title: Laser excitation studies of the product release steps in the catalytic cycle of the light-driven enzyme, protochlorophyllide oxidoreductase.
abstract: The latter stages of the catalytic cycle of the light-driven enzyme, protochlorophyllide oxidoreductase, have been investigated using novel laser photoexcitation methods. The formation of the ternary product complex was initiated with a 6-ns laser pulse, which allowed the product release steps to be kinetically accessed for the first time. Subsequent absorbance changes associated with the release of the NADP+ and chlorophyllide products from the enzyme could be followed on a millisecond timescale. This has facilitated a detailed kinetic and thermodynamic characterization for the interconversion of all the various bound and unbound product species. Initially, NADP+ is released from the enzyme in a biphasic process with rate constants of 1210 and 237 s(-1). The rates of both phases show a significant dependence on the viscosity of the solvent and become considerably slower at higher glycerol concentrations. The fast phase of this process exhibits no dependence on NADP+ concentration, suggesting that conformational changes are required prior to NADP+ release. Following NADP+ release, the NADPH rebinds to the enzyme with a maximum rate constant of approximately 72 s(-1). At elevated temperatures (>298 K) chlorophyllide is released from the enzyme to yield the free product with a maximum rate constant of 20 s(-1). The temperature dependencies of the rates of each of these steps were measured, and enthalpies and entropies of activation were calculated using the Eyring equation. A comprehensive kinetic and thermodynamic scheme for these final stages of the reaction mechanism is presented.
authors.affiliation: Manchester Interdisciplinary Biocentre, Faculty of Life Sciences, University of Manchester, 131 Princess Street, Manchester M1 7DN, United Kingdom. derren.heyes@manchester.ac.uk
authors.forename: Derren J
authors.initials: DJ
authors.lastname: Heyes
authors.forename: Michiyo
authors.initials: M
authors.lastname: Sakuma
authors.forename: Nigel S
authors.initials: NS
authors.lastname: Scrutton
mesh_terms: D002384:Catalysis; D000458:Cyanobacteria / Q000201:enzymology* / Q000378:metabolism; D007700:Kinetics; D007834:Lasers; D009249:NADP / Q000378:metabolism; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000737:chemistry* / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D011994:Recombinant Proteins; D009249:NADP; D044925:Oxidoreductases Acting on CH-CH Group Donors; C023886:protochlorophyllide reductase