Alanine from tryptophan

Certain amino acids and their derivatives, although not found in proteins, nonetheless are biochemically important. A few of the more notable examples are shown in Figure 4.5. y-Aminobutyric acid, or GABA, is produced by the decarboxylation of glutamic acid and is a potent neurotransmitter. Histamine, which is synthesized by decarboxylation of histidine, and serotonin, which is derived from tryptophan, similarly function as neurotransmitters and regulators. /3-Alanine is found in nature in the peptides carnosine and anserine and is a component of pantothenic acid (a vitamin), which is a part of coenzyme A. Epinephrine (also known as adrenaline), derived from tyrosine, is an important hormone. Penicillamine is a constituent of the penicillin antibiotics. Ornithine, betaine, homocysteine, and homoserine are important metabolic intermediates. Citrulline is the immediate precursor of arginine. 

Ai-Stearoylamino acids and their methyl esters were synthesized from enantiomeric and racemic forms of tyrosine, serine, alanine, and tryptophan (Fig. 16). Analogs of these molecules were investigated initially over 30 years ago by Zeelen and Havinga, who found stereochemical differentiation in the monolayer HjA isotherms of these materials (Zeelen, 1956 Zeelen and Havinga, 1958). We have extended this study using more sensitive Langmuir balances, a wider array of dynamic and equilibrium techniques, and the A-stearoyl methyl esters of the amino acids (Harvey et al., 1989 Harvey and Arnett, 1989). 

Several amino acids are broken down by de-carbo qflation. This reaction gives rise to what are known as biogenic amines, which have various functions. Some of them are components of biomolecules, such as ethanolamine in phospholipids (see p. 50). Cysteamine and T-alanine are components of coenzyme A (see p.l2) and of pantetheine (see pp. 108, 168). Other amines function as signaling substances. An important neurotransmitter derived from glutamate is y-aminobutyrate (GABA, see p.356). The transmitter dopamine is also a precursor for the catecholamines epinephrine and norepinephrine (see p.352). The biogenic amine serotonin, a substance that has many effects, is synthesized from tryptophan via the intermediate 5-hydroxytryptophan. 

Fig.4.40. Separation of some DNS-amino acids on a small-particle silica-gel column (see text for details). Peaks 1 = inert 2 = unknown 3 isoleucine 4 = valine 5 = leucine 6 = tyrosine 7 = alanine 8 = tryptophan 9 = glycine 10 = histidine 11= lysine. (From ref. 81 with permission of the American Chemical Society, Washington.)...

Echinulin.—Detailed work40 has been carried out in the biosynthesis of echinulin (115). This fungal metabolite arises from tryptophan and alanine through the... 

Thus we designed and synthesized a bicyclic pyridoxamine derivative carrying an oriented catalytic side arm (16) [11], Rates for conversion of the ketimine Schiff base into the aldimine, formed with 26 (below) and a-ketovaleric acid, indolepyruvic acid, or pyruvic acid, were enhanced 20-30 times relative to those carried out in the presence of the corresponding pyridoxamine derivatives without the catalytic side arm. With a-ketovaleric acid, 16 underwent transamination to afford D-norvaline with 90% ee. The formation of tryptophan and alanine from indolepyruvic acid and pyruvic acid, respectively, showed a similar preference. A control compound (17), with a propylthio group at the same stereochemical position as the aminothiol side arm in 16, produced a 1.5 1 excess of L-norvaline, in contrast to the large preference for D-amino acids with 16. Therefore, extremely preferential protonation seems to take place on the si face when the catalytic side arm is present as in 16. 

Miscellaneous Indole Alkaloids.—Echinulin (38) is known to be assembled from tryptophan, mevalonic acid, and alanine.24-26 The indole derivative (37) has now been shown27 to be converted efficiently by Aspergillus amstelodami into echinulin and is, therefore, probably an intermediate. 

