Saturday, May 2, 2009

Kinetic trapping of oxygen in cell respiration

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 weak2,3. 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.

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