Pyridoxal enzymes

Over 250 analogues of the B vitamers have been reported (11,100). Nearly all have low vitamin B activity and some show antagonism. Among these are the 4-deshydroxy analogue, pyridoxine 4-ethers, and 4-amino-5-hydroxymeth5i-2-methyipyrimidine, a biosynthetic precursor to thiamine. StmcturaHy unrelated antagonists include dmgs such as isoniazid, cycloserine, and penicillamine, which are known to bind to pyridoxal enzyme active sites (4). 

At the same time, Snell and coworkers used model systems to achieve most of the reactions of the pyridoxal enzymes (Metzler and Snell, 1952a,b Olivard et al., 1952 Ikawa and Snell, 1954a,b Metzler et al 1954a,b Longnecker and Snell, 1957). They too developed the modern mechanisms for the series of reactions and demonstrated the role of the coenzyme as an electron sink by substituting alternative catalysts for pyridoxal phosphate. In particular, they showed that 2-hydroxy-4-nitrobenzaldehyde (Ikawa and Snell, 1954) functioned in their model systems just as did the vitamin its electronic structure is really quite similar (3). 

Arsonoalanine was not a substrate or inhibitor of aspartate aminotransferase under the conditions tested (88), but it inactivated it progressively. Many alanine derivatives in which there is a good leaving group on C-3 do this (95-97), both to this enzyme and other pyridoxal enzymes (96), with some interesting results, e.g., the difference in reac-... 

Hayashi, H. (1995) Pyridoxal enzymes mechanistic diversity and uniformity. J. Biochem. 118, 463-473. 

NH2 (lysine) Acetoacetate decarboxylase, aldolase, transaldolase, pyridoxal enzymes Schiff base... 

The alanine racemization catalyzed by alanine racemase is considered to be initiated by the transaldimination (Fig. 8.5).26) In this step, PLP bound to the active-site lysine residue forms the external Schiff base with a substrate alanine (Fig. 8.5, 1). The following a-proton abstraction produces the resonance-stabilized carbanion intermediates (Fig. 8.5, 2). If the reprotonation occurs on the opposite face of the substrate-PLP complex on which the proton-abstraction proceeds, the antipodal aldimine is formed (Fig. 8.5,3). The subsequent hydrolysis of the aldimine complex gives the isomerized alanine and PLP-form racemase. The random return of hydrogen to the carbanion intermediate is the distinguishing feature that differentiates racemization from reactions catalyzed by other pyridoxal enzymes such as transaminases. Transaminases catalyze the transfer of amino group between amino acid and keto acid, and the reaction is initiated by the transaldimination, followed by the a-proton abstraction from the substrate-PLP aldimine to form a resonance-stabilized carbanion. This step is common to racemases and transaminases. However, in the transamination the abstracted proton is then tranferred to C4 carbon of PLP in a highly stereospecific manner The re-protonation occurs on the same face of the PLP-substrate aldimine on which the a-proton is abstracted. With only a few exceptions,27,28) each step of pyridoxal enzymes-catalyzed reaction proceeds on only one side of the planar PLP-substrate complex. However, in the amino acid racemase... 

Katunuma N, Kominami E, and Kominami S (1971) A new enzyme that specifically inactivates apo protein of pyridoxal enzymes. Biochemical and Biophysical Research Communications AS, 70-5. 

The UV/visible spectroscopic probe consists of protein residues (e.g. Trp, Tyr), cofactors/coenzyes (e.g. pyridoxal and flavin coenzymes) and substrates or their analogs as target chromophores. For example, pyridoxal-5 -phosphate (PLP) of pyridoxal enzymes undergo different stages of transformations that can be monitored spectroscopically (Metzler, 1979) as shown in Figure 11.7. 

Figure 11.7 Spectral changes accompanying pyridoxal enzyme reaction...

Transferase, isomerase and lyase pyridoxal enzymes. Pyridoxal-5 -phosphate (PLP) is an obligate coenzyme for the great majority of enzymes catalyzing... 

All pyridoxal enzymes catalyzing chanical transformations of a-amino acids function via ... 

Figure 11.13 Reactions at a-carbon of a-amino acids catalyzed by pyridoxal enzymes All three substituents at C are subject to labilization in the three types of a-carbon reactions. The hydrogen is labilized in recemization reactions, the amino group is labUized in the transamination and the carboxyl group is labilized in decarboxylation. a-Amino acid condenses with pyridoxal phosphate to yield pyridoxylidene imino acid (an aldimine). The common intermediate, aldimine and distinct ketimines leading to the production of oxo-acid (in transamination), amino acid (in racemization) and amine (in decarboxylation) are shown. The catalytic acid (H-A-) and base (-B ) are symbolic both can be from the same residue such as Lys258 in aspartate aminotransferase.

Pyridoxal enzymes, 31, 37, see also specific enzymes Pyridoxal phosphate, 90, 163, 432 Pyridoxal-5 -phosphate analogs, 441-447 Pyridoxal phosphate enzymes, 163, 164, 427... 

Chmielewski, J. and Breslow, R., Increasing the racemase activity versus transaminase activity of a pyridoxal enzyme model by the attachment of a rigid base. Heterocycles, 1987,25, 533. 

Liu, L. and Breslow, R., Polymeric and dendrimeric pyridoxal enzyme mimics, Bioorg. Med. Chem., 2004,... 

The following results on models are evidence of the existence of carbanion intermediates in pyridoxal enzymes. For instance, NMR studies by Gansow and Holm (302) in 1969 on pyridoxamine in the presence of Zn(II) or Al(III) ions in H20, pH 1-13 range, showed the condensation of substrate (pyruvate) with a B vitamin (pyridoxamine). The rate of exchange of hydrogen atoms increases with pH. 

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