Conformational changes in an ultrafast light-driven enzyme determine catalytic activity

The role of conformational changes in explaining the huge catalytic power of enzymes is currently one of the most challenging questions in biology(1-7). Although it is now widely regarded that enzymes modulate reaction rates by means of short- and long- range protein motions(3-7), it is almost impossible to distinguish between conformational changes and catalysis. We have solved this problem using the chlorophyll biosynthetic enzyme NADPH: protochlorophyllide ( Pchlide) oxidoreductase, which catalyses a unique light- driven reaction involving hydride and proton transfers(8). Here we report that prior excitation of the enzyme- substrate complex with a laser pulse induces a more favourable conformation of the active site, enabling the coupled hydride and proton transfer reactions to occur. This effect, which is triggered during the Pchlide excited-state lifetime and persists on a long timescale, switches the enzyme into an active state characterized by a high rate and quantum yield of formation of a catalytic intermediate. The corresponding spectral changes in the mid- infrared following the absorption of one photon reveal significant conformational changes in the enzyme, illustrating the importance of flexibility and dynamics in the structure of enzymes for their function.