Decarboxylation, carbohydrate

The principal steps in the mechanism of polyisoprene formation in plants are known and should help to improve the natural production of hydrocarbons. Mevalonic acid, a key intermediate derived from plant carbohydrate via acetylcoen2yme A, is transformed into isopentenyl pyrophosphate (IPP) via phosphorylation, dehydration, and decarboxylation (see Alkaloids). IPP then rearranges to dimethylaHyl pyrophosphate (DMAPP). DMAPP and... 

Itaconic 2Lcid[97-65-4] (methylenebutanedioic acid, methylenesuccinic acid) is a crystaUine, high, melting acid (mp = 167-168) produced commercially by fermentation of carbohydrates (1 4). Itaconic acid is produced in the broth from citric acid (qv). Isolated from the pyrolysis products of citric acid in 1836, this a-substituted acryUc acid received its name by rearrangement of aconitic, the acid from which it is formed by decarboxylation. 

Lipoic acid is an acyl group carrier. It is found in pyruvate dehydrogenase zard a-ketoglutarate dehydrogenase, two multienzyme complexes involved in carbohydrate metabolism (Figure 18.34). Lipoie acid functions to couple acyl-group transfer and electron transfer during oxidation and decarboxylation of a-keto adds. 

The citric acid cycle is the final pathway for the oxidation of carbohydrate, Upid, and protein whose common end-metabolite, acetyl-CoA, reacts with oxaloacetate to form citrate. By a series of dehydrogenations and decarboxylations, citrate is degraded, releasing reduced coenzymes and 2CO2 and regenerating oxaloacetate. 

Thiamin has a central role in energy-yielding metabo-hsm, and especially the metabohsm of carbohydrate (Figure 45-9). Thiamin diphosphate is the coenzyme for three multi-enzyme complexes that catalyze oxidative decarboxylation reactions pymvate dehydrogenase in carbohydrate metabolism a-ketoglutarate dehydro-... 

The shikimate pathway is the major route in the biosynthesis of ubiquinone, menaquinone, phyloquinone, plastoquinone, and various colored naphthoquinones. The early steps of this process are common with the steps involved in the biosynthesis of phenols, flavonoids, and aromatic amino acids. Shikimic acid is formed in several steps from precursors of carbohydrate metabolism. The key intermediate in quinone biosynthesis via the shikimate pathway is the chorismate. In the case of ubiquinones, the chorismate is converted to para-hydoxybenzoate and then, depending on the organism, the process continues with prenylation, decarboxylation, three hydroxy-lations, and three methylation steps. - ... 

Thiamine catalyzes decarboxylations, as of pyruvic acid and trans-ketolations in carbohydrate metabolism. Free thiamine is carried by the... 

Photolysis of three 2,4-dinitroanilino-substituted carbohydrates, compounds that differ considerably from each other in photochemical reactivity, has been reported.150,151 l-Deoxy-l-(2,4-dinitroanilino)-D-glucitol (73) is photochemically unreactive in contrast, sodium 2-deoxy-2-(2,4-dinitroanilino)-D-gluconate (74) produces D-arabinose in 52% yield upon irradiation.150 The behavior of compounds 73 and 74 indicates that oxidative loss of the 2,4-dinitroanilino group during photolysis is only possible when it is accompanied by simultaneous decarboxylation. The evidence gathered from the considerable study of this reaction for noncarbohydrate systems suggested that this process is quite complex. Although useful, mechanistic proposals have... 

Besides Szent-Gyorgi and Krebs, other groups were attacking the problem of carbohydrate oxidation. Weil-Malherbe suggested It is probable that the further oxidation of succinic acids passes through the stages of fumaric, malic, and oxaloacetic acid pyruvic acid is formed by the decarboxylation of the latter and the oxidative cycle starts again. K.A.C. Elliott, from the Cancer Research Laboratories at the University of Pennsylvania, also proposed a cycle via some 6C acid. 

The tricarboxylic acid cycle was therefore validated, having been tested not only in pigeon-breast muscle but also with brain, testis, liver, and kidney. The nature of the carbohydrate fragment entering the cycle was still uncertain. The possibility that pyruvate and oxaloacetate condensed to give a 7C derivative which would be decarboxy-lated to citrate, was dismissed partly because the postulated compound was oxidized at a very low rate. Further, work on the oxidation of fatty acids (see Chapter 7) had already established that a 2C fragment like acetate was produced by fatty acid oxidation, en route for carbon dioxide and water. It therefore seemed likely that a similar 2C compound might arise by decarboxylation of pyruvate, and thus condense with oxaloacetate. For some considerable time articles and textbooks referred to this unknown 2C compound as active acetate. ... 

Scheme 25 C-disaccharides by anodic decarboxylation of carbohydrate carboxylic acids.

Thiamine diphosphate (TDP) is an essential coenzyme in carbohydrate metabolism. TDP-dependent enzymes catalyze carbon-carbon bond-breaking and -forming reactions such as a-keto acid decarboxylations (oxidative and non-oxidative) and condensations, as well as ketol transfers (trans- and phospho-ketolation). Some of these processes are illustrated in Fig. 12. 

