TPP-dependent enzyme

TPP-dependent enzymes are involved in oxidative decarboxylation of a-keto acids, making them available for energy metabolism. Transketolase is involved in the formation of NADPH and pentose in the pentose phosphate pathway. This reaction is important for several other synthetic pathways. It is furthermore assumed that the above-mentioned enzymes are involved in the function of neurotransmitters and nerve conduction, though the exact mechanisms remain unclear. 

Answer E. Most important TPP-dependent enzymes include pyruvate dehydrogenase, a-keto-glutarate dehydrogenase, and transketolase. Transketolase is in the HMP shunt and is not strictly essential for glucose oxidation. 

TPP-dependent enzymes catalyze either simple decarboxylation of a-keto acids to yield aldehydes (i.e. replacement of C02 with H+), or oxidative decarboxylation to yield acids or thioesters. The latter type of reaction requires a redox coenzyme as well (see below). The best known example of the former non-oxidative type of decarboxylation is the pyruvate decarboxylase-mediated conversion of pyruvate to acetaldehyde and C02. The accepted pathway for this reaction is shown in Scheme 10 (69MI11002, B-70MI11003, B-77MI11001>. 

Benzoylformate decarboxylase (BFD), a TPP-dependent enzyme, features a high degree of sequence similarity with a whole family of other TPP-dependent decarboxylases, among them pyruvate decarboxylase (PDC), one of the most important enzymes in metabolism. 

Transketolase is a TPP-dependent enzyme found in the cytosol of many tissues, especially hver and blood cells, in which principal carbohydrate pathways exist. In the pentose phosphate pathway, which additionally supplies reduced nicotinamide-adenine dinucleotide phosphate (NADPH) necessary for biosynthetic reactions, this enzyme catalyzes the reversible transfer of a glycoaldehyde moiety from the first two carbons of a donor ketose phosphate to the aldehyde carbon of an aldose phosphate. 

This is the most studied TPP-dependent enzyme, and it is likely that mechanisms of other thiamin pyrophosphate-dependent decarboxylations are similar. Editor s note Other aspects of thiamin pyrophosphate chemistry are presented in Chapter 7 by Kluger.)... 

Transketolase. Another group of enzymes that catalyze the stereospecific formation and cleavage of carbohydrates in vivo are the transketolases and transaldolases. Transketolase (E.C. 2.2.1.1) is a thiamin pyrophosphate (TPP) dependent enzyme that catalyzes the transfer of a hydroxyketo group from a ketose phosphate to an aldose phosphate in the pentose pathway (Scheme 12) (36). 

It was recognized in 1953 that TK is a TPP-dependent enzyme [1, 2]. Since the work of Breslow who succeeded in directly localizing the catalytic site of thiamine [3] a considerable knowledge on the mechanism of TPP catalyzed reactions has accumulated [4-6], Very little, however, is known about the interaction of TPP with its corresponding apoenzyme, which will be the subject of this paper. 

The a-keto acid decarboxylases such as pyruvate (E.C. 4.1.1.1) and benzoyl formate (E.C. 4.1.1.7) decarboxylases are a thiamine pyrophosphate (TPP)-dependent group of enzymes, which in addition to nonoxidatively decarboxylating their substrates, catalyze a carboligation reaction forming a C-C bond leading to the formation of a-hydroxy ketones.269-270 The hydroxy ketone (R)-phenylacetylcarbinol (55), a precursor to L-ephedrine (56), has been synthesized with pyruvate decarboxylase (Scheme 19.35). BASF scientists have made mutations in the pyruvate decarboxylase from Zymomonas mobilis to make the enzyme more resistant than the wild-type enzyme to inactivation by acetaldehyde for the preparation of chiral phenylacetylcarbinols.271... 

