Excited state dynamics and catalytic mechanism of the light-driven enzyme protochlorophyllide oxidoreductase

The reduction of protochlorophyllide (Pchlide) to chlorophyllide, catalysed by the enzyme protochlorophyllide oxidoreductase (POR), is the penultimate step in the chlorophyll biosynthetic pathway and is a key light-driven reaction that triggers a profound transformation in plant development. As POR is light-activated it can provide new information on the way in which light energy can be harnessed to power enzyme reactions. Consequently, POR presents a unique opportunity to study catalysis at low temperatures and on ultrafast timescales, which are not usually accessible for the majority of enzymes. Recent advances in our understanding of the catalytic mechanism of POR illustrate why it is an important model for studying enzyme catalysis and reaction dynamics. The reaction involves the addition of one hydride and one proton, and catalysis is initiated by the absorption of light by the Pchlide substrate. As the reaction involves the Pchlide excited state, a variety of ultrafast spectroscopic measurements have shown that significant parts of the reaction occur on the picosecond timescale. A number of excited state Pchlide species, including an intramolecular charge transfer complex and a hydrogen bonded intermediate, are proposed to be required for the subsequent hydride and proton transfers, which occur on the microsecond timescale. Herein, we review spectroscopic investigations, with a particular focus on time-resolved transient absorption and fluorescence experiments that have been used to study the excited state dynamics and catalytic mechanism of POR.