Using EPR up close and from afar: Elucidating mechanisms in haem copper oxidases

Haem copper oxidases constitute the terminal complex of the respiratory chain and catalyse the reduction of oxygen to water. This exergonic redox reaction is coupled to proton pumping across the inner mitochondrial or bacterial membrane. O2 reduction occurs at the binuclear haem-CuB centre. Despite high resolution X-ray crystallographic structures, the properties of the catalytic redox states of the metal centres and their relation to protonation states within this class of enzyme remain still poorly understood. Modern EPR techniques (also in combination with magneto-optical studies) enable us to probe different catalytic intermediate states either directly or indirectly. From afar pulsed ELDOR spectroscopy, a technique for accurately measuring inter spin distances in the range 2–8 nm, is used to resolve subtle structural changes when applied to spin-labelled systems trapped in different intermediate states (e.g. P, R & F states) and which allows the study of local conformational changes in great detail. Using this technique conformational change within the proton uptake channels is discussed. Up close both EPR and magneto-optical techniques (Magnetic Circular Dichroism) are used to address the nature of the metal ligands in the binuclear centre as well as transiently formed radical species from different intermediate states as well as in oxidases from different species.