'N CH3 pyridoxamine phosphate nuclear binding, where again it is the carbonyl group that is important.

The Role of Pyridoxal Phosphate in Amino Acid Metabolism

The various reactions of pyridoxal phosphate in amino acid metabolism (Figure 2) all depend on the same chemical principle—the ability to stabilize amino acid carbanions, and hence to weaken bonds about the a-carbon of the substrate. This is achieved by reaction of the a-amino group with the carbonyl group of the coenzyme to form a Schiff base (aldimine).

Pyridoxal phosphate is bound to enzymes, in the absence of the substrate, by the formation of an internal Schiff base to the e-amino group of a lysine residue at the active site. Thus the first reaction between the substrate and the coenzyme is transfer of the aldimine linkage from this e-amino group to the a-amino group of the substrate.

The ring nitrogen of pyridoxal phosphate exerts a strong electron-withdrawing effect on the aldimine, and this leads to weakening of all three bonds about the a-carbon of the substrate. In nonenzymic model systems, all the possible pyridoxal-catalyzed reactions are observed: a-decarboxylation, amino-transfer, racemization, and side chain elimination and replacement reactions. By contrast, enzymes show specificity for the reaction pathway followed; which bond is cleaved will depend on the orientation of the Schiff base relative to reactive groups of the catalytic site.

a-Decarboxylation If the electron-withdrawing effect of the ring nitrogen is primarily centered on the a-carbon-carboxyl bond, the result is decarbo-xylation of the amino acid with the release of carbon dioxide. The resultant carbanion is then protonated, and the primary amine corresponding to the amino acid is displaced by the lysine residue at the active site, with reformation of the internal Schiff base.

A number of the products of the decarboxylation of amino acids are important as neurotransmitters and hormones—5-hydroxytryptamine, the catecho-lamines dopamine, noradrenaline, and adrenaline, and histamine and 7-aminobutyrate (GABA)—and as the diamines and polyamines involved in the regulation of DNA metabolism. The decarboxyla-tion of phosphatidylserine to phosphatidylethanola-mine is important in phospholipid metabolism.

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