title: D2O solvent isotope effects suggest uniform energy barriers in ribulose-1,5-bisphosphate carboxylase/oxygenase catalysis.
abstract: d-Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the most abundant enzyme on Earth and is responsible for the fixation of atmospheric CO(2) into biomass. The reaction consists of incorporation of CO(2) and solvent H(2)O into d-ribulose 1,5-bisphosphate (RuBP) to yield 3-phospho-d-glycerate. The reaction involves several proton-dependent events: abstraction and protonation during enolization of RuBP and hydrolysis and reprotonation of the six-carbon reaction intermediate (carboxyketone). Although much is known about Rubisco structure and diversity, fundamental aspects of the reaction mechanism are poorly documented. How and when are protons exchanged among substrate, amino acid residues, and solvent water, and could alterations of proton exchange influence catalytic turnover? What is the energy profile of the reaction? To answer these questions, we measured catalytic rates and the (12)CO(2)/(13)CO(2) isotope effect in isotopic waters. We show that with increasing D(2)O content, the maximal carboxylation velocity (k(cat)(c)) decreased linearly and was 1.7 times lower in pure D(2)O. By contrast, the isotope effect on the apparent Michaelis constant for CO(2) (K(c)) was unity, suggesting that H/D exchange might have occurred with the solvent in early steps thereby slowing the overall catalysis. Calculations of kinetic commitments from observed isotope effects further indicate that (1) enolization and processing of the carboxyketone are similarly rate-limiting and (2) the tendency of the carboxyketone to go backward (decarboxylation) is likely exacerbated upon deuteration. Our results thus suggest that Rubisco catalysis is achieved by a rather equal distribution of energy barriers along the reaction.
authors.affiliation: Institut de Biologie des Plantes, CNRS UMR 8618, Université Paris-Sud 11, Orsay cedex, France. guillaume.tcherkez@u-psud.fr
authors.forename: Guillaume G B
authors.initials: GG
authors.lastname: Tcherkez
authors.forename: Camille
authors.initials: C
authors.lastname: Bathellier
authors.forename: Hilary
authors.initials: H
authors.lastname: Stuart-Williams
authors.forename: Spencer
authors.initials: S
authors.lastname: Whitney
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Gout
authors.forename: Richard
authors.initials: R
authors.lastname: Bligny
authors.forename: Murray
authors.initials: M
authors.lastname: Badger
authors.forename: Graham D
authors.initials: GD
authors.lastname: Farquhar
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism; D017666:Deuterium Oxide / Q000378:metabolism*; D007700:Kinetics; D018515:Plant Leaves / Q000378:metabolism; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism*; D012274:Ribulosephosphates / Q000378:metabolism; D018724:Spinacia oleracea / Q000201:enzymology* / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D012274:Ribulosephosphates; D002245:Carbon Dioxide; C001933:ribulose-1,5 diphosphate; D012273:Ribulose-Bisphosphate Carboxylase; D017666:Deuterium Oxide