Coherent reaction dynamics in a bacterial cytochrome c oxidase

Biological reactions in protein complexes involve structural dynamics spanning many orders of magnitude in time. In standard descriptions of catalysis by enzymes, the transition state between reactant and product is reached by thermal, stochastic motion. In the ultrashort time domain, however, the protein moiety and cofactor motions leading to altered conformations can be coherent rather than stochastic in nature. Such coherent motions may play a key role in controlling the accessibility of the transition state and explain the high efficiency of the reaction. Here we present evidence for coherent population transfer to the product state during an ultrafast reaction catalysed by a key enzyme in aerobic organisms. Using the enzyme cytochrome c oxidase aa₃ from the bacterium Paracoccus denitrificans, we have studied haem dynamics during the photo-initiated ultrafast transfer of carbon monoxide from haem a₃ to CuB by femtosecond spectroscopy. The ground state of the unliganded a₃ species is populated in a stepwise manner in time, indicating that the reaction is mainly governed by coherent vibrations (47 cm^-1). The reaction coordinate involves conformational relaxation of the haem group and we suggest that ligand transfer also contributes.