S11/4 Fast ligand and electron transfer dynamics in oxidases and cytochrome c

Abstract

The active site of heme-copper oxidases contains two cofactors, heme a3 and CuB, which can both bind external ligands as the substrate O2 and signalling molecules NO and CO, and which are both involved in electron transfer processes. Over the last few years we have exploited the fact that the heme-ligand bond can be dissociated by a short light pulse to explore the dynamics of CO and NO in the active site and the interaction between the two cofactors using ultrafast spectroscopic techniques. For example, we have time-resolved the CO transfer from heme a3 to CuB and shown that it occurs in a ballistic way in ∼ 500 fs, which presumably reflects rigidity of the active site. Heme a is located close to heme a3 (∼ 7 A edge-to-edge) and acts as electron donor for the active site. Using mixed valence oxidases we have extended the 'reverse electron flow' technique to the ultrafast regime and demonstrated that this electron transfer process occurs in only 1.2 ns. The process is activationless and associated with a very low reorganization energy (< 200 meV), in contrast to common assumptions but in general agreement with the hydrophobic environment of the reactants. Finally, ligand dynamics in native and modified cytochrome c reflects the rigidity required for optimal electron transfer properties.