Serine pyridoxal reaction

Siibstitution. A modification of the jS-elimination reaction may be substitution of a new group. This has been proposed as the mechanism of action of tryptophan desmolase, in which indole is substituted for the OH of serine. This reaction was found to proceed to a measurable extent in model reactions, in spite of the competing 8-elimination reactions of both serine and tryptophan and other side reactions of indole compounds. Additional substitution reactions of biological significance are the formation of cystathionine from homocysteine and serine and the formation of (Sf-methylcysteine from methyl mercaptan and serine. These reactions are catalyzed by enzymes that require pyridoxal phosphate as a cofactor. 

The metabolism of P-hydroxy-a-amino adds involves pyridoxal phosphate-dependent enzymes, dassified as serine hydroxymethyltransferase (SHMT) (EC 2.1.2.1) or threonine aldolases (ThrA L-threonine selective = EC 4.1.2.5, L-aHo-threonine selective = EC 4.1.2.6). Both enzymes catalyze reversible aldol-type deavage reactions yielding glycine (120) and an aldehyde (Eigure 10.45) [192]. 

This pyridoxal-phosphate-dependent enzyme [EC 4.2.1.22] (also known as serine sulfhydrase, /3-thionase, and methylcysteine synthase) catalyzes the reaction of homocysteine with serine to produce cystathionine and water. 

This pyridoxal-phosphate-dependent enzyme [EC 2.6.1.52] catalyzes the reaction of O-phospho-L-serine with a-ketoglutarate (or, 2-oxoglutarate) to produce 3-phosphonooxypyruvate and L-glutamate. 

L-Serine dehydratase [EC 4.2.1.13], also known as serine deaminase and L-hydroxyaminoacid dehydratase, catalyzes the pyridoxal-phosphate-dependent hydrolysis of L-serine to produce pyruvate, ammonia, and water. In a number of organisms, this reaction is also catalyzed by threonine dehydratase. 

This pyridoxal-phosphate-dependent enzyme [EC 2.1.2.1], which has a recommended EC name of glycine hydroxymethyltransferase, catalyzes the reversible reaction of 5,10-methylenetetrahydrofolate with glycine and water to produce tetrahydrofolate and L-serine. The enzyme will also catalyze the reaction of glycine with acetaldehyde to form L-threonine as well as with 4-tri-methylammoniobutanal to form 3-hydioxy-N, N, N -trimethyl-L-lysine. 

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. 

Non-pyridoxal Phosphate Dependent. Figure 2 depicts the postulated mechanism for a non-pyridoxal phosphate catal) zed decarboxylation of histidine to histamine involving a pyruvoyl residue instead of pyridoxal -5 - phosphate (20). Histidine decarboxylases from Lactobacillus 30a and a Micrococcus sp. have been shown to contain a covalently bound pyruvoyl residue on the active site. The pyruvoyl group is covalently bound to the amino group of a phenylalanine residue on the enzyme, and is derived from a serine residue (21) of an inactive proenzyme (22). The pyruvoyl residue acts in a manner similar to pyridoxal phosphate in the decarboxylation reaction. 

Pyridoxal phosphate is the coenzyme for the enzymic processes of transamination, racemization and decarboxylation of amino-acids, and for several other processes, such as the dehydration of serine and the synthesis of tryptophan that involve amino-acids (Braunstein, 1960). Pyridoxal itself is one of the three active forms of vitamin B6 (Rosenberg, 1945), and its biochemistry was established by 1939, in considerable part by the work of A. E. Braunstein and coworkers in Moscow (Braunstein and Kritzmann, 1947a,b,c Konikova et al 1947). Further, the requirement for the coenzyme by many of the enzymes of amino-acid metabolism had been confirmed by 1945. In addition, at that time, E. E. Snell demonstrated a model reaction (1) for transamination between pyridoxal [1] and glutamic acid, work which certainly carried with it the implication of mechanism (Snell, 1945). 

The enzyme serine hydroxymethyltransferase requires pyridoxal phosphate as cofactor. Propose a mechanism for the reaction catalyzed by this enzyme, in the direction of serine degradation (glycine production). (Hint See Figs 18-19 and 18-20b.)... 

The second part of the reaction requires pyridoxal phosphate (Fig. 22-18). Indole formed in the first part is not released by the enzyme, but instead moves through a channel from the a-subunit active site to the jS-subunit active site, where it condenses with a Schiff base intermediate derived from serine and PLP. Intermediate channeling of this type may be a feature of the entire pathway from chorismate to tryptophan. Enzyme active sites catalyzing different steps (sometimes not sequential steps) of the pathway to tryptophan are found on single polypeptides in some species of fungi and bacte-... 