Thiamine pyrophosphate is a coenzyme for several enzymes involved in carbohydrate metabolism. These enzymes either catalyze the decarboxylation of oi-keto acids or the rearrangement of the carbon skeletons of certain sugars. A particularly important example is provided by the conversion of pyruvic acid, an oi-keto acid, to acetic acid. The pyruvate dehydrogenase complex catalyzes this reaction. This is the key reaction that links the degradation of sugars to the citric acid cycle and fatty acid synthesis (chapters 16 and 18) ... 

Carbohydrate resources. Carbohydrate resourees, sueh as hydrolyzed stareh and suerose as well as xylose and glueose, ean be proeessed into hydroearbons in a proeess similar to the one performed with bio-oils as described above (section 3.2.2), i.e. by using a HZSM-5 catalyst operated at around 510 °C and ambient pressure." This process is perhaps a little surprising since carbohydrates do not resemble the desired hydrocarbon product as much as the bio-oils do. However, formation of hydrocarbon compounds was found to occur as a result of oxygen removal from the carbohydrate by decarbonylation and decarboxylation reactions." This process is probably one of the first attempts to conduct catalytic cracking of biomass. 

CpPNO in carbohydrate metabolism is not yet known, a Nar/-like hydroge-nase has been identified in both C. parvum (Stejskal et al. 2003 Abrahamsen et al. 2004) and C. hominis (Xu et al. 2004), which may function to oxidize the NADPH produced by PNO during pyruvate decarboxylation. Unlike other amitochondriate protists (Entamoeba, Giardia, Trichomonas), neither of these Cryptosporidia possesses an [FeFe]-hydrogenase capable of transferring electrons produced during the oxidation of PFO (Horner et al. 2000). It is proposed that the acetyl-CoA resulting from the decarboxylation of pyruvate in C. parvum may then be converted to malonyl-CoA (Templeton et al. 

Also unsuccessful have been all attempts to decarboxylate fluoroformates of various carbohydrates, but it has been shown that an excess of reagent, solvent, temperature and time of reaction are all of great importance in obtaining high yields for the fluorination of carbohydrate chloroformates (Table l).5... 

Biotin is involved in many carboxylation and decarboxylation reactions in carbohydrate, fatty acid, protein, and nucleic acid metabolism. Milk is a fairly good source of this vitamin, generally providing about 3/xg/100 g. Pasteurization has a minimal effect on the biotin content of milk. 

The reductive carboxylation of acetyl-CoA to pyruvate (Eq. 17-47) occurs only in a few types of bacteria. For most species, from microorganisms to animals, the oxidative decarboxylation of pyruvate to acetyl-CoA is irreversible. This fact has many important consequences. For example, carbohydrate... 

Vitamin Influences. The involvement of NAD and NADP in many carbohydrate reactions explains the importance of nicotinamide in carbohydrate melaholism. Thiamine, in the form or thiamine pyrophosphate (cocarboxylase), is the cofaclor necessary in the decarboxylation of pyruvic acid, in the iraru-kelolase-calalyzed reactions of the pentose phosphaie cycle, and in the decarboxylation of alpha-keloglutaric acid in the citric acid cycle, among other reactions. Biotin is a hound cofaclor in the fixation of carbon dioxide to form nxalacetic acid from pyruvic acid. Pantothenic acid is a part of the C oA molecule. There are separate alphabetical entries in this volume on the various specific vitamins as well as a review entry on Vitamin. 

Thiamine, biotin and pyridoxine (vitamin B) coenzymes are grouped together because they catalyze similar phenomena, i.e., the removal of a carboxyl group, COOH, from a metabolite. However, each requires different specific circumstances. Thiamine coenzyme decarboxylates only alpha-keto acids, is frequently accompanied by dehydrogenation, and is mainly associated with carbohydrate metabolism. Biotin enzymes do not require the alpha-keto configuration, are readily reversible, and are concerned primarily with lipid metabolism. Pyridoxine coenzymes perform nonoxidative decarboxylation and are closely allied with amino acid metabolism. 

TPP also mediates the oxidative decarboxylation of a-ketoglutaric acid, another intermediate of carboxydrate metabolism in the citric acid cycle. The nutritional requirement for thiamine increases as dietary carbohydrate increases because of a greater demand for TPP. 

Acetyl-CoA is the only compound that can enter the TCA cycle when the cycle is operating purely oxidatively, but one molecule of oxaloacetate must enter for each molecule of citrate, a-ketoglutarate, or succinyl-CoA that is removed for use in biosynthesis. It follows that pyruvate is a major metabolic branchpoint in a cell that is living on carbohydrate. The partitioning of pyruvate between decarboxylation to acetyl-CoA and carboxylation to oxaloacetate is, in effect, partitioning between the two major metabolic uses of pyruvate oxidation of carbon for regeneration of ATP and conversion to starting materials for biosynthesis. 

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