Pyridoxal analogs

The ultraviolet spectrum of vitamin Be, or pyridoxine, measured in aqueous ethanol varies with the composition of the solvent indicating that this compound is in equilibrium with the zwitterion form 38. The equilibrium constant in pure water was obtained by extrapolation. Prior to this, equilibria which involved tautomers of type 39 had been suggested for vitamin Be, but see Section VI,A. In the case of pyridoxal, an additional equilibrium, 40 41, occurs (cf. Section VIII) other pyridoxal analogs have also been studied (Table II). 

Kuzuhara et al. synthesized an optically resolved pyridoxal analog having an ansa chain" between the 2 - and 5 -positions (45) [46]. The aldolase-type reaction of 45 and glycine with either acetaldehyde or propionaldehyde afforded the corresponding P-hy-droxy-a-amino acid with 27-77% ee. The erythro isomers were 1.2-1.8 times dominant over threo ones. The (S) -enantiomer of the pyridoxal derivative furnished the (S)-amino acid in excess. Accordingly, the reaction occurred on the same face as was occupied by the ansa chain. We have confirmed these results [47]. 

Further studies on pyridoxal analogs showed XXVIIil45 and XXIX146 to have activity against E. acervulina. A series of pyran-3(4H)-ones, exemplified by XXX, were active against E. acervulina and E. tenella.147... 

Pyridoxal phosphate mainly serves as coenzyme in the amino acid metabolism and is covalently bound to its enzyme via a Schiff base. In the enzymatic reaction, the amino group of the substrate and the aldehyde group of PLP form a Schiff base, too. The subsequent reactions can take place at the a-, (3-, or y-carbon of the respective substrate. Common types of reactions are decarboxylations (formation of biogenic amines), transaminations (transfer of the amino nitrogen of one amino acid to the keto analog of another amino acid), and eliminations. 

The deamination of primary amines such as phenylethylamine by Escherichia coli (Cooper et al. 1992) and Klebsiella oxytoca (Flacisalihoglu et al. 1997) is carried out by an oxidase. This contains copper and topaquinone (TPQ), which is produced from tyrosine by dioxygenation. TPQ is reduced to an aminoquinol that in the form of a Cu(l) radical reacts with O2 to form H2O2, Cu(ll), and the imine. The mechanism has been elucidated (Wihnot et al. 1999), and involves formation of a Schiff base followed by hydrolysis in reactions that are formally analogous to those involved in pyridoxal-mediated transamination. 

Work in the Imperiali laboratory has also focused on exploring the ability of minimal peptide scaffolds to augment the rate of coenzyme-mediated transaminations [22-25]. To accomplish this, a strategy has been developed in which the core functionality of the coenzyme is incorporated as an integral constituent of an unnatural coenzyme amino acid chimera construct. Thus, non-cova-lent binding of the coenzyme to the peptide or protein scaffold is unnecessary. Both the pyridoxal and pyridoxamine analogs have been synthesized in a form competent for Fmoc-based solid phase peptide synthesis (SPPS) (Fig. 7) [23,24]. 

The pyridoxal amino acid analog (Pal) was stereoselectively synthesized from a readily available pyridoxol derivative and the residue was incorporated into peptides at the alcohol oxidation state in protected form. Oxidation of the 4 -alcohol group to the desired aldehyde was achieved post-synthetically on free. 

This enzyme [EC 4.1.99.1], also known as L-tryptophan indole-lyase, catalyzes the hydrolysis of L-tryptophan to generate indole, pyruvate, and ammonia. The reaction requires pyridoxal phosphate and potassium ions. The enzyme can also catalyze the synthesis of tryptophan from indole and serine as well as catalyze 2,3-elimination and j8-replacement reactions of some indole-substituted tryptophan analogs of L-cysteine, L-serine, and other 3-substituted amino acids. 

Affinity Labeling with UDP-pyridoxal. Read and Delmer (21) utilized the substrate analog UDP-pyridoxal to inhibit mung bean glucan synthase. This affinity label inhibited glucan synthase at micromolar levels and inhibition was protected against with UDP-glucose. A 42 kD polypeptide could be labelled with [3H]UDP-pyridoxal. 

Several studies published since March 1996 have expanded the list of in vitro integrase inhibitors effective at IC50 values below 100 pM. These include two dicaffeoylquinic acids obtained from medicinal plants and a synthetic analog, L-chicoric acid [68], the HIV protease inhibitors NSC 117027 and NSC 158393 [69], certain anthraquinone derivatives [70], coumermycin, and pyridoxal phosphate [71]. In addition to exhibiting in vitro inhibition, the dicaffeoylquinic acids effectively inhibited HIV-1 replication in T-lymphoblastoid cell lines [68]. 

