Conformations of cytochrome oxidase: thermodynamic evaluation of the interconversion of the 418- and 428-nm forms

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Abstract
Oxidized [bovine heart] cytochrome c oxidase exists in 2 reasonably well-defined conformations, a high-spin conformation with maximal absorption at 418 nm and a low-spin conformation with maximal absorption at 428 nm. The equilibrium between these 2 conformations was studied as a function of pH, pressure and temperature. pH affects the equilibrium between the 2 conformations, the maximum fraction of the 418-nm form being found at about pH 6.8. Increasing pressure displaced the equilibrium toward the 428-nm form; the molar volume changes are independent of pH but strongly dependent on temperature. Increasing temperature over the range -20 to 25.degree. C displaces the equilibrium toward the 428-nm form; the van''t Hoff plots that result show a discontinuity at about 10.degree. C. Above 10.degree. C, .DELTA.H is relatively constant as a function of pH; below 10.degree. C, .DELTA.H is strongly pH dependent. .DELTA.G, .DELTA.H, .DELTA.S and .DELTA.V were evaluated for the equilibrium.

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