Coenzymes thiamine diphosphate

Aldehydes and ketones are converted into alkenes by means of a nucleophilic addition called the Wittig reaction. The reaction has no direct biological counterpart but is important both because of its wide use in the laboratory and drug manufacture and because of its mechanistic similarity to reactions of the coenzyme thiamin diphosphate, which well see in Section 29.6. 

In a rare autosomal recessive condition (discovered in 1954) the urine and perspiration has a maple syrup odor/ High concentrations of the branched-chain 2-oxoacids formed by transamination of valine, leucine, and isoleucine are present, and the odor arises from decomposition products of these acids. The branched-chain amino acids as well as the related alcohols also accumulate in the blood and are found in the urine. The biochemical defect lies in the enzyme catalyzing oxidative decarboxylation of the oxoacids, as is indicated in Fig. 24-18. Insertions, deletions, and substitutions may be present in any of the subunits (Figs. 15-14,15-15). The disease which may affect one person in 200,000, is usually fatal in early childhood if untreated. Children suffer seizures, mental retardation, and coma. They may survive on a low-protein (gelatin) diet supplemented with essential amino acids, but treatment is difficult and a sudden relapse is apt to prove fatal. Some patients respond to administration of thiamin at 20 times the normal daily requirement. The branched-chain oxoacid dehydrogenase from some of these children shows a reduced affinity for the essential coenzyme thiamin diphosphate.d... 

The coenzymes thiamin diphosphate and lipoamide are involved in the last step. 

The coenzyme thiamin diphosphate (ThDP, I in Scheme 16.1), the biologically active form of vitamin Bi, is used by different enzymes that perform a vide range of catalytic functions, such as the oxidative and nonoxidative decarboxylation of a-ketoacids, the formation of acetohydroxyacids and ketol transfer bet veen sugars. 

Thiamin (thiamine, vitamin Bi), also formerly called aneurin (antineuritic factor), is a water-soluble vitamin of the B group. Like other B vitamins, thiamin is the precursor of an important coenzyme, thiamin diphosphate (ThDP), required for the oxidative decarboxylation of 2-oxo acids. However, in contrast to other B vitamins, non-cofactor roles have been proposed for thiamin derivatives. These could be mediated by two triphosphate derivatives, thiamin triphosphate (ThTP) and the recently discovered adenosine thiamin triphosphate (AThTP) (Bettendorff et al. 2007). 

The decarboxylation and oxidation of pyruvate to form acetyl CoA requires the coenzyme thiamin diphosphate, which is formed from vitamin (section 11.6.2). In thiamin deficiency, this reaction is impaired, and deficient subjects are unable to metabolize glucose normally. Especially after a test dose of glucose or moderate exercise they develop high blood concentrations of pyruvate and lactate. In some cases this may be severe enough to result in life-threatening acidosis. 

The structures of thiamin and the coenzyme thiamin diphosphate are shown in Figure 11.12. 

Figure 11.12 Thiamin vitamin B ) and the coenzyme thiamin diphosphate.

Following absorption, thiamin is transported to the liver where it is phosphorylated under the action of ATP to form the coenzyme thiamin diphosphate (formerly called thiamin pyrophosphate or cocarboxylase), (see Fig. T-9) although this phosphorylation occurs rapidly in the liver, it is noteworthy that all nucleated cells appear to be capable of bringing about this conversion. 

In the coenzyme thiamin diphosphate (thiamin pyrophosphate) the-OH group is replaced by O O... 

Step 4 of Figure 29.12 Oxidative Decarboxylation The transformation of cr-ketoglutarate to succinyl CoA in step 4 is a multistep process just like the transformation of pyruvate to acetyl CoA that we saw in Figure 29.11. In both cases, an -keto acid loses C02 and is oxidized to a thioester in a series of steps catalyzed by a multienzynie dehydrogenase complex. As in the conversion of pyruvate to acetyl CoA, the reaction involves an initial nucleophilic addition reaction to a-ketoglutarate by thiamin diphosphate vlide, followed by decarboxylation, reaction with lipoamide, elimination of TPP vlide, and finally a transesterification of the dihydrolipoamide thioester with coenzyme A. 

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-... 

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 (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... 

Now this reaction is effectively a repeat of the pyruvate acetyl-CoA oxidative decarboxylation we saw at the beginning of the Krebs cycle. It similarly requires thiamine diphosphate, lipoic acid, coenzyme A and NAD+. A further feature in common with that reaction is that 2-oxoglutarate dehydrogenase is also an enzyme complex comprised of three separate enzyme activities. 2-Oxoglutarate is thus transformed into succinyl-CoA, with the loss of... 

