Coherent reaction dynamics in a bacterial cytochrome c oxidase

Biological reactions in protein complexes involve structural dynamics spann ing many orders of magnitude in time. In standard descriptions of catalysis by enzymes, the transition state between reactant and product is reached b y thermal, stochastic motion. In the ultrashort time domain, however, the p rotein moiety and cofactor motions leading to altered conformations can be coherent rather than stochastic in nature(1-4). Such coherent motions may p lay 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 react ion catalysed by a key enzyme in aerobic organisms. Using the enzyme cytoch rome c oxidase aa(3) from the bacterium Paracoccus denitrificans, we have s tudied haem dynamics during the photo-initiated ultrafast transfer of carbo n monoxide from haem a(3) to CuB by femtosecond spectroscopy. The ground st ate of the unliganded a(3) species is populated in a stepwise manner in tim e, indicating that the reaction is mainly governed by coherent vibrations ( 47 cm(-1)). The reaction coordinate involves conformational relaxation of t he haem group and we suggest that ligand transfer also contributes.