Amino acid pyridoxal phosphate catalysis

The biologically active form of vitamin Bg is pyridoxal-5-phosphate (PEP), a coenzyme that exists under physiological conditions in two tautomeric forms (Figure 18.25). PLP participates in the catalysis of a wide variety of reactions involving amino acids, including transaminations, a- and /3-decarboxylations, /3- and ") eliminations, racemizations, and aldol reactions (Figure 18.26). Note that these reactions include cleavage of any of the bonds to the amino acid alpha carbon, as well as several bonds in the side chain. The remarkably versatile chemistry of PLP is due to its ability to... 

The addition of cofactors to antibodies is a sure means to confer a catalytic activity to them insofar as this cofactor is responsible for the activity. Indeed for many enzymes, the interaction with cofactors such as thiamins, flavins, pyridoxal phosphate, and ions or metal complexes is absolutely essential for the catalysis. It is thus a question there of building a new biocatalyst with two partners the cofactor responsible for the catalytic activity, and the antibody which binds not only the cofactor but also the substrate that it positions in a specific way one with respect to the other, and can possibly take part in the catalysis thanks to some of its amino acids. 

Unlike other pyridoxal phosphate-dependent enzymes, in which it is the carbonyl group that is essential for catalysis, the internal Schiff base between pyridoxal phosphate and lysine in glycogen phosphorylase can be reduced with sodium borohydride without affecting catalytic activity. Thus, while pyridoxal phosphate is essential for phosphorylase activity, it does not act by the same kind of mechanism as in amino acid metabolism. 

Fig. 8.13 Reactive sites of pyridoxal phosphate. The functional group of pyridoxal phosphate is a reactive aldehyde (shown in blue) that forms a covalent intermediate with amino groups of amino acids (a Schiff base). The positively charged pyridine ring is a strong electron-withdrawing group that can pull electrons into it (electrophilic catalysis).

Like homogeneous catalysis, the removal of a-hydrogen of the amino acid fragment by OH ions, the local concentration of which is apparently high in the polymer phase, is probably the rate-determining step of heterogeneous racemization. Under similar conditions, the rate of a-amino acid racemization decreases in the sequence Ala = Ser>Phe>Nva>Lys>Val, and correlates with the rate of substrate racemization in the presence of Schiff bases and transamination of amino acids by pyridoxal phosphate. 

Schiff base formation between pyridoxal phosphate and amino acids are the basis for most enzymatic transformations of amino acids including transamination, decarboxylation, and racemization. Schiff bases formed between amino acids and pyridoxal phosphate or other heteroaromatic or aromatic aldehydes are, however, not only transformed enzymatically, but can, without enzymatic catalysis, undergo a large number of reactions, although at lower rate and/or higher temperatures than those for the corresponding enzymatic reactions. The enzymatic reactions require metal ions as cofactors and in analogy the nonenzymatic reaction are also catalyzed by metal ions, most effectively by cupric ions. 

Much of the investigation of the mechanism and catalysis of C=N— forming condensation reactions has been designed to gain insight into the catalytic activity of enzymes which require pyridoxal phosphate (37), for their activity and are involved in the metabolism of amino acids. 

The metabolically active vitamer is pyridoxal phosphate, which is involved in many reactions of amino acid metabolism, where the carbonyl group is the reactive moiety, in glycogen phosphorylase, where it is the phosphate group that is important in catalysis, and in the release of hormone receptors from tight nuclear binding, where again it is the carbonyl group that is important. 

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