Mechanism of cleavage of carbamate anions

Abstract

Carbamates and monothiocarbamates of basic aliphatic amines undergo rate-determining C-N cleavage after a rapid equilibrium protonation step, as shown most directly by inverse solvent deuterium isotope effects of kD/kH = 3.6-4.8 for 0.0-and 0,s-N-n-butylcarbamates and by rapid acid-catalyzed exchange of the N H proton of n-BuNHCOS-with k e x c h = 5 X lo7 M-I s-I. The lifetimes of substituted N-protonated carbamates have been estimated to range down to <1O-Io s. It is concluded that general acid catalysis of the cleavage of carbamates of weakly basic anilines ( a = 0.84) occurs through an enforced preassociation mechanism with hydrogen bonding to the leaving protonated nitrogen atom and C-N cleavage in the rate-determining step. There is more proton transfer in the transition state (larger a ) and a smaller / ?I, with more basic amines and upon substitution of sulfur for oxygen. The low pK, values of N-protonated carbamates and monothiocarbamates illustrate the strong electron-accepting ability of -COO-and -COS-.

Formylmethanofuran dehydrogenases from methanogenic Archaea Substrate specificity, EPR properties and reversible inactivation by cyanide of the molybdenum or tungsten iron‐sulfur proteins

“…This appears to exclude the possibility that the formyl group of N-formylmethanofuran is transferred prior to oxidation to an amino or hydroxyl group of the enzyme, since such a transfer starting from free formate is thermodynamically unfavorable. Rather, it suggests that the formyl group is dehydrogenated while still bound to the primary amino group of methanofuran yielding N-carboxymethanofuran as product (reaction b) which should break down non-enzymically to CO, and methanofuran (Ewing et al, 1980) : 0 R Reaction (b) indicates that formylmethanofuran dehydrogenase belongs to the group of molybdenum enzymes that catalyze an insertion of an oxygen atom derived from H,O into a C-H bond (Pilato and Stiefel, 1993). Enzymes belonging to this group are xanthine dehydrogenases and xanthine oxidases (Bray, 1988;Wootton et al, 1991), molybdenum-containing formate dehydrogenases (Adams and Mortenson, 1985;Barber et al, 1986;Friedebold and Bowien, 1993), formate-ester dehydrogenase (van Ophem et al, 1992), aldehyde oxidase (Branzoli and Massey, 1974), aldehyde dehydrogenase (Poels et al, 1987), aldehyde oxidoreductase (White et al, 1993), nicotine dehydrogenase (Freudenberg et al, 1988), nicotinate dehydrogenase and 6-hydroxynicotinate dehydrogenase (Nagel and Andreesen, 1990), isonicotinate dehydrogenase and 2-hydroxyisonicotinate dehydrogenase (Kretzer and Andreesen, 1991), quinoline oxidoreductase (Hettrich et al, 1991), quinoline-4-carboxylic acid oxidoreductase (Bauer and Lingens, 19921, quinaldine oxidoreductase (de Beyer and Lingens, 1993), quinaldic acid 4-oxidoreductase (Fetzner and Lingens, 1993), picolinate dehydrogenase (Siegmund et al, 1990), 2-furoyl-coenzyme A dehydrogenase , and pyrimidine oxidase and pyridoxal oxidase (Burgmayer and Stiefel, 1985).…”

Use of Carbon Oxysulfide, a Structural Analog of CO2, to Study Active CO2 Transport in the Cyanobacterium Synechococcus UTEX 625

“…COS is an alternate substrate for Rubisco and the products of the reaction of COS with ribulose-1 ,5-bisphosphate are 3-phosphoglycerate and 3-phospho-l-thioglycerate (13). Interestingly, in terms of possible mechanisms for CO2 and COS transport, COS forms thiocarbamates with uncharged amines (9). The ratios of the rate constants for the addition to amines for CO2:COS:CS2 are in the order of 105:103:1, respectively (9).…”

“…Interestingly, in terms of possible mechanisms for CO2 and COS transport, COS forms thiocarbamates with uncharged amines (9). The ratios of the rate constants for the addition to amines for CO2:COS:CS2 are in the order of 105:103:1, respectively (9). Thus, COS reacts at a reasonable rate with amines compared to C02, whereas CS2 does not.…”

“…The observation that only COS inhibited CO2 transport may be a reflection of a covalent bonding reaction obligatory for transport. A possibility for such a reaction would be that of carbamate formation with amine N (9).…”

Crystal Structures of Vertebrate Dihydropyrimidinase and Complexes from Tetraodon nigroviridis with Lysine Carbamylation

“…Lysine carbamylation does not need an additional enzyme to achieve the covalent modification, a process that has been reported to be a spontaneous and reversible (15,16). By comparison, additional specific enzymes or catalysts, such as methyltransferases and acetyltransferases, are required to catalyze methylation and acylation reactions, respectively (17,18), both with high kinetic barriers for the covalent bond formation.…”