Tryptophan is oxidized to produce alanine (from the non-ring carbons), formate, and acetyl CoA. Tryptophan is, therefore, both glucogenic and ketogenic (Fig. 39.19). 

One extreme view of chemical introduction of an extrinsic fluorescent probe is found in the case ofthe alanine derivative of the fluorophore 6-dimethylamino-2-acylnaphthalene (DAN) (Figure 4.23). This derivative fluorophore, given the trivial name Aladan, is incorporated into a polypeptide by solid-phase synthetic chemistry (although a molecular biology technique known as nonsense suppression is now available for the introduction of unnatural amino-acid residues into recombinant proteins). The fluorescent emission maximum (Tnax) of Aladan shifts dramatically on different solvent exposures, from 409 nm in heptane to 542 nm in water, yet at the same time remains only mildly changed by variations in pH or salt concentration. This compares to a maximum environment-mediated shift of around 40 nm for intrinsic tryptophan fluorescence. In addition, there is little spectral overlap between extrinsic Aladan fluorescence and intrinsic fluorescence from tryptophan or tyrosine. 

III. Diketopiperazine Metabolites Derived from Tryptophan and Alanine... 

Few fungal metabolites consist of a diketopiperazine ring derived from tryptophan and an amino acid other than alanine and proline. Those known in Aspergillus species include ditryptophenaline and D-valyl-L-tryptophan anhydride. 

Tryptoquivalines appear to be tetrapeptides derived from tryptophan, an-thranilic acid, valine, and alanine (or methylalanine). Deoxynortryptoquivalone (FTN) may be the first metabolite formed in the pathway of tryptoquivaline biosynthesis. Oxidation of the secondary amine to the hydroxyamine would form nortryptoquivalone. If the isobutyl side chain from the above tryptoquivalones is lost by further oxidation, FTJ and FTE would result. On the other hand, if reduction of the carbonyl group following acetylation occurs on the side chain, nortryptoquivaline (FTD) or deoxynortryptoquivaline respectively, would be... 

This conversion has been found to be catalyzed by an enzyme isolated from the liver and kidney of several mammalian species (6Jir67), from tryptophan-adapted Pseudomonas (68, 69), and from Neurospora (60). The enzyme appears to be identical with the system which causes the hydrolysis of 3-kynurenine to anthranilic acid. The products of the hydrolysis of hydroxykynurenine are hydroxyanthranilic acid and alanine. Pyri-doxal phosphate is required for the reaction. 

Consideration of the structures of sporidesmin (25) and its cometabolites suggests a biosynthetic derivation from tryptophan and alanine, although, clearly, an extensive series of peripheral changes to the carbon skeleton must take place. 

The lability of tryptophan under the conditions usually employed for acid hydrolysis of proteins (6N HCl, 24 hrs, in sealed tubes under vacuum) is not due to instability of the indole nucleus under such conditions, but to side reactions involving non-proteinaceus material, like carbohydrates, or to the presence of particular amino acids in the acidic hydrolysis mixture. In fact, gramicidin A, a pentadecapeptide containing, apart from tryptophan, only purely aliphatic amino acids like glycine, alanine, leucine and valine, gives a quantitative recovery of tryptophan after acid hydrolysis in 6N HCl (l20). 

Enzymatic Process. Chemically synthesized substrates can be converted to the corresponding amino acids by the catalytic action of an enzyme or the microbial cells as an enzyme source, t - Alanine production from L-aspartic acid, L-aspartic acid production from fumaric acid, L-cysteine production from DL-2-aminothiazoline-4-catboxyhc acid, D-phenylglycine (and D-/> -hydtoxyphenylglycine) production from DL-phenyUiydantoin (and DL-/)-hydroxyphenylhydantoin), and L-tryptophan production from indole and DL-serine have been in operation as commercial processes. Some of the other processes shown in Table 10 are at a technical level high enough to be useful for commercial production (24). Representative chemical reactions used ia the enzymatic process are shown ia Figure 6. 

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