Oh

H3C N Pyridoxic acid

Pyridoxamine 5'-phosphate (PMP)

EC 1.4.3.5 pyridoxal 5'-phosphate synthase EC 2.7.1.35 pyridoxal kinase EC 3.1.3.74 pyridoxal phosphatase EC 1.2.3.1 aldehyde oxidase a cotransaminase in the transamination of alanine to form pyruvic acid and as a codecarboxylase in the decarboxylation of dihydroxyphenylalanine (DOPA) to form dopamine. Other biological transformations of amino acids in which pyridoxal can function are racemization, elimination of the «-hydrogen together with a ¡-sub-stituent (i.e., OH or SH) or a y-substituent, and probably the reversible cleavage of ¡-hydroxyamino acids to glycine and carbonyl compounds.

An electromeric displacement of electrons from bonds with the «-carbon (see Fig. 28.36) results in the release of a cation (H, R', or COOH) and, subsequently, leads to various reactions observed with pyridoxal. The extent to which one of these displacements predominates over others depends on the structure of the amino acid and the environment (pH, solvent, catalysts, enzymes, etc.). When this mechanism applies in vivo, the pyridoxal component is linked to the enzyme through the phosphate of the hydroxymethyl group by hydrogen bond networks and/or ionic interactions.

Figure 28.37 shows the role of PLP in the transamination reaction. The first half-reaction involves the initial proton abstraction from the external aldimine at C« by the active site lysine, yielding a quinonoid intermediate. Lysin ammonium, thus formed, protonates the C4 of the quinonoid intermediate to generate a ketimine, which is subsequently hydrolyzed to release «-ketogluterate and enzyme-PMP complex (E-PMP).

Diacylglycine decarboxylase (DGD) is an unusual PLP-dependent enzyme that performs both decarboxylation, in the first half-reaction, and transamination, in the second half-reaction (Fig. 28.38). In the first half-reaction, a dialkylamino acid is decarboxylated producing a ketone and releasing the E-PMP form, which in the second half-reaction catalyzes transamination to convert an «-keto acid (e.g., pyruvic acid) into an amino acid (e.g., alanine), regenerating the DGD-PLP form.

h3c n

Figure 28.35 • Pyridoxine-S'-jS-D-glucoside.

Figure 28.36 • Generalized vitamin B6 reactions.

Figure 28.37 • Transamination reactions of pyridoxine.

Internal aldimine (E-PLP)

Figure 28.37 • Transamination reactions of pyridoxine.

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