Transport thiamin diphosphate

Pyruvate oxidase. The soluble flavoprotein pyruvate oxidase, which was discussed briefly in Chapter 14 (Fig. 14-2, Eq. 14-22), acts together with a membrane-bound electron transport system to convert pyruvate to acetyl phosphate and C02.319 Thiamin diphosphate is needed by this enzyme but lipoic acid is not. The flavin probably dehydrogenates the thiamin-bound intermediate to 2-acetylthiamin as shown in Eq. 15-34. The electron acceptor is the bound FAD and the reaction may occur in two steps as shown with a thiamin diphosphate radical intermediate.3193 Reaction with inorganic phosphate generates the energy storage metabolite acetyl phosphate. 

Thiamine is absorbed in the intestine by both active transport mechanisms and passive diffusion. The active form of the cofactor, thiamine pyrophosphate (thiamine diphosphate, TPP), is synthesized by an enzymatic transfer of a pyrophosphate group from ATP to thiamine (Figure 15-1). The resulting TPP has a reactive carbon on the thiazole ring that is easily ionized to form a carbanion, which can undergo nucleophilic addition reactions. 

Thiamin-related diseases are the result of either insulficient thiamin intake (thiamin deficiency), poisoning by antithiamins, or of mutations in thiamin transporters or thiamin diphosphate-dependent enzymes. 

Pyruvate dehydrogenase of propionibacteria differs from that of aerobic bacteria in that it does not depend on lipoate and has a similarity with pyruvate cytochrome b oxidoreductase. Castberg and Morris (1978) reported the isolation from cells of P. shermanii of pyruvate oxidase (reducing 2,6-dichlorophenolindophenol (DCIP)) and pyruvate dehydrogenase system (reduces NAD). The pyruvate oxidase could not use NAD as an electron acceptor, and the NAD-dependent enzyme did not transport electrons to DCIP. Unlike the pyruvate oxidase of E. coli, the enzyme from P, shermanii was not activated by phosphatydylcholine in the presence of SDS, and the presence of thiamine diphosphate and Mg " was not required for the activity of purified preparations. 

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. 

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