It is well over 40 years since Pfeiffer discovered that certain reactions of a-amino acid esters, in particular, ester exchange, racemization and oxygenation, are effected very readily when their Schiff bases with salicylaldehyde are complexed to a transition metal ion (most notably Cu11). The Schiff bases result from a condensation reaction between a reactive carbonyl group and the amino group of the amino acids. Snell and his co-workers43 were also one of the first to point out that similar reactions also occurred if pyridoxal was used instead of salicylaldehyde, and that there is a close analogy with pyridoxal phosphate-promoted enzymic reactions of a-amino acid metabolism. Since then much work has been due on these and other similar systems and their reactivities. 

The /3 decarboxylation of aspartate (equation 7) proceeds by elimination of a /3-carbanionic intermediate like that in figure 10.4c/ from the ketimine, analogous to the intermediate produced by loss of the a proton from the aldimine of aspartate with pyridoxal-5 -phosphate. 

A subclass of lyases, involved in /J-hydroxy-a-amino acid metabolism, utilizes pyridoxal 5 -phosphate (PLP) as a prosthetic group for imine/enamine type activation. These enzymes are not only of interest for the synthesis of the naturally occurring prototypes L-serine (Sect. 6.8) or L-threonine, but also offer a potential entry to rare or non-natural analogs. 

Asymmetric transamination.2 This planar chiral pyridoxamine analog in the presence of Zn(C104), (l/Zn(C104)2 = 1.0.5) converts a-keto acids into (R)-amino tieids in 60 96%ee. Use of (R)-l in place of (S)-l produces (S)-amino acids with the wime elliciency. Chemical yields range from 50 75%. The preferred solvent is tnel li.mol. The pyridoxal-type analog is recovered in 75-85%yield. The transamination is considered to involve kinetically controlled stereoselective protonation of an octahedral Ztr 1 chelate intermediate. 

View of an aj /3 pair. The a subunit (blue) is shown with the eight strands of the central barrel (white) surrounding the bound substrate analog (indole propanol phosphate) at the active center. Residues 57-59 ( disordered loop ) and 177-191 ( Flexible loop ) are shown in darker blue. The /3 subunit N-domain (yellow) and C-domain (orange) are shown surrounding the coenzyme pyridoxal phosphate at the active site. The central /3-sheet strands of each domain are also shown in white. Note the region of the C-domain which does not have a well-defined secondary structure and which is in contact with the a subunit. 

Transamination. Transamination with pyridoxal or analogs requires a metal catalyst such as Cu(II). However, transamination can be effected with DPL in combination with hexadecyltrimethylammonium chloride. By using these two reagents phenylglycine undergoes transamination with 2-oxoglutaric acid (equation I). 

Pyridoxal-5-phosphate (PLP), an analog of 3PGA or phosphorylated sugars that can be covalently bound to the enzyme by reduction with NaBH ... 

Hayon and co-workers have studied, by pulse radiolysis, 1-hydropyridinyl radicals ranging from those formed from simple C-acylpyridines to those from pyridoxine and pyridoxal phosphate.210-212 Structural, spectroscopic, kinetic, and thermodynamic data have been presented. Various mechanisms have been adduced which involve 35 and analogous species from quinolines,... 

Studies on the reactivation of apoglycogen phosphorylase with a variety of analogs of pyridoxal phosphate have shown that the catalytic moiety is the 5 -phosphate group - only analogs with a reversibly protonatable dianion in this position have any activity In the nonactivated form of phosphorylase b, the phosphate is monoprotonated (-OPO3H ) when the enzyme has been activated, either allosterically or by phosphorylation (phosphorylase a), it is dianionic (-OPOa ). A glutamate residue in the active site acts as the proton acceptor or donor for this transition between the inactive and active forms of the cofactor. 

Pyridoxal-5-phosphate (PLP) could be considered to have some structural analogy to 3-PGA, and it was found to activate both the enzymes from spinach leaf and Anabaena. In spinach ADP-Glc PPase, PLP bound at Lys440, which is very close to the C-terminal of the small subunit, and bound to three Lys residues in the large subunit. Binding to these sites was prevented by the allosteric effector 3-PGA, which indicated that they are close to or are involved directly in the binding of this activator (100, 101). 

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