Pyridoxai phosphate

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). 

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. 

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

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. 

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.

This is a pyridoxai phosphate protein which can react with phenylalanine as well as tyrosine. 

Figure 2.17 The removal of ammonia from an amino-acid by pyridoxai phosphate (PLP) and a transaminase enzyme The PLP is held tightly to the enzyme by a lysine and ionic bonding of the phosphate group. B indicates some general base...

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