Aromatic amino acids pyridoxal phosphate

By contrast, the cytoplasmic decarboxylation of dopa to dopamine by the enzyme dopa decarboxylase is about 100 times more rapid (Am 4x 10 " M) than its synthesis and indeed it is difficult to detect endogenous dopa in the CNS. This enzyme, which requires pyridoxal phosphate (vitamin B6) as co-factor, can decarboxylate other amino acids (e.g. tryptophan and tyrosine) and in view of its low substrate specificity is known as a general L-aromatic amino-acid decarboxylase. 

Following the synthesis of 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase, the enzyme aromatic amino acid decarboxylase (also known as 5-HTP or dopa decarboxylase) then decarboxylates the amino acid to 5-HT. L-Aromatic amino acid decarboxylase is approximately 60% bound in the nerve terminal and requires pyridoxal phosphate as an essential enzyme. 

This enzyme [EC 2.6.1.57] catalyzes the reversible reaction of an aromatic amino acid with a-ketoglutarate to generate an aromatic oxo acid and glutamate. Pyridoxal phosphate is a required cofactor. Methionine can also act as a weak substrate, substituting for the aromatic amino acid. Oxaloacetate substitutes for a-ketoglutarate. 

This pyridoxal-phosphate-dependent enzyme [EC 4.1.1.53] catalyzes the conversion of L-phenylalanine to phenethylamine and carbon dioxide. Tyrosine and other aromatic amino acids can also serve as substrates. 

Pyridoxine (vitamin Bg, 18) (Fig. 13) assists in the balancing of sodium and potassium as well as promoting red blood cell production. A lack of pyridoxine can cause anemia, nerve damage, seizures, skin problems, and sores in the mouth. It is required for the production of the monoamine neurotransmitters serotonin, dopamine, norepinephrine, and epinephrine, as it is the precursor to pyridoxal phosphate, which is the cofactor for the aromatic amino acid decