Accelerating Unimolecular Decarboxylation by Preassociated Acid Catalysis in Thiamin-Derived Intermediates: Implicating Brønsted Acids as Carbanion Traps in Enzymes
C-Alkyl pyridine acids provide similar catalysis while other acids have no effect. This suggests that an enzyme that generates an aldehyde from a 2-ketoacid should have functional Brønsted acids in their active sites that would trap the carbanion, as does benzoylformate decarboxylase. Enzymes that give nonaldehydic products from decarboxylation of thiamindiphosphate conjugates containing an associated
Making Thiamin Work Faster: Acid-Promoted Separation of Carbon Dioxide
作者:Qingyan Hu、Ronald Kluger
DOI:10.1021/ja054165p
日期:2005.9.1
The conjugate of thiamin and benzoylformate, mandelylthiamin (MTh), undergoes decarboxylation about 106 times slower than the analogous enzymic intermediate. It has now been discovered that the decarboxylation of MTh is accelerated by the acid component of pyridine and 4-picoline buffers. There is no role for a proton donor to stabilize the transition state for decarboxylation: catalysis must be achieved by the acid's trapping the product carbanion, preventing recarboxylation. This requires that diffusion of CO2 is rate-determining, and that protonation of the carbanion allows this to occur. This interpretation correctly predicts that the same acid components will prevent a fragmentation reaction by protonating the intermediate, which fragments only as the conjugate base.
Decomposition of 2-(1-Hydroxybenzyl)thiamin in Neutral Aqueous Solutions: Benzaldehyde and Thiamin Are Not the Products
作者:R. Kluger、J.F. Lam、C.S. Kim
DOI:10.1006/bioo.1993.1023
日期:1993.9
Base-Catalyzed Decarboxylation of Mandelylthiamin: Direct Formation of Bicarbonate as an Alternative to Formation of CO<sub>2</sub>
作者:Graeme W. Howe、Michael Bielecki、Ronald Kluger
DOI:10.1021/ja310952a
日期:2012.12.26
The decarboxylation of mandelylthiamin is subject to general base catalysis (β = 0.26), an outcome that is inconsistent with the expected dissociative transition state in which CO(2) forms along with a residual carbanion. The results implicate a previously unrecognized associative route in which addition of water to a carboxylate followed by base-catalyzed proton transfer and C-C cleavage produces