Decarboxylation reactions biochemical

Biochemical reactions include several types of decarboxylation reactions as shown in Eqs. (1)-(5), because the final product of aerobic metabolism is carbon dioxide. Amino acids result in amines, pyruvic acid and other a-keto acids form the corresponding aldehydes and carboxylic acids, depending on the cooperating coenzymes. Malonyl-CoA and its derivatives are decarboxylated to acyl-CoA. -Keto carboxylic acids, and their precursors (for example, the corresponding hydroxy acids) also liberate carbon dioxide under mild reaction conditions. 

In biochemical decarboxylation reactions where the reactant contains a 3-keto group, the e-amino group of a lysyl side chain of the protein backbone can form an iminium derivative with the substrate.82 Upon loss of carbon dioxide, the delocalized, weakly basic product will not react faster than carbon dioxide can separate. Benner83 showed that the stereochemical consequence of decarboxylation of acetoacetate by acetoacetate decarboxylase involves protonation of the product from either face, consistent with a passive, uncatalyzed step, which is consistent with the view we have presented. 

Pre-association by a proton donor or electron acceptor can be a novel catalytic component in decarboxylation reactions. The mechanism is readily available in biochemical systems but is difficult to achieve in solution as the preassociation requires specific interactions. As instances are found, they provide important information on the mode of catalysis. 

All microbes, including chemoorganohetero-trophs, possess some ability to engage in reversible carboxylation (i.e., CO2-C assimilation into an organic compound) and decarboxylation reactions, some of which lead to the incorporation of a significant amount of CO2 (Wood, 1985). Here, we briefly consider the biochemical pathways that photo- and chemolithotrophic bacteria deploy in order to produce the majority of their biomass. The four major C02-fixing pathways are the Calvin cycle, the acetyl-CoA pathway, the reductive tricarboxylic acid (TCA) cycle, and the 3-hydroxypriopionate cycle. 

All aspects of the biochemical role of biotin have not yet been clarified. The vitamin has been implicated in the metabolism of carbohydrates, lipids, proteins, and nucleic acids. Available evidence indicates that biotin acts as a CO2 carrier in a number of carboxyla-tion and decarboxylation reactions connected with carbohydrate and fatty acid metabolism. A number of experimental procedures are used to establish the participation of biotin in a given biochemical reaction (1) the study of enzyme activity in biotin-deficient animals (2) the effect of avidin administered in vivo or added to the incubation mixture on the activity of the enzyme under study and (3) purification of the enzyme and demonstration of the existence of enzyme-bound biotin. Studies of this kind have established that biotin is required for the carboxylases of jS-methyl-crotonyl CoA, acetyl-CoA, propionyl CoA, and oxaloacetic transcarboxylase. Only some of the results are presented here [74-76]. 

The shortcomings of the latter scheme are that no examples are known of the decarboxylation by biochemical reactions of a saturated dicar-boxylic acid without a prior oxidation step in the molecule, and, secondly, it is difficult to understand how, if acetate were the primary product and it condensed to form acetoacetate, the resulting acetoacetate would also not be labeled to some extent in the carbonyl carbon. 

DECARBOXYLATION REACTIONS The Hunsdiecker Reaction Biochemical Decarboxylation Reactions... 

Among the biochemical reactions that ammo acids undergo is decarboxylation to amines Decarboxylation of histidine for example gives histamine a powerful vasodila tor normally present m tissue and formed m excessive amounts under conditions of trau matic shock... 

Pterin-6-cafboxylic acid, 3,8-dimethyl-rearrangements, 3, 309 Pterincarboxylic acids occurence, 3, 323 Pterin-6-carboxylic acids acidity, 3, 277 methylation, 3, 297 synthesis, 3, 295, 304 Pterin-7-carboxylic acids acidity, 3, 277 methylation, 3, 297 synthesis, 3, 295 Pterin coenzymes biochemical pathways, 1, 260-263 Pterin-6,7-dicarboxylic acid decarboxylation, 3, 304 reactions, 3, 304... 

Among the biochemical reactions that anino acids undergo is decarboxylation to fflnines. Decar boxylation of histidine, for example, gives histamine, a powerful vasodilator nonnally present in tissue and fonned in excessive fflnounts under conditions of traumatic shock. 

