Pyridoxai

Figure 31-5. Conversion oftyrosine to epinephrine and norepinephrine in neuronai and adrenai ceiis. (PLP, pyridoxai phosphate.)...

Neural cells convert tyrosine to epinephrine and norepinephrine (Figure 31—5). While dopa is also an intermediate in the formation of melanin, different enzymes hydroxylate tyrosine in melanocytes. Dopa decarboxylase, a pyridoxai phosphate-dependent enzyme, forms dopamine. Subsequent hydroxylation by dopamine P-oxidase then forms norepinephrine. In the adrenal medulla, phenylethanolamine-A -methyltransferase uti-hzes S-adenosyhnethionine to methylate the primary amine of norepinephrine, forming epinephrine (Figure 31-5). Tyrosine is also a precursor of triiodothyronine and thyroxine (Chapter 42). 

Scheme 2 Reaction mechanism of the conversion of SAM to ACC catalyzed by ACS. ACS-Lys denotes active site lysine of ACS, PLP denotes pyridoxai 5 -phosphate, and Opi-Op4 denotes oxygens of the pyridoxal 5 -phosphate moiety.

Vitamin Be is again a small family of related compounds having the same biological activity. These include pyridoxine, pyridoxai, and pyridoxamine. In humans, these molecules are readily interconverted, accounting for their equivalence as vitamins. The stuff in your vitamin pill is likely to be pyridoxine. The actual molecule that functions as a coenzyme in metabolism is pyridoxai phosphate, in which a phosphate group has been added to pyridoxai in an ATP-dependent reaction. 

The active form of vitamin Be, pyridoxai phosphate, is the most important coenzyme in the amino acid metabolism (see p. 106). Almost all conversion reactions involving amino acids require pyridoxal phosphate, including transaminations, decarboxylations, dehydrogenations, etc. Glycogen phosphory-lase, the enzyme for glycogen degradation, also contains pyridoxal phosphate as a cofactor. Vitamin Be deficiency is rare. 

S.G. Withers, i.P. Street, M.D. Percivai, Fiuorinated carbohydrates as probes of enzyme specificity and mechanism, ACS Symposium Series 374 (1988) 59-77. W.G. Stirtan, S.G. Withers, Phosphonate and aipha-fluorophosphonate analogue probes of the ionization state of pyridoxai 5 -phosphate (PLP) in glycogen phosphor-ylase. Biochemistry 35 (1996) 15057-15064. 

Hayashi H, Wada H, Yoshimura T, Esaki N, and Soda K (1990) Recent topics in pyridoxai 5 -phosphate enzyme studies. Annual Reviews of Biochemistry 59, 87-110. 

John RA (1995) Pyridoxai phosphate-dependent enzymes. Biochimica et Biophysica Acta 1248, 81-96. 

Marteii AE (1982) Reaction pathways and mechanisms of pyridoxai cataiysis. Advances in Enzymology and Related Areas of Molecular Biology 53,163-99. 

Schiff base between amino acid and pyridoxai phosphate... 

Snell, E. E, (1970), Analogues of pyrldoxal or pyridoxai phosphate Relation of structure to binding with apocnzymcs and to catalytic activity. W(, f JerhF . 28, 265-Z9D. 

Vinylglycine is a natural amino acid found in mushrooms. It is an inhibitor of pyridoxai-linked aspartate aminotransferase, and has also been postulated as an intermediate in the enzymatic conversion of homoserine to threonine and o-ketobutyrate. Protected vinylglycine is also a versatile asymmetric starting material for synthesis.7 Variants have been prepared in racemic. " optically active, optically pure,2.15-17 and isotopically labeled form.4b.i8-20 xhis procedure is derived from our earlier publication and contains improvements in procedure and scale-up. 

There are several enzymes that form a Schiff base between their substrates and either a lysine s-amino group from the protein, the aldehyde electrophile of pyridoxaI-5-phosphate or the keto group of a covalently bound pyruvyl moiety. The electron sink under all these conditions is either the imine or its protonated iminium form, that in many cases leads to the formation of an enamine by the variety of pathways outlined below. 

Reagent layer buffer at pH 8.0 lactate dehydrogenase NADH pyridoxai-S-phosphale ... 

Figure 5 Schematic representation of the reactions cataiyzed by the glycine cleavage system. The P-protein (acetoin dehydrogenase) binds pyridoxai 5 -phosphate (PLP) the T-protein (dihydrolipoyl transacetylase) contains lipoamide the L-protein (dihydroiipoyi dehydrogenase) binds fiavin adenine dinucleotide (FAD).

R. E. Hill I. D. Spenser, The Biosynthesis of Vitamin Bg. In Pyridoxai Phosphate Chemicai, Biochemicai and Medicai Aspeds] D. Dolphin, R. Poulson, 0. Avramovic, Eds. John Wiley Sons, Inc. New York, 1986 pp 417-476. 

Figure 28 The structure and mechanism of threonine phosphate decarboxyiase, CobD. (a) This enzyme of the anaerobic pathway identifies the active site by the presence of its pyridoxai phosphate cofactor (stick representation), which is attached to an active site iysine. (b) The mechanism of the enzyme invoives the binding of the threonine phosphate to the pyridoxai phosphate cofactor via a Schiff base. Foiiowing decarboxyiation, the product is released.

S/. no. Organism Growth after 4 days Pyridoxine Pyridoxamine Pyridoxai... 

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