title: Excited-state charge separation in the photochemical mechanism of the light-driven enzyme protochlorophyllide oxidoreductase.
abstract: The unique light-driven enzyme protochlorophyllide oxidoreductase (POR) is an important model system for understanding how light energy can be harnessed to power enzyme reactions. The ultrafast photochemical processes, essential for capturing the excitation energy to drive the subsequent hydride- and proton-transfer chemistry, have so far proven difficult to detect. We have used a combination of time-resolved visible and IR spectroscopy, providing complete temporal resolution over the picosecond-microsecond time range, to propose a new mechanism for the photochemistry. Excited-state interactions between active site residues and a carboxyl group on the Pchlide molecule result in a polarized and highly reactive double bond. This so-called "reactive" intramolecular charge-transfer state creates an electron-deficient site across the double bond to trigger the subsequent nucleophilic attack of NADPH, by the negatively charged hydride from nicotinamide adenine dinucleotide phosphate. This work provides the crucial, missing link between excited-state processes and chemistry in POR. Moreover, it provides important insight into how light energy can be harnessed to drive enzyme catalysis with implications for the design of light-activated chemical and biological catalysts.
authors.affiliation: Manchester Institute of Biotechnology and Photon Science Institute, University of Manchester, 131 Princess Street, Manchester M1 7DN (UK). Derren.heyes@manchester.ac.uk.
authors.forename: Derren J
authors.initials: DJ
authors.lastname: Heyes
authors.forename: Samantha J O
authors.initials: SJ
authors.lastname: Hardman
authors.forename: Tobias M
authors.initials: TM
authors.lastname: Hedison
authors.forename: Robin
authors.initials: R
authors.lastname: Hoeven
authors.forename: Greg M
authors.initials: GM
authors.lastname: Greetham
authors.forename: Michael
authors.initials: M
authors.lastname: Towrie
authors.forename: Nigel S
authors.initials: NS
authors.lastname: Scrutton
mesh_terms: D055162:Biocatalysis; D002735:Chlorophyllides / Q000737:chemistry / Q000378:metabolism; D004579:Electron Transport; D008027:Light; D009249:NADP / Q000737:chemistry; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000737:chemistry / Q000378:metabolism*; D055668:Photochemical Processes; D011521:Protochlorophyllide / Q000737:chemistry / Q000378:metabolism; D013055:Spectrophotometry, Infrared
chemical_list: D002735:Chlorophyllides; D011521:Protochlorophyllide; D009249:NADP; D044925:Oxidoreductases Acting on CH-CH Group Donors; C023886:protochlorophyllide reductase
keywords: charge transfer; enzyme catalysis; excited states; photochemistry; protochlorophyllide