Most coenzymes have aromatic heterocycles as major constituents. While enzymes possess purely protein structures, coenzymes incorporate non-amino acid moieties, most of them aromatic nitrogen het-erocycles. Coenzymes are essential for the redox biochemical transformations, e.g., nicotinamide adenine dinucleotide (NAD, 13) and flavin adenine dinucleotide (FAD, 14) (Scheme 5). Both are hydrogen transporters through their tautomeric forms that allow hydrogen uptake at the termini of the quinon-oid chain. Thiamine pyrophosphate (15) is a coenzyme that assists the decarboxylation of pyruvic acid, a very important biologic reaction (Scheme 6). 

The second function, and the one pertinent to this section, is the decarboxylation of oxalosuccinic acid to 2-oxoglutaric acid. This is simply a biochemical example of the ready decarboxylation of a P-ketoacid, involving an intramolecular hydrogen-bonded system. This reaction could occur chemically without an enzyme, but it is known that isocitric acid, the product of the dehydrogenation, is still bound to the enzyme isocitrate dehydrogenase when decarboxylation occurs. 

Relatively little is known concerning the oxidation of azolium salts. Most of the publications deal with thiazolium salts due to the significant biochemical role of thiamin as a coenzyme in a variety of enzyme-catalyzed decarboxylations and aldol-type condensations. The chemistry of thiamin has been extensively reviewed (83MI1). Depending on the reaction conditions, thio-chrome (197) and the disulfide 198 are formed by oxidation of thiamin (57JA4386). 

The addition of an enolate anion to C02 to form a (3-oxoacid represents one of the commonest means of incorporation of C02 into organic compounds. The reverse reaction of decarboxylation is a major mechanism of biochemical formation of C02. The equilibrium constants usually favor decarboxylation but the cleavage of ATP can be coupled to drive carboxylation when it is needed, e.g., in photosynthesis. 

Many additional examples of the elucidation of prostereoisomerism in biochemical reactions could be given, for example the elegant elucidation by Comforth and coworkers 111, n8,137) of the biosynthesis of squalene, which was recognized by the Nobel prize in chemistry in 1975, or the recent studies of the enzymatic decarboxylation of tyrosine 138) and histidine 139) and of the condensation of homoserine with cysteine to give lanthionine 140), but the examples already provided should illustrate the principles and techniques involved in such studies. 

This reaction is clearly analogous to that of NAD-IDHs, NADP-IDHs, and NAD-IMDHs. Early studies showed that the enzyme of S. cerevisiae was separated from the NAD-IDH and has a different pH optimum [33], The molecular mass of the enzyme is 48 kDa. These results suggest that NAD-HDH is a novel member of the P-decarboxylating dehydrogenase family. In spite of a number of biochemical and genetic studies, the gene encoding the enzyme has not been identified. 

This is the main reaction of MLR Chemically it consists of a simple decarboxylation of the L-malic acid in wine into L-lactic acid. Biochemically, it is the result of activity of the malolactic enzyme, characteristic of lactic acid bacteria. This transformation has a dual effect. On the one hand, it deacidifies the wine, in other words, it raises the pH, an effect that is greater at higher initial quantities of malic acid. It also gives the wine a smoother taste, replacing the acidic and astringent flavour of the malic acid, by the smoother flavour of the lactic acid. 

Grape compounds which can enter the yeast cell either by diffusion of the undissociated lipophilic molecule or by carrier-mediated transport of the charged molecule across the cell membrane are potentially subject to biochemical transformations by enzymatic functions. A variety of biotransformation reactions of grape compounds that have flavour significance are known. One of the earlier studied biotransformations in yeast relates to the formation of volatile phenols from phenolic acids (Thurston and Tubb 1981). Grapes contain hydroxycinnamic acids, which are non-oxidatively decarboxylated by phenyl acryl decarboxylase to the vinyl phenols (Chatonnet et al. 1993 Clausen et al. 1994). 

Free radical reactions of purines with amines gave similar products to those produced in alcohol solution although deamination may also occur, probably at the post- rather than the pre-adduct stage. Whereas purine and n-propylamine afforded 6-n-propylpurine (71MI40907), adenine and caffeine produced both the 8-aminoalkyl and corresponding 9-alkyl derivatives (74MI40904). Also irradiation of 8-aminoalkylpurines in methanol furnished the 8-alkyl derivatives. Amino acids as an amine source are of special biochemical interest. They also tend to produce 8-alkylpurines by concomitant deamination and decarboxylation (69CC905). 

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