Carbohydrate metabolism thiamin

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-... 

Rice bran is the richest natural source of B-complex vitamins. Considerable amounts of thiamin (Bl), riboflavin (B2), niacin (B3), pantothenic acid (B5) and pyridoxin (B6) are available in rice bran (Table 17.1). Thiamin (Bl) is central to carbohydrate metabolism and kreb s cycle function. Niacin (B3) also plays a key role in carbohydrate metabolism for the synthesis of GTF (Glucose Tolerance Factor). As a pre-cursor to NAD (nicotinamide adenine dinucleotide-oxidized form), it is an important metabolite concerned with intracellular energy production. It prevents the depletion of NAD in the pancreatic beta cells. It also promotes healthy cholesterol levels not only by decreasing LDL-C but also by improving HDL-C. It is the safest nutritional approach to normalizing cholesterol levels. Pyridoxine (B6) helps to regulate blood glucose levels, prevents peripheral neuropathy in diabetics and improves the immune function. 

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

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) ... 

Thiamine (vitamin Bi), in the form of thiamine diphosphate (TPP), is a coenzyme of some considerable importance in carbohydrate metabolism. Dietary deficiency leads to the condition beriberi, characterized by neurological disorders, loss of appetite, fatigue, and muscular weakness. We shall study a number of... 

TPP thus plays a very important role in carbohydrate metabolism. The parent alcohol thiamine is one of the B group vitamins, namely vitamin Bi dietary... 

Thiamine pyrophosphate Tight Thiamine (B ) Cofactor for oxidative removal of CO2 in several reactions of carbohydrate metabolism... 

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. 

Thiamin is proportionately linked with carbohydrate metabolism and the antioxidant vitamins A, C and E can aid the elimination of free radicals formed within muscle at an increased rate during strenuous activity (Davies et al., 1982). Vitamins can thus be of value during post-activity recovery. 

Carbohydrate metabolism provides the main energy source in coccidia. Diets deficient in thiamin, riboflavin, or nicotinic acid—all cofactors in carbohydrate metabolism—result in suppression of parasitic infestation of chickens by E tenella and E acervulina. A thiamin analog, amprolium—1-[(4-amino-2-propyl-5-pyrimidinyl)-methyl]-2-picolinium chloride—has long been used as an effective anticoccidial agent in chickens and cattle with relatively low host toxicity. The antiparasitic activity of amprolium is reversible by thiamin and is recognized to involve inhibition of thiamin transport in the parasite. Unfortunately, amprolium has a rather narrow spectrum of antiparasitic activity it has poor activity against toxoplasmosis, a closely related parasitic infection. 

Metabolic abnormalities under normal conditions, such as impaired carbohydrate metabolism in thiamin deficiency (Section 6.5) or excretion of methylmalonic acid in vitamin B12 deficiency (Section 10.10.3). 

Thiamine deficiency results in inhibition of carbohydrate metabolism causing accumulation of Pyruvate. 

It is apparent from the centred role of thieunin in carbohydrate metabolism that the requirement will depend on CMbohydrate inteike to a considerable extent. In practice, requirements Me cedculated on the basis of total energy intedce, assuming that the average diet provides 40% of energy from fat. For diets that are lower in fat, emd hence higher in CMbohydrate and protein, thiamin requirements will be somewhat higher. 

Thiamine pyrophosphate has two important coenzyme roles, both of which focus mostly on carbohydrate metabolism (Figs. 8.26 and 8.27). The active portion of the coen- rae is the thiazole ring. The first step in the oxidative decarboxylation of a-keto acids requires TPP. The two most common examples are pyruvate and a-ketoglutarate, oxidatively decarboxyatedto acetyl CoA and succinyl CoA, respectively. The same reaction is found in the metabolism of the branched-chain amino acids valine, isoleucine, leucine, and methionine. In all cases, TPP is a coenzyme in a mitochondrial multienzyme complex, consisting of TPP, lipoic acid, coenzyme A, FAD, and NAD. Note the number of vitamins required for the oxidative decarboxylation of a-keto acids thiamine (TPP), pantothenic acid (coenzyme A), riboflavin (FAD),and niacin (NAD). 

Thiamin is involved in the stabilization of the acyl carbanion intermediate and is particularly important in carbohydrate metabolism [87]. 

Thiamine pyrophosphate is also an important cofactor for the transketolase reactions in the pentose phosphate pathway of carbohydrate metabolism (Fignre 15-3). These reactions are important in the reversible transformation of pentoses into the glycolytic intermediates fructose 6-phosphate and glyc-eraldehyde 3-phosphate. Again, it is the reactive carbon on the thiazole ring of TPP that reacts with a ketose phosphate (xylnlose 5-phosphate) to canse the release of an aldose phosphate with two fewer carbons (glyceraldehyde 3-phosphate). The TPP-bonnd glycoaldehyde unit is then transferred to a different aldose phosphate (ribose 5-phosphate or erythrose 4-phosphate) to produce a ketose phosphate that has two carbons more (sedoheptulose 7-phosphate or fructose 6-phosphate). 

Thiamine pyrophosphate is also an important cofactor for many dehydrogenase reactions as well as the transketolase reactions in the pentose phosphate pathway of carbohydrate metabolism. 

VITAMIN Bj (Thiamine, aneurine) Vitamin Bj has an important function in carbohydrate-metabolism as a prosthetic group in the enzyme co-carboxylase. 

