Thermodynamic regulation of cytochrome oxidase.

It was concluded that cytochrome oxidase was a strange enzyme for three reasons. (1) The thermodynamic flux-force relationship of this enzyme was inverse in some conditions: flux decreased when force increased. (2) The flux-force relationship was not unique and depended on the way in which the thermodynamic span of cytochrome oxidase was changed. (3) The regulation of cytochrome oxidase was different in the same conditions when different external parameters (energy demand, oxygen concentration) were changed. It was also shown that the flux control coefficient of cytochrome oxidase, small at saturating oxygen concentration, increases when oxygen pressure diminishes, approaching unity at very low oxygen concentrations.