title: Catalyzing separation of carbon dioxide in thiamin diphosphate-promoted decarboxylation.
abstract: Thiamin diphosphate-dependent decarboxylases form addition intermediates between thiamin diphosphate (ThDP) and 2-ketoacids. Although it appears that the intermediate should react without the intervention of catalysts, evidence has clearly shown that Brønsted acid catalysis occurs through a pre-associated system. This can promote separation of carbon dioxide from the residual carbanion by protonation of the carbanion. Proteins operate through pre-association and may readily promote the separation of carbon dioxide by protonating or oxidizing the nascent carbanion. Alternatively, a nucleophilic side chain may trap carbon dioxide as an unstable hemi-carbonate. Mutagenesis experiments by others have shown that enhanced activity due to the protein in the presence of thiamin diphosphate does not depend on the presence of any one proton donor, consistent with pooled activity within the active site. This form of catalysis has not been widely recognized, but should be considered an integral aspect of enzyme-promoted decarboxylation.
authors.affiliation: Davenport Chemical Laboratories, Department of Chemistry, University of Toronto, Canada. rkluger@chem.utoronto.ca
authors.forename: Ronald
authors.initials: R
authors.lastname: Kluger
authors.forename: Steven
authors.initials: S
authors.lastname: Rathgeber
mesh_terms: D002245:Carbon Dioxide / Q000302:isolation & purification*; D002384:Catalysis; D003653:Decarboxylation / Q000187:drug effects*; D004058:Diffusion; D019277:Entropy; D004798:Enzymes / Q000378:metabolism; D005415:Flavins / Q000737:chemistry; D007700:Kinetics; D010880:Piperidines / Q000737:chemistry; D011725:Pyridines / Q000737:chemistry; D013831:Thiamine / Q000031:analogs & derivatives; D013835:Thiamine Pyrophosphate / Q000494:pharmacology*
chemical_list: D004798:Enzymes; D005415:Flavins; D010880:Piperidines; D011725:Pyridines; D002245:Carbon Dioxide; C023666:pyridine; D013835:Thiamine Pyrophosphate; D013831:Thiamine