Transketolases thiamin diphosphate coenzyme

Later studies established the coenzyme role of thiamin diphosphate in transketolase in the pentose phosphate pathway. More recent studies have shown that thiamin triphosphate acts to regulate a chloride channel in nerve tissue. 

Konig, S., Schellenberger, A., Neee, H., Schneider, G. (1994), Specificity of coenzyme binding in thiamin diphosphate-dependent enzymes. Crystal structures of yeast transketolase in complex with analogs of thiamin diphosphate, J. Biol. Chem. 269, 10879-10882. 

Thiamine diphosphate is of significance as the coenzyme of x-keto acid decarboxylases (C 4) and transketolases. Thiamine is a vitamin for animals and human beings (vitamin E 2.1). 

Thiamine derivatives Thiamine diphosphate (D 10.4.5) Coenzyme of keto acid decarboxylase (C 4) and transketolases, in humans formed from thiamine (vitamin Bj)... 

Thiamin pyrophosphate (or thiamin diphosphate) is a coenzyme involved in (1) the oxidative decarboxylation of pyruvate to acetyl coenzyme A (enzyme pyruvate dehydrogenase), (2) the oxidative decarboxylation of a-ketoglutarate to succinyl coenzyme A (a-ketoglutarate dehydrogenase) in the tricarboxylic add cycle, (3) the pentose phosphate pathway (transketolase) and (4) the synthesis of branched-chain amino acids such as valine (branched-chain ketoacid dehydrogenase) in bacteria, yeasts and plants. 

Protein was determined by the method of Lowry, using a calibration curve which had been obtained with a TK solution standardized by the Biuret method. TPP was determined by radioactivity measurements. The transketolase assay was carried out in the presence and absence of added thiamine diphosphate/Mg++. A stimulation of 13 % of enzymatic activity was found when the coenzyme was added, indicating a loss of bound coenzyme upon gel filtration. Assuming a linear relationship between enzymatic activity and thiamine diphosphate molecules bound to apotransketolase the value found for thiamine diphosphate was corrected by these 13 %. 

Thiamin diphosphate is also the coenzyme for transketolase, in the pentose phosphate pathway of carbohydrate metabolism (section 5.4.2). 

The enzyme is a transketolase and the process is called transketolation (keto-carrying). Transketolase uses thiamin diphosphate as a coenzyme to bring about the transfer of a 2-carbon unit from one sugar (a 2-keto sugar) to aldoses (monosaccharides with the characteristic aldehyde group [-CHO]). 

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