Pyridoxine Vitamin B6

Vitamin B6 was discovered in the 1930s and named pyridoxine because of its structural resemblance to pyridine (Figure 3). Pyridoxine's principal involvement is with a family of enzymes known collectively as amino transferases. These enzymes exchange amine groups from amino acids to a-keto acids. Familiar names include serum glutamate-oxalate transaminase (SGOT). The coenzyme, pyridoxal-5'

Pyridoxine Pyridoxal Pyridoxal 5'-phosphate Pyridoxamine 5'-phosphate

Figure 3 Pyridoxine (vitamin B6) and its coenzymes. The coenzyme form is capable of receiving (pyridoxal) and donating (pyridoxamine) NH3 groups.

Pyridoxine Pyridoxal Pyridoxal 5'-phosphate Pyridoxamine 5'-phosphate

Figure 3 Pyridoxine (vitamin B6) and its coenzymes. The coenzyme form is capable of receiving (pyridoxal) and donating (pyridoxamine) NH3 groups.

phosphate (PLP), is the predominant form and is synthesized in a two-step reaction involving the oxidation of the hydroxymethyl group in the para position on the pyridine ring to an aldehyde, and the phosphorylation of the hydroxymethyl group on the 5 position. PLP is also a coenzyme for glycogen phosphorylase and ornithine decarboxylase. With tetrahydrofolate (see below), PLP takes part in serine to glycine interconversion. On the enzyme surface, the reactive species is not an aldehyde, but rather an aldamine formed by a Schiff base bond between the aldehyde and an e-amino group of lysine in the active site.

Reactivity The reactivity of PLP is due to a number of features. First, the carbonyl (aldehyde) on the ring is positioned to engage the amino groups from amino acids and tether the amino acid to the enzyme. Through a series of electron rearrangements promoted by the PLP, the nitrogen on the amino acid substrate is disengaged and the carbon skeleton (an a-keto acid) is set free retaining the amine group on the coenzyme (pyridoxamine-5' phosphate). The enzyme then binds a second a-keto acid and transfers the amino group generating a new amino acid and restoring the carbonyl function on the PLP. Other changes can also occur to a tethered structure. An electron rearrangement can result in the loss of a carboxyl group (as CO2) or a molecule of H2O. Thus, PLP enzymes also take part in decarboxyla-tion and dehydration reactions. In the glycogen phos-phorylase reaction, the phosphate group of the coenzyme acts as a general catalyst, promoting the attack of phosphate on the glycosidic bond of glycogen.

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