Transmembrane proton transfer in biological systems : an investigation of bacterial and archaeal haem-copper oxidases

Sammanfattning: Haem-copper oxidases are membrane-bound protein complexes that catalyse the reduction of dioxygen to water in the respiratory chain. These enzyme complexes are molecular machines that couple the free energy released from this reaction to pump protons across the membrane. Pumped protons as well as protons used for dioxygen reduction to water are taken up from the inside of the membrane, which contributes to maintaining an electrochemical gradient across the membrane. This gradient is used for other energy-requiring processes in the cell, among them the formation of ATP.This thesis focuses on the proton coupled reactions and the timing of the proton pumping events.Specific pathways provide rapid transfer of protons through haem-copper oxidases. Since the intermediates formed during reduction of dioxygen must rapidly and continuously be supplied with protons, the proton uptake from the outside bulk solution must be maintained at a high rate. One part of this thesis concerns the effect of altering the surrounding of the entrance of one of the proton pathways on the proton-uptake rates.The directionality of proton transfer through the membrane must be controlled by the enzyme to prevent the dissipation of the electrochemical gradient. A specific pump element within one of the proton pathways has been proposed to control this directionality. A part of this thesis addresses the proposed pump element.The timing of proton pumping, monitored using a pH sensitive dye present in the outside bulk solution, was investigated in oxidase incorporated into closed artificial membranes. The results show that proton release occurs on a millisecond-time scale. Since the results from other studies were interpreted to indicate also faster proton pumping events, it is speculated that the membrane surface plays an important role in the delay of the proton release to the bulk solution.