Vitamin Bi is an essential co-factor for several enzymes of carbohydrate metabolism such as transketolase, pyruvate dehydrogenase (PDH), pyruvate decarboxylase and a-ketoglutarate dehydrogenase. To become the active co-factor thiamin pyrophosphate (TPP), thiamin has to be salvaged by thiamin pyrophosphokinase or synthesized de novo. In Escherichia coli and Saccharomyces cerevisiae thiamin biosynthesis proceeds via two branches that have to be combined. In the pyrimidine branch, 4-amino-5-hydroxymethy-2-methylpyrimidine (PIMP) is phosphorylated to 4-amino-2-methyl-5-hydroxymethyl pyrimidine diphosphate (PIMP-PP) by the enzyme HMP/HMP-P kinase (ThiD) however, the step can also be catalyzed by pyridoxine kinase (PdxK), an enzyme also responsible for the activation of vitamin B6 (see below). The second precursor of thiamin biosynthesis, 5-(2-hydroxyethyl)-4-methylthiazole (THZ), is activated by THZ kinase (ThiM) to 4-methyl-5-(2-phosphoethyl)-thiazole (THZ-P), and then the thia-zole and pyrimidine moieties, HMP-PP and THZ-P, are combined to form thiamin phosphate (ThiP) by thiamin phosphate synthase (ThiE). The final step, pyrophosphorylation, yields TPP and is carried out by thiamin pyrophosphorylase (TPK). 

Thiamine, a soluble vitamin, is an essential nutrient for humans and is important in carbohydrate metabolism, maintaining normal neural activity and preventing beriberi. Various analytical techniques have been reported for the determination of thiamine in pure form, in pharmaceutical preparations, or in biological fluids. Spectrophotometric methods suffer from poor sensitivity (mg/L detection limit). Spectrofluorometric methods usually involve the conversion of thiamine to thiochrome.2 High performance liquid chromatography requires a post-column derivatization step3 and instrumentation for electrophoresis-based methods is expensive.4... 

Thiamin transporter Carbohydrate metabolism is the primary energy source in coc-cidia. Inhibition of the cellular transport of thiamin by the structurally similar agent ampro-lium leads to a deficiency of this cofactor in coccidia. 

I. Pharmacology. Thiamine (vitamin B ) is a water-soluble vitamin that acts as an essential cofactor for various pathways of carbohydrate metabolism. Thiamine also acts as a cofactor in the metabolism of glyoxylic acid (produced in ethylene glycol intoxication). Thiamine deficiency may result in beriberi and Wemicke-Korsakoff syndrome. Thiamine is rapidly absorbed after oral, intramuscular, or intravenous administration. However, parenteral administration is recommended for initial management of thiamine deficiency syndromes. 

ANAEROBIC CARBOHYDRATE METABOLISM Yeasts growing in media containing high concentrations of fermentable carbohydrate invariably metabolize it fermentatively to produce ethanol and CO2. If air is present, and when the sugar concentration has been lowered, the ethanol is respired using the metabolic routes described above. Under the anaerobic conditions of a brewery fermentation the hexoses derived from wort fermentable carbohydrates are catabolized by the EMP pathway (Fig. 17.2) to pyruvic acid. The pyruvate produced is decarboxylated by the enzyme pyruvate decarboxylase, with the formation of acetaldehyde and CO2. The enzyme requires the cofactor thiamine pyrophosphate (TPP) for activity and the reaction is shown in Fig. 17.10. The acetaldehyde formed acts (in the absence of the respiratory chain) as an electron acceptor and is used to oxidize NADH with the formation of ethanol ... 

Transketolase (EC 2.2.1.1) an enzyme that catalyses transketolation, an important process of carbohydrate metabolism, especially in the Pentose phosphate cycle (see) and Calvin cycle (see). T. has been found in a wide variety of cells and tissues, including mammalian liver, green plants and many bacterial species. The enzyme contains divalent metal cations and the coenzyme, thiamin pyrophosphate. Transketolation involves transfer of a C2-unit (often called active glycolaldehyde or a ketol moiety) from a ketose to Cl of an aldose. Only ketoses with L-configuration at C3 and preferably irons configuration on the next carbon (i.e. Cl, 2, 3 and preferably 4 as in fructose) can serve as donors of the C2-unit. The acceptor is always an aldose. Thins-ketolation is reversible. Details of the reaction in which xylulose S-phosphate serves as the donor of... 

Thiamin (V. Bj) 1926 1.2 Carbohydrate metabolism, transfer of active aldehyde antineuritic V. 

V.B] results in disturbances of carbohydrate metabolism, accompanied by an increase in the concentration of blooid oxoadds (mostly pyruvate), which reflects the role of thiamin pyrophosphate as a co-enzyme of pyruvate dehydrogenase. The typical defi-dency disease, beriberi, results from a diet excludvely of polished rice. It is characterized by disturbances of the central and peripheral nervous system (polyneitri-tis) and of cardiac function. The daily requirement for th min is about 1 mg. 

The fact that rats maintained on a carbohydrate-free diet can survive for a long time, even if their diet is completely devoid of thiamine, demonstrates that the main effect of thiamine deficiency is on carbohydrate metabolism. As soon as 5-10% carbohydrates are added to the diet, polyneuritis and death occur. On the contrary, if sorbitol is added as a source of carbohydrate, the animals survive, and this is... 

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