Phenylalanine decarboxylation

Kaminaga, Y, Schnepp, ]., Peel, G., Kish, C.M., Ben-Nissan, G., Weiss, D., Orlova, L, Lavie, O., Rhodes, D., Wood, K., Porterfield, D.M., Cooper, A.J.L., Schloss, J.V., Pichersky, E., Vainstein, A. and Dudareva, N. (2006) Plant phenylacetaldehyde synthase is a bifunctional homotetrameric enzyme that catalyzes phenylalanine decarboxylation and oxidation.. Biol. Chem., 281,23357-66. 

Kaminaga, Y, et al. (2006) Plant phenylacetal-dehyde synthase is a bifrinctional homotetra-meric enzyme that catalyzes phenylalanine decarboxylation and oxidation. J. Biol. Chem. 281, 23357-23366... 

This is the branch-poiat differentiatiag phenylalanine (25, R = H) from tyrosiae (25, R = OH). Both phenylalanine and tyrosiae contain an aryl ring, a three-carbon side chain (a Cg—Cg fragment), and a nitrogen. Decarboxylation yields a two-carbon side chain (a Cg—Cg fragment), eg, 2-phenethylamine (59, R = H) from phenylalanine and tyramine (59, R = OH) from tyrosiae, although it is not certain that ia all cases decarboxylation must precede use ia alkaloid constmction. 

FIGURE 27.5 Tyrosine is the biosynthetic precursor to a number of neurotransmitters. Each transformation is enzyme-catalyzed. Hydroxy-lation of the aromatic ring of tyrosine converts it to 3,4-dihydroxy phenylalanine (L-dopa), decarboxylation of which gives dopamine. Hy-droxylation of the benzylic carbon of dopamine converts it to norepinephrine (noradrenaline), and methy-lation of the amino group of norepinephrine yields epinephrine (adrenaline). 

A new development is the industrial production of L-phenylalanine by converting phenylpyruvic add with pyridoxalphosphate-dependent phenylalanine transaminase (see Figure A8.16). The biotransformation step is complicated by an unfavourable equilibrium and the need for an amino-donor (aspartic add). For a complete conversion of phenylpyruvic add, oxaloacetic add (deamination product of aspartic add) is decarboxylated enzymatically or chemically to pyruvic add. The use of immobilised . coli (covalent attachment and entrapment of whole cells with polyazetidine) is preferred in this process (Figure A8.17). 

Decarboxylation of histidine to histamine is catalyzed by a broad-specificity aromatic L-amino acid decarboxylase that also catalyzes the decarboxylation of dopa, 5-hy-droxytryptophan, phenylalanine, tyrosine, and tryptophan. a-Methyl amino acids, which inhibit decarboxylase activity, find appfication as antihypertensive agents. Histidine compounds present in the human body include ergothioneine, carnosine, and dietary anserine (Figure 31-2). Urinary levels of 3-methylhistidine are unusually low in patients with Wilson s disease. 

The pathway for synthesis of the catecholamines dopamine, noradrenaline and adrenaline, illustrated in Fig. 8.5, was first proposed by Hermann Blaschko in 1939 but was not confirmed until 30 years later. The amino acid /-tyrosine is the primary substrate for this pathway and its hydroxylation, by tyrosine hydroxylase (TH), to /-dihydroxyphenylalanine (/-DOPA) is followed by decarboxylation to form dopamine. These two steps take place in the cytoplasm of catecholaminereleasing neurons. Dopamine is then transported into the storage vesicles where the vesicle-bound enzyme, dopamine-p-hydroxylase (DpH), converts it to noradrenaline (see also Fig. 8.4). It is possible that /-phenylalanine can act as an alternative substrate for the pathway, being converted first to m-tyrosine and then to /-DOPA. TH can bring about both these reactions but the extent to which this happens in vivo is uncertain. In all catecholamine-releasing neurons, transmitter synthesis in the terminals greatly exceeds that in the cell bodies or axons and so it can be inferred... 

The anaerobic metabolism of L-phenylalanine by Thauera aromatica under denitrifying conditions involves several steps that result in the formation of benzoyl-CoA (a) conversion to the CoA-ester by a ligase, (b) transamination to phenylacetyl-CoA, (c) a-oxidation to phenylglyoxalate, and (d) decarboxylation to benzoyl-CoA (Schneider et al. 1997). 

The question of the stability of the biomolecules is a vital one. Could they really have survived the tremendous energies which would have been set free (in the form of shock waves and/or heat) on the impact of a meteorite Blank et al. (2000) developed a special technique to try and answer this question. They used an 80-mm cannon to produce the shock waves the shocked solution contained the two amino acids lysine and norvaline, which had been found in the Murchison meteorite. Small amounts of the amino acids survived the bombardment , lysine seeming to be a little more robust. In other experiments, the amino acids aminobutyric acid, proline and phenylalanine were subjected to shock waves the first of the three was most stable, the last the most reactive. The products included amino acid dimers as well as cyclic diketopiperazine. The kinetic behaviour of the amino acids differs pressure seems to have a greater effect on the reaction pathway than temperature. As had been recognized earlier, the effect of pressure would have slowed down certain decomposition reactions, such as pyrolysis and decarboxylation (Blank et al., 2001). 