In the biosynthesis of serine from glycine, (25) serves as the methylene donor. The reverse of this reaction is important in the catabolism of serine and provides a major source of the one-carbon units needed in biosynthesis (80MI11003). In addition to tetrahydrofolate, pyridoxal phosphate is required as a coenzyme in this transformation. The topic will be taken up again in the next section. 

The pyridoxal-5 -phosphate dependent serine hydroxymethyltransferase (SHMT EC 2.1.2.1) in vivo catalyzes the interconversion of L-serine 158 and glycine 149 by transfer of the /1-carbon of L-serine to tetrahydrofolate (THF) by which the activated formaldehyde is physiologically made available as a C,-pool. The reaction is fully reversible and provides a means for the stereoselective synthesis of 158 in vitro from donor 149 and formaldehyde. Economical yields (88-94%) of L-serine have thus been obtained on a multimolar scale using raw cell extracts of recombinant Klebsiella aerogenes or E. coll in a controlled bioreactor at final product concentrations > 450 gl 1 [461,462], Several SHMTs have been purified and characterized from various organisms including animal tissues [463,464], eucaryotic [465] and procaryotic... 

Fig. 7.1 Reactions catalyzed at the active sites of the a subunit (a reaction) and of the 0 subunit (0 reaction) and the coupled, physiological reaction (a0 reaction). In the a0 reaction, indole produced by cleavage of indole-3-glycerol phosphate at the a site diffuses through an intramolecular tunnel to the 0 site 25-30 A distant where it undergoes a pyridoxal phosphate-dependent /3-replacement reaction with L-serine to form L-tryptophan. Abbreviations used IGP, indole-3-glyceroI phosphate G-3-P, o-glyceraldehyde 3-phosphate, IND, indole [IND], indole intermediate PLP, pyridoxal phosphate.

The chromophoric pyridoxal phosphate coenzyme provides a useful spectrophotometric probe of catalytic events and of conformational changes that occur at the pyridoxal phosphate site of the P subunit and of the aiPi complex. Tryptophan synthase belongs to a class of pyridoxal phosphate enzymes that catalyze /3-replacement and / -elimination reactions.3 The reactions proceed through a series of pyridoxal phosphate-substrate intermediates (Fig. 7.6) that have characteristic spectral properties. Steady-state and rapid kinetic studies of the P subunit and of the aiPi complex in solution have demonstrated the formation and disappearance of these intermediates.73-90 Fig. 7.7 illustrates the use of rapid-scanning stopped-flow UV-visible spectroscopy to investigate the effects of single amino acid substitutions in the a subunit on the rate of reactions of L-serine at the active site of the P subunit.89 Formation of enzyme-substrate intermediates has also been observed with the 012P2 complex in the crystalline state.91 ... 

There are several types of evidence that the L-serine derivative that activates the a reaction is the Schiff base formed between aminoacrylate and pyridoxal phosphate (ES III in Fig. 7.6). (1) Amino acids including l- or D-tryptophan and glycine that form tetrahedral,... 

Murakami et al. utilized catalytic bilayer membranes to catalyze the (1-replacement reaction of serine with indoles [44], The bilayer vesicle formed with 32 and 36 drastically accelerated the (1-replacement reaction by 51-fold (krel) relative to pyridoxal in homogeneous aqueous solution. They attributed this to the hydrophobic microenvironmental effect provided by the bilayer vesicle, which affords effective incorporation of indole molecules and elimination of water molecules in the reaction site. The imida-zolyl group of 33 enhanced the reaction further, krd being 130, possibly due to general acid-base catalysis by the imidazolyl group. Copper(n) ions also improved the reaction. 

The glycine-dependent aldolases are pyridoxal 5-phosphate dependent enzymes that catalyze the reversible aldol reaction, where glycine and an acceptor aldehyde form a (i-hydroxy-a-amino acid (Scheme 5.47).74 Serine hydroxymethyltransferases, SHMT (EC 2.1.2.1), and threonine aldolases, two types of glycine dependent aldolases, have been isolated. In... 

Transamination Reactions of Other Pyridoxal Phosphate Enzymes Inaddition to theirmainreactions, anumberofpyridoxalphosphate-dependent enzymes also catalyze the half-reaction of transamination. Such enzymes include serine hydroxymethyltransferase (Section 10.3.1.1), several decarboxylases, and kynureninase (Section 8.3.3.2). 

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