C%) centre. The simpler monomeric system, having only a single functional unit, have significant advantages in may determining the electron/f.u. ratio required to trigger rapid inhibition by cyanide 1. 2.
T. S.M. Taha and S. Ferguson-Mi 1ler Biochemistry, 31, 9090-9097 (1992) F. MaZatesta, G. Antonini, P. Sarti and M. Brunori Biochem. J., 234, 569-572 (1986)
DO48 STUDIES
ON MONOMERIC
315
C YTOCEROMEi C OXIDASES
A.E.Aitken, M.T.Wilson Department of Chemistry and Biological Chemistry, University of Essex, Wivenhoe Park, Colchester, Essex, U.K. Cytochrome c oxidase (E.C.1.9.3.1.) the terminal electron acceptor of the mitochondrion has been isolated from a number of eukaryote species. The enzyme from beef, sheep, horse and pig is isolated as dimers and higher aggregates of the functional unit (f.u.) but may be rendered monomeric by removal of the large hydrophobic subunit III. The hammerhead shark enzyme is isolated as the monomer while retaining subunit III. All monomers exhibit biphasic Eadie-Hofstee plots at low ionic strength (Km values for horse cyt c binding to the I1high affinity lVsite are :beef 0.19w; horse 0.04w; sheep 0.08w; pig 0.05m; shark O.l7@I) and, therefore, explanations of such kinetic behaviour in terms of models of the dimeric state of the enzyme, in which cytochrome c binding to monomer within a dimer shows negative cooperativity, may be eliminated, in agreement with Taha et al [l]. The electron transfer between reactions cytochrome c and either dimeric or monomeric oxidase(s) are essentially similar (both at the level of cytochrome a and Cu,) indicating that the electron entry site on the enzyme is unmodified by monomerisation [2]. Partial reduction of the monomer or dimer initiates a transition in the enzyme which permits rapid cyanide binding to the binuclear (cytochrome a3.C%) centre. The simpler monomeric system, having only a single functional unit, have significant advantages in may determining the electron/f.u. ratio required to trigger rapid inhibition by cyanide 1. 2.
T. S.M. Taha and S. Ferguson-Mi 1ler Biochemistry, 31, 9090-9097 (1992) F. MaZatesta, G. Antonini, P. Sarti and M. Brunori Biochem. J., 234, 569-572 (1986)