Serine dehydration

The enzyme serine dehydratase employs pyridoxal phosphate (PEP) in the direct deamination of serine. In this reaction, the a-carbon of serine undergoes a two-electron oxidation through a-elimination. Show how PEP participates in the process by writing a mechanism for serine dehydration and deamination. 

A number of nonprotein amino acids with unsaturated side chains have been isolated. Many of these contain alkene side chains, but some alkyne side chains containing amino acids have also been identified. Nonprotein dehydroamino acids do not have an a-stereocenter these amino acids are still classified under this category. Dehydroamino acids are generally biosynthesized by the enzymatic elimination of a leaving group at the /3-carbon. For example, serine and threonine are enzymatically dehydrated to give dehydroalanine and dehydrobutyrine, respectively. A similar biosynthetic pathway is hypothesized for dehydroamino acids found in nonribosomal peptides, such as nodularins and microcystins. ... 

Figure 4 The biosynthesis of nisin A as a representative example of the posttranslational maturation process of lantibiotics. Following ribosomal synthesis, NisB dehydrates serine and threonine residues in the structural region of the prepeptide NisA. NisC subsequently catalyzes intramolecular addition of cysteine residues onto the dehydro amino acids in a stereo- and regioselective manner. Subsequent transport of the final product across the cell membrane by NisT and proteolytic cleavage of the leader sequence by NisP produces the mature lantibiotic. For the sequence of the leader peptide, see Figure 6. Adapted with permission from J. M. Willey W. A. van der Donk, Annu. Rev. Microbiol. 2007, 61, 477-501.

The regulation of bacteriocin synthesis by PPs goes back to mid-1990s when it was shown that the Class II bacteriocins (nonmodified heat-stable bacteriocins) as well as Class I bacteriocins most often referred to as lantibiotics (post-translatory modified containing most frequently lanthionine, methyl-lanthionine, dehydrated serine, and threonine) were regulated by small ribosomally synthesized peptides either resembling a peptide... 

The biosynthesis 237,5381 involves enzymatic dehydration of serine and threonine residues in a manner similar to the formation of thiazoles and dihydrothiazoles vide supra) with or without subsequent oxidation to yield the 2-(l-aminoalkyl)oxazole-4-carboxylic acid and 2-(l-aminoalkyl)dihydrooxazole-4-carboxylic acid shown in Scheme 38. These cyclic peptides display interesting physiological properties such as cytotoxicity/541, 569,5831 antitumor activities, or antineoplastic effects/523,5291 but as for the sulfur-containing compounds the mechanism of action is not yet understood despite extensive SAR studies. 515,521,540,5431... 

In sharp contrast to the development of the [4 + 2] cycloaddition reactions of electron-rich 2-azadienes, reports dealing with the chemistry of electron-poor 2-azadienes remained unknown until a few years ago. In fact, the first cycloaddition of an electron-withdrawing substituted 2-azadiene was observed in 1986 by Wulff and Bohnke [86AG(E)90] while they were preparing dehydroaminoacid derivatives (Scheme 48). They isolated AL(arylmethylene)dehydroalanine methyl esters 208 by dehydration of the Schiff base of the serine methyl ester 207 and found that it dimerized through a [4 + 2] cycloaddition to give tetrahydropyridine derivative 209 in 56% yield as a sole diasteroisomer. 

Izumiya and co-workers (77JA8346) have undertaken a systematic study of the chiral induction on hydrogenation (H2,l atm., Pd black, MeOH) of a series of cyclodipeptides of the type cyclo(L-aminoacyl-Dha), where Dha represents dehydroalanine. These were synthesized from the corresponding L-serine derivatives the final dehydration was accomplished by Photaki s procedure (63JA1123). Contrary to the report of the earlier workers, the de in the product cyclo(L-aminoacyl-Ala) [(S,S) over (S,R) was the highest with valine (98.4%) and the least with proline (84.8%). Leucine gave about 95% de. 

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). 

MECHANISM FIGURE 22-18 Tryptophan synthase reaction. This enzyme catalyzes a multistep reaction with several types of chemical rearrangements. An aldol cleavage produces indole and glyceraldehyde 3-phosphate this reaction does not require PLP. Dehydration of serine forms a PLP-aminoacrylate intermediate. In steps and this condenses with indole, and the product is hydrolyzed to release tryptophan. These PLP-facilitated transformations occur at the /3 carbon (C-3) of the amino acid, as opposed to the a-carbon reactions described in Figure 18-6. The /3 carbon of serine is attached to the indole ring system. Tryptophan Synthase Mechanism... 