The proposed catalytic mechanism of the ferredoxin oxidoreductase [32] is shown in Fig. 4, a similar mechanism existing for the analogous citric acid cycle enzyme, 2-oxoglutarate oxidoreductase. In outline, the 2-oxoacid is decarboxylated in a TPP-dependent reaction to give an hydroxyalkyl-TPP. From this, one electron is abstracted and transferred to the enzyme-bound iron-sulphur cluster, generating a free-radical-TPP species. This intermediate can then interact direct with coenzyme-A to form acyl-CoA, the iron-cluster receiving the second electron. In each case, ferredoxin serves to re-oxidise the enzyme s redox centre. 

The most reliable method for assessing thiamin status involves the measurement of red blood cell transketolase. This enzyme is measured with and without the addition of TPP to the enzyme assay mixtures. In dietary thiamin deficiency, synthesis of transketolasc continues, but conversion of the apoet zyme to the holoenzyme in the cell is inhibited, resulting in the accumulation of the enzyme in the apoenzyme form. Addition of TPP to cell homogenates results in the conversion of apoenzyme to holoenzyme. This conversion can easily be detected by enzyme assays. The amount of shmulation of enzyme activity by the added TPP is used to assess thiamin status. A deficiency is indicated by a shmulation of over 20%, The TPP-dependent stimulation, using red blood cells from normal subjects, ranges from 0 to 15%. 

It has been postulated that the 2-ketoglutarate undergoes a TPP-dependent decarboxylation with the formation of succinic semialdehyde anion of TPP (13) and a requirement for TPP in the reaction was shown. ° The mechanism of decarboxylation is identical to that catalyzed by the first enzyme of the... 

Wernicke Korsakoff syndrome (WKS) is the neurological disorder most clearly linked to thiamine deficiency in humans. WK develops in a subset of chronic alcoholics, who are vitamin deficient because so many calories are consumed as alcohol instead of normal diet, and a diet rich in carbohydrates increases the metabolic demand for thiamine. Thiamine dependent enzymes were diminished in the brains of patients who died with WKS, but not in alcoholic controls (Butterworth et al., 1993). Transketolase in fibroblasts from those patients who develop WKS syndrome binds TPP more avidly than the control lines. The Km was nearly ten times higher in patients with WKS. Thus, these patients have an abnormahty of transketolase that would be clinically unimportant if the diet was adequate (Blass and Gibson, 1977, 1979). The latter demonstrate a predisposing biochemical mutation to a neurological diseases that is only revealed by inadequate diet. 

Mechanism of thiamine pyrophosphate action. Intermediate (a) is represented as a resonance-stabilized species. It arises from the decarboxylation of the pyruvate-thiamine pyrophosphate addition compound shown at the left of (a) and in equation (2). It can react as a carbanion with acetaldehyde, pyruvate, or H+ to form (b), (c), or (d), depending on the specificity of the enzyme. It can also be oxidized to acetyl-thiamine pyrophosphate (TPP) (e) by other enzymes, such as pyruvate oxidase. The intermediates (b) through (e) are further transformed to the products shown by the actions of specific enzymes. 

Most early clinical descriptions of apparent thiamine-responsive PDC deficiency were not characterized biochemically to ascertain true thiamine dependence. In subsequent reports, immunochemical analyses have demonstrated varied patterns of a- and P-subunit expression, and in vitro studies of cultured cells have sometimes found altered El enzyme kinetics (high Km, low Vmax) for TPP. When molecular genetic analyses have been undertaken, different mutations have been identified within the conserved TPP-binding motif that are considered to lead to diminished binding affinity for TPP or to decreased stability of the oc2P2 tetramer. 

These two 5-C sugars, R-5-P and Xu-5-P, are now interconverted to a 7-C sugar, Sedoheptulose-7-P, and a 3-C sugar, Glyceraldehyde-3-P. This reaction is catalyzed by Transketolase, a Thiamine pyrophosphate dependent enz)rme which catalyzes the transfer of C2 units. In the first part of this reaction the TPP carbanion makes a nucleophilic attack on the carbonyl group of xylulose. In the resulting intermediate the C2-C3 bond is destabilized and cleavage takes place to yield the enzyme bound 2-(l,2-dihydroxyethyl)-TPP resonance stabilized carbanion ... 

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