The intermediary metabolism has multienzyme complexes which, in a complex reaction, catalyze the oxidative decarboxylation of 2-oxoacids and the transfer to coenzyme A of the acyl residue produced. NAD" acts as the electron acceptor. In addition, thiamine diphosphate, lipoamide, and FAD are also involved in the reaction. The oxoacid dehydrogenases include a) the pyruvate dehydrogenase complex (PDH, pyruvate acetyl CoA), b) the 2-oxoglutarate dehydrogenase complex of the tricarboxylic acid cycle (ODH, 2-oxoglutarate succinyl CoA), and c) the branched chain dehydrogenase complex, which is involved in the catabolism of valine, leucine, and isoleucine (see p. 414). 

The five different coenzymes involved are associated with the enzyme components in different ways. Thiamine diphosphate is non-covalently bound to El, whereas lipoamide is covalently bound to a lysine residue of E2 and FAD is bound as a prosthetic group to E3. NAD" and coenzyme A, being soluble coenzymes, are only temporarily associated with the complex. 

D. 4. Pumping Ions with the Help of Biotin Thiamin Diphosphate 735. .. Table 14-2 Enzymes Dependent upon Thiamin Diphosphate as a Coenzyme... 

Compounds, often derivatives of vitamins that, while in the active site of the enzyme, alter the structure of a substrate in a way that permits it to react more readily. Coenzyme A, pyridoxal phosphate, thiamin diphosphate, and vitamin B12 coenzymes fall into this group. 

Why do we need vitamins Early clues came in 1935 when nicotinamide was found in NAD+ by H. von Euler and associates and in NADP+ by Warburg and Christian. Two years later, K. Lohman and P. Schuster isolated pure cocarboxylase, a dialyz-able material required for decarboxylation of pyruvate by an enzyme from yeast. It was shown to be thiamin diphosphate (Fig. 15-3). Most of the water-soluble vitamins are converted into coenzymes or are covalently bound into active sites of enzymes. Some lipid-soluble vitamins have similar functions but others, such as vitamin D and some metabolites of vitamin A, act more like hormones, binding to receptors that control gene expression or other aspects of metabolism. 

Thiamin is synthesized in bacteria, fungi, and plants from 1-deoxyxylulose 5-phosphate (Eq. 25-21), which is also an intermediate in the nonmevalonate pathway of polyprenyl synthesis. However, thiamin diphosphate is a coenzyme for synthesis of this intermediate (p. 736), suggesting that an alternative pathway must also exist. Each of the two rings of thiamin is formed separately as the esters 4-amino-5-hydroxy-methylpyrimidine diphosphate and 4-methyl-5-((i-hydroxyethyl) thiazole monophosphate. These precursors are joined with displacement of pyrophosphate to form thiamin monophosphate.92b In eukaryotes this is hydrolyzed to thiamin, then converted to thiamin diphosphate by transfer of a diphospho group from ATP.92b c In bacteria thiamin monophosphate is converted to the diphosphate by ATP and thiamin monophosphate kinase.92b... 

We see that the essence of the action of thiamin diphosphate as a coenzyme is to convert the substrate into a form in which electron flow can occur from the bond to be broken into the structure of the coenzyme. Because of this alteration in structure, a bond breaking reaction that would not otherwise have been possible occurs readily. To complete the catalytic cycle, the electron flow has to be reversed again. The thiamin-bound cleavage product (an enamine) from either of the adducts in Eq. 14-20 can be reconverted to the thiazolium dipolar ion and an aldehyde as shown in step b of Eq. 14-21 for decarboxylation of pyruvate to acetaldehyde. 

Enzymes Dependent upon Thiamin Diphosphate as a Coenzyme... 

The oxidative cleavage of an a-oxoacid is a major step in the metabolism of carbohydrates and of amino acids and is also a step in the citric acid cycle. In many bacteria and in eukaryotes the process depends upon both thiamin diphosphate and lipoic acid. The oxoacid anion is cleaved to form C02 and the remaining acyl group is combined with coenzyme A (Eq. 15-33). 

Schellenberger A, Neef H, Golbig R, Hiibner G, Konig S (1990) Mechanistic aspects of thiamine pyrophosphate enzymes via site-directed substitutions of the coenzyme structure. In Bisswanger H, Ullrich H (eds) Biochemistry and physiology of thiamin diphosphate enzymes. VCH, Weinheim, p 3... 

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