Both enzymes are inhibited by sodium borohydride and also by nitromethane. After reduction with NaB3H4 and hydrolysis, 3H-containing alanine was isolated. This suggested that they contain dehydroalanine, which could arise by dehydration of a specific serine residue.286,287 For phenylalanine ammonia-lyase from Pseudomonas putida this active site residue has been identified as S143. Replacement by cysteine in the S143C mutant also gave active enzyme while S143A... 

The role of the iron-sulfur clusters in many of the proteins that we have just considered is primarily one of single-electron transfer. The Fe-S cluster is a place for an electron to rest while waiting for a chance to react. There may sometimes be an associated proton pumping action. In a second group of enzymes, exemplified by aconitase (Fig. 13-4), an iron atom of a cluster functions as a Lewis acid in facilitating removal of an -OF group in an a,P dehydration of a carboxylic acid (Chapter 13). A substantial number of other bacterial dehydratases as well as an important plant dihydroxyacid dehydratase also apparently use Fe-S clusters in a catalytic fashion.317 Fumarases A and B from E. coli,317 L-serine dehydratase of a Pepto-streptococcus species,317-319 and the dihydroxyacid... 

Peptide thioesters (Section 15.1.10) are generally prepared by coupling protected amino acids or peptides with thiols and are used for enzymatic hydrolysis. Peptide dithioesters, used to study the structures of endothiopeptides (Section 15.1.11), may be prepared by the reaction of peptide nitriles with thiols followed by thiolysis (Pinner reaction). Peptide vinyl sulfones (Section 15.1.12), inhibitors of various cysteine proteases, are prepared from N-protected C-terminal aldehydes with sulfonylphosphonates. Peptide nitriles (Section 15.1.13) prepared by dehydration of peptide amides, acylation of a-amino nitriles, or the reaction of Mannich adducts with alkali cyanides, are relatively weak inhibitors of serine proteases. 

Glyoxal can be formed by oxidation of glycolaldehyde (e.g., in Scheme 2.5), but it can also be formed by autoxidation of unsaturated fats and by enzymic degradation of serine.60 2-Oxopropanal can be obtained by retroaldolisation of 1- and 3-deoxyglucosone or by hydrolysis of diacetylformoin (see Scheme 2.5). Butanedione can also be derived from diacetylformoin, but by reduction, dehydration, and hydrolysis (see Scheme 2.5). 2,3-Pentanedione can be formed from butanedione by aldol reaction with formaldehyde, dehydration, and reduction or by aldol condensation of hydroxyacetone and acetaldehyde, followed by dehydration. 

P-Elimination and then replacement reaction of an a-amino acid with a nucleophile is very attractive from the viewpoint of synthetic organic chemistry because various P-substituted alanines may be prepared from a simple a-amino acid, such as serine, and nucleophiles. A reaction catalyzed by tryptophan synthase - the formation of tryptophan from serine and indole - is one of the most well-known P-elimination and replacement reactions (Scheme 2.7). Here, an aldimine Schiff base is derived from reaction of the enzyme-bound PLP with serine, which then dehydrates to give the Schiff base of PLP with 2-aminoacrylate. Indole then adds to the vinyl Schiff base, generating tryptophan after lysine aminolysis of the Schiff base product. 

Such chemical changes may lead to compounds that are not hydrolyzable by intestinal enzymes or to modifications of the peptide side chains that render certain amino acids unavailable. Mild heat treatments in the presence of water can significantly improve the protein s nutritional value in some cases. Sulfur-containing amino acids may become more available and certain antinutritional factors such as the trypsin inhibitors of soybeans may be deactivated. Excessive heat in the absence of water can be detrimental to protein quality for example, in fish proteins, tryptophan, arginine, methionine, and lysine may be damaged. A number of chemical reactions may take place during heat treatment including decomposition, dehydration of serine and threonine, loss of sulfur from cysteine, oxidation of cysteine and methio-... 

Alternatively, over very long time periods, L-amino acids can racemize to produce D-amino acids. However, measured d/l ratios for certain amino acids cannot be achieved based on known racemization rates of amino acids in seawater. For example, Lee and Bada (1977) calculated d/l ratios of 0.01 and 0.004 for aspartic acid and alanine, respectively, assuming an oceanic residence time of 3,400 years for these amino acids. These calculated values are much lower than the measured values and led Lee and Bada (1977) to conclude that the enhanced D-amino acid concentrations in marine DOM must be derived from a bacterial source. In a later paper, Bada et al. (1982) suggested that the near-racemic mixture (50% each of the d and l enantiomer) of alanine at depth in the ocean was a result of the dehydration of serine or threonine to produce racemic alanine. These authors also detected near racemic a-amino- -butyric acid (ABA), which can be produced from the dehydration of threonine. This mechanism of D-alanine formation... 

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