Phenylalanine derivatives, substituted either with 3,4,5-trimethoxy or 2,5-dimethoxy-4-methyl substituents did not show activity (46). Although these would yield potentially active phenethylamines if decarboxylated in vivo, neither is likely to be a substrate for decarboxylases in the CNS (62). Several additional side chain alkylated compounds are discussed in the next section as rigid analogs. 

Experiments by Kenyon and Blois, with samples of phenylalanine labelled with i4C at each of the three aliphatic carbon positions, showed that the molecule could photolyse at each of the three exocyclic carbon-carbon bonds. Decarboxylation was also thought to be an important process, but unfortunately no resulting phenylethylamine was detected during this work. Mechanisms for the production of the observed products were suggested [24],... 

Some rather important indole derivatives influence our everyday lives. One of the most common ones is tryptophan, an indole-containing amino acid found in proteins (see Section 13.1). Only three of the protein amino acids are aromatic, the other two, phenylalanine and tyrosine being simple benzene systems (see Section 13.1). None of these aromatic amino acids is synthesized by animals and they must be obtained in the diet. Despite this, tryptophan is surprisingly central to animal metabolism. It is modified in the body by decarboxylation (see Box 15.3) and then hydroxylation to 5-hydroxytryptamine (5-HT, serotonin), which acts as a neurotransmitter in the central nervous system. 

Methyldopa (dopa = dihydroxy-phenylalanine), as an amino acid, is transported across the blood-brain barrier, decarboxylated in the brain to a-methyldopamine, and then hydroxylat-ed to a-methyl-NE The decarboxylation of methyldopa competes for a portion of the available enzymatic activity, so that the rate of conversion of L-dopa to NE (via dopamine) is decreased. The false transmitter a-methyl-NE can be stored however, unlike the endogenous mediator, it has a higher affinity for a2- than for ai-receptors and therefore produces effects similar to those of clonidine. The same events take place in peripheral adrenergic neurons. 

L-Dopa. Dopamine itself cannot penetrate the blood-brain barrier however, its natural precursor, L-dihydroxy-phenylalanine (levodopa), is effective in replenishing striatal dopamine levels, because it is transported across the blood-brain barrier via an amino acid carrier and is subsequently decarboxy-lated by DOPA-decarboxylase, present in striatal tissue. Decarboxylation also takes place in peripheral organs where dopamine is not needed, likely causing undesirable effects (tachycardia, arrhythmias resulting from activation of Pi-adrenoceptors [p. 114], hypotension, and vomiting). Extracerebral production of dopamine can be prevented by inhibitors of DOPA-decarboxylase (car-bidopa, benserazide) that do not penetrate the blood-brain barrier, leaving intracerebral decarboxylation unaffected. Excessive elevation of brain dopamine levels may lead to undesirable reactions, such as involuntary movements (dyskinesias) and mental disturbances. 

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. 

Notably, nitrile-degrading enzymes (e.g. nitrilase that converts the CN group to carboxylic acid, and nitrile hydratase that produces an amide function) have been described, and they co-exist with aldoxime-degrading enzymes in bacteria (Reference 111 and references cited therein). Smdies in this area led to the proposal that the aldoxime-nitrile pathway, which is implemented in synthesis of drugs and fine chemicals, occurs as a natural enzymic pathway. It is of interest that the enzyme responsible for bacterial conversion of Af-hydroxy-L-phenylalanine to phenacetylaldoxime, an oxidative decarboxylation reaction, lacks heme or flavin groups which are found in plant or human enzymes that catalyze the same reaction. Its dependency on pyridoxal phosphate raised the possibility that similar systems may also be present in plants . 

True alkaloids derive from amino acid and they share a heterocyclic ring with nitrogen. These alkaloids are highly reactive substances with biological activity even in low doses. All true alkaloids have a bitter taste and appear as a white solid, with the exception of nicotine which has a brown liquid. True alkaloids form water-soluble salts. Moreover, most of them are well-defined crystalline substances which unite with acids to form salts. True alkaloids may occur in plants (1) in the free state, (2) as salts and (3) as N-oxides. These alkaloids occur in a limited number of species and families, and are those compounds in which decarboxylated amino acids are condensed with a non-nitrogenous structural moiety. The primary precursors of true alkaloids are such amino acids as L-ornithine, L-lysine, L-phenylalanine/L-tyrosine, L-tryptophan and L-histidine . Examples of true alkaloids include such biologically active alkaloids as cocaine, quinine, dopamine, morphine and usambarensine (Figure 4). A fuller list of examples appears in Table 1. 

Racemic jS-fluoroalkyl tyrosines and phenylalanines have been prepared by classical methods starting from the corresponding fluoroacetophenones. Synthesis of the nonracemic compounds is much more difficult, as exemplified by the preparation of jS-difluoromethyl meta-tyrosines (Figure 5.14). jS-Trifluoromethyl tryptophan is prepared by alkylation of ethyl acetamido malonate with indolyl-2,2-trifluoroethanol. Surprisingly, the decarboxylation reaction leads stereoselectively to the syn isomer (Figure 5.15). ... 

Although 2-phenylethanol can be synthesised by normal microbial metabolism, the final concentrations in the culture broth of selected microorganisms generally remain very low [110, 111] therefore, de novo synthesis cannot be a strategy for an economically viable bioprocesses. Nevertheless, the microbial production of 2-phenylethanol can be greatly increased by adding the amino acid L-phenylalanine to the medium. The commonly accepted route from l-phenylalanine to 2-phenylethanol in yeasts is by transamination of the amino acid to phenylpyruvate, decarboxylation to phenylacetaldehyde and reduction to the alcohol, first described by Ehrlich [112] and named after him (Scheme 23.8). 

Whilst the term biogenic amine strictly encompasses all amines of biological origin, for the purpose of this article it will be employed to refer to the catecholamine (dopamine, noradrenaline) and serotonin group of neurotransmitters. These neurotransmitters are generated from the amino acid precursors tyrosine and tryptophan, respectively, via the action of the tetrahydrobiopterin (BH4)-dependent tyrosine and tryptophan hydroxylases. Hydroxylation of the amino acid substrates leads to formation of 3,4-dihydroxy-l-phenylalanine ( -dopa) and 5-hydroxytryptophan, which are then decarboxylated via the pyridoxalphosphate-dependent aromatic amino acid decarboxylase (AADC) to yield dopamine and serotonin [4]. In noradrenergic neurones, dopamine is further metabolised to noradrenaline through the action of dopamine-jS-hydroxylase [1]. 

In individuals with PKU, a secondary, normally little-used pathway of phenylalanine metabolism comes into play. In this pathway phenylalanine undergoes transamination with pyruvate to yield phenylpyruvate (Fig. 18-25). Phenylalanine and phenylpyruvate accumulate in the blood and tissues and are excreted in the urine—hence the name phenylketonuria. Much of the phenylpyruvate, rather than being excreted as such, is either decarboxylated to phenylacetate or reduced to phenyllactate. Phenylacetate imparts a characteristic odor to the urine, which nurses have traditionally used to detect PKU in infants. The accumulation of phenylalanine or its metabolites in early life impairs normal development of the brain, causing severe mental retardation. This may be caused by excess phenylalanine competing with other amino acids for transport across the blood-brain barrier, resulting in a deficit of required metabolites. 

Serotonin, also called 5-hydroxytryptamine, is synthesized and stored at several sites in the body (Figure 21.18). By far the largest amount of serotonin is found in cells of the intestinal mucosa. Smaller amounts occur in platelets and in the central nervous system. Serotonin is synthesized from tryptophan, which is hydroxy-lated in a reaction analogous to that catalyzed by phenylalanine hydroxylase. The product, 5-hydroxytryptophan, is decarboxylated to serotonin. Serotonin has multiple physiologic roles, including pain perception, affective disorders, and regulation of sleep, temperature, and blood pressure. 

The mechanism for synthesis of alcohols and aldehydes from amino acids has been discussed in a review by Morgan (1976). Both S. lactis and its malty variant can reversibly form keto acids from the amino acids valine, leucine, isoleucine, methionine, and phenylalanine. However, unlike S. lactis, S. lactis var. maltigenes can decarboxylate these keto acids to form aldehydes and reduce the aldehydes to their corresponding alcohols through the action of alcohol dehydrogenase in the presence of NADH. 

Figure 25-5 shows the principal catabolic pathways, as well as a few biosynthetic reactions, of phenylalanine and tyrosine in animals. Transamination to phenylpyruvate (reaction a) occurs readily, and the product may be oxidatively decarboxylated to phen-ylacetate. The latter may be excreted after conjugation with glycine (as in Knoop s experiments in which phenylacetate was excreted by dogs after conjugation with glycine, Box 10-A). Although it does exist, this degradative pathway for phenylalanine must be of limited importance in humans, for an excess of phenylalanine is toxic unless it can be oxidized to tyrosine (reaction b, Fig. 25-5). Formation of phenylpyruvate may have some function in animals. The enzyme phenylpyruvate tautomerase, which catalyzes interconversion of enol and oxo isomers of its substrate, is also an important immunoregulatory cytokine known as macrophage migration inhibitory factor.863...

Deamination 17 Examples of deamination and decarboxylation include conversion of amino acids to fusel oil (leucine to isoamyl alcohol, isoleucine to amyl alcohol, and phenylalanine to phenyl ethanol). Fusel oil formation is a normal function of all yeast fermentations (in alcoholic beverages, levels range from trace to 2200 parts per million). Deamination Glutamic acid to gamma-OH-butyric acid (S. cerevisiae). 

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