Hydroxyethyl thiamin diphosphate

ADH CD 3D HEThDP Mes nh PAC PDC PDCS.c. PDCS.u. PDCZ.w. So.5 ThDP v/S v max wt alcohol dehydrogenase circular dichroism three-dimensional 2-(hydroxyethyl)thiamine diphosphate 4-morpholineethanesulfonsaure Hill-coefficient phenylacetyl carbinol pyruvate decarboxylase PDC from Saccharomyces cerevisiae PDC from Saccharomyces uvarum PDC from Zymomonas mobilis substrate concentration necessary for half-maximal velocity thiamine diphosphate velocity vs substrate concentration maximal velocity wild-type... 

ScHEME 35. Lactylthiamin diphosphate is converted to (hydroxyethyl)thiamin diphosphate. 

The biosynthetic pathway of tetramethylpyrazine requires two pyruvate units, one of which is transferred to the thiamine diphosphate (TPP) cofactor under decarboxylation to give 2-(l-hydroxyethyl)thiamine diphosphate 55. The latter adds an acetyl group to the second pyruvate unit by acetolactate synthase (AS) to give (5)-2-acetolactate 56 (Figure 6.70). Subsequent decarboxylation converts (5)-2-acetolactate 56 to acetoin 57. Oxidation of the latter catalyzed by acetoin dehydrogenase (AD) forms butanedione 59. Transamination of butanedione 59 generates 3-aminobutanone 60. Alternatively, a transamination reaction of acetoin 57 proceeds... 

Figure 5.4 Structural formulae of thiamin phosphate esters. At present, five natural thiamin phosphate derivatives have been described thiamin monophosphate (ThMP) thiamin diphopshate (ThDP) thiamin triphosphate (ThTP) adenosine thiamin diphosphate (AThDP) and adenosine thiamin triphosphate (AThTP). Catalytic intermediates, such as for instance a-hydroxyethyl thiamine diphosphate formed by the action of yeast pyruvate decarboxylase (EC 4.1.1.1), are not considered here.

Thiamine is present in cells as the free form 1, as the diphosphate 2, and as the diphosphate of the hydroxyethyl derivative 3 (Scheme 1) in variable ratio. The component heterocyclic moieties, 4-amino-5-hydroxymethyl-2-methylpyrimidine (4) and 4-methyl-5-(2-hydroxyethyl)thiazole (5) are also presented in Scheme 1, with the atom numbering. This numbering follows the rules of nomenclature of heterocyclic compounds for the ring atoms, and is arbitrary for the substituents. To avoid the use of acronyms, compound 5 is termed as the thiazole of thiamine or more simply the thiazole. This does not raise any ambiguity because unsubstituted thiazole is encountered in this chapter. Other thiazoles are named after the rules of heterocyclic nomenclature. Pyrimidine 4 is called pyramine, a well established name in the field. A detailed account of the present status of knowledge on the biosynthesis of thiamine diphosphate from its heterocyclic moieties can be found in a review by the authors.1 This report provides only the minimal information necessary for understanding the main part of this chapter (Scheme 2). 

Initially, pyruvate dehydrogenase [El] catalyzes the decarboxylation of pyruvate and the transfer of the resulting hydroxyethyl residue to thiamine diphosphate (TPP, la). The same enzyme then catalyzes oxidation of the TPP-bound hydroxyethyl group to yield an acetyl residue. This residue and the reducing equivalents obtained are then transferred to lipoamide (1b). 

R. L. Baxter, and L. Sawyer. Biotin synthesis requires three other enzymes (steps b, c, d). Step b is catalyzed by a PLP-dependent transaminase. At the left is thiamin diphosphate, in the form of its 2-(1 -hydroxyethyl) derivative, an intermediate in the enzyme pyruvate decarboxylase (Dobritzsch et al.,. Biol. Chem. 273,20196-20204,1998). Courtesy of Guoguang Lu. Thiamin diphosphate functions in all living organisms to cleave C-C bonds adjacent to C=O groups. 

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... 

Tittmann, K., Wille, G., Golbik, R., Weidner, a., Ghisia, S., Hubner, G. (2005b), Radical phosphate transfer mechanism for the thiamin diphosphate-and FAD-dependent pyruvate oxidase from Lactobacillus plantarum. Kinetic coupling of intercofactor electron transfer with phosphate transfer to acetyl-thiamin diphosphate via transient FAD semiquinone/hydroxyethyl-ThDP radical pair. Biochemistry 44, 13291-13303. 

The mechanism suggested by Kerscher and Oesterhelt is indicated in Scheme 46 for the enzyme from H. halobium (213). The initial step is identical to that of the 2-oxoacid dehydrogenase complexes and involves binding of pyruvate to thiamin diphosphate and subsequent decarboxylation yielding hydroxyethylthia-min diphosphate. This intermediate undergoes one-electron transfer to the [4Fe-4S] cluster to form the stable free radical. The cluster is then reoxidized by ferredoxin or oxygen to give the enzyme-intermediate complex. Reaction with CoA initiates the second electron transfer to the iron-sulfur cluster, acyl transfer, followed by reoxidation of the enzyme by ferredoxin or O2 to complete the cycle. Two basic questions are yet unanswered (1) What is the mechanism of the enzymic reaction between CoASH and hydroxyethyl-TPP in the absence... 

Thiamine is relatively stable in acidic solutions (pH < 5). Thiamine diphosphate is unstable in weakly acidic and neutral solutions, and its hydrolysis yields thiamine monophosphate and thiamine. In neutral and alkaline solutions, thiamine exists as the fi ee base, which is very unstable. It is hydrolysed to 4-amino-5-hydroxymethyl-2-methylpyrimidine and 5-(2-hydroxyethyl)-... 

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). 

Vitamin Bi (also called thiamine) is a water-soluble vitamin of B complex present in many foods as a natural nutrient. It is a biologically and pharmaceutically important compound containing a pyrimidine and a thiazole moiety such as 4-amino-5-hydroxymethyl-2-methyl-pyrimidine and 5-(2-hydroxyethyl)-4-methylthiazole linked by a methylene bridge (Figure 15.1). It is necessary for carbohydrate metabolism and for the maintenance of neural activity because most of the humans and mammals cannot synthesize vitamin Bi. Nerve cells need vitamin BI for their normal function because vitamin Bi has diphosphate-active sites which serve as a cofactor for several enzymes (Leopold et al. 2005). Vitamin Bi is employed for the prevention and treatment of beriberi, neuralgia, etc. and played a vital role in enzymatic mitochondrial... 

Thiamine (vitamin Bj, also formerly known as aneurine) contains a pyrimidine ring (4-amino-2-methylpyrimidine) attached by the methylene group at C-5 to the nitrogen of 5-(2-hydroxyethyl)-4-methylthiazole. Thiamine (5-51) occurs primarily as a free compound and in the form of phosphate esters (5-52), the monophosphate, diphosphate (pyrophosphate called cocarboxylase) and triphosphate. 

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