KINETIC TRAPPING OF OXYGEN IN CELL RESPIRATION

CELL respiration in eukaryotes is catalysed by the mitochondri`al enzy me cytochrome c oxidase. In bacteria there are many variants of this e nzyme, all of which have a binuclear haem iron-copper centre at which O-2 reduction occurs, and a low-spin haem, which serves as the immedia te electron donor to this centre(1). It is essential that the componen ts of the cell respiratory system have a high affinity for oxygen beca use of the low concentrations of dissolved O-2 in the tissues; however , the binding of O-2 to the respiratory haem-copper oxidases is very w eak(2,3). This paradox has been attributed to kinetic trapping during fast reactions of O-2 bound within the enzyme's binuclear haem iron-co pper centre(2). Our earlier work(3) indicated that electron transfer f rom the low-spin haem to the oxygen-bound binuclear centre may be nece ssary for such kinetic oxygen trapping. Here we show that a specific d ecrease of this haem-haem electron transfer rate in the respiratory ha em-copper oxidase from Escherichia coli leads to a corresponding decre ase in the enzyme's operational steady-state affinity for O-2. This de monstrates directly that fast electron transfer between the haem group s is a key process in achieving the high affinity for oxygen in cell r espiration.