Kinetic trapping of oxygen in cell respiration

Abstract

CELL respiration in eukaryotes is catalysed by the mitochondrial enzyme cytochrome c oxidase. In bacteria there are many variants of this enzyme, all of which have a binuclear haem iron–copper centre at which O₂ reduction occurs, and a low-spin haem, which serves as the immediate electron donor to this centre¹. It is essential that the components of the cell respiratory system have a high affinity for oxygen because of the low concentrations of dissolved O₂ in the tissues; however, the binding of O₂ to the respiratory haem–copper oxidases is very weak^2,3. This paradox has been attributed to kinetic trapping during fast reactions of O₂ bound within the enzyme's binuclear haem iron–copper centre². Our earlier work³ indicated that electron transfer from the low-spin haem to the oxygen-bound binuclear centre may be necessary for such kinetic oxygen trapping. Here we show that a specific decrease of this haem–haem electron transfer rate in the respiratory haem–copper oxidase from Escherichia coli leads to a corresponding decrease in the enzyme's operational steady-state affinity for O₂. This demonstrates directly that fast electron transfer between the haem groups is a key process in achieving the high affinity for oxygen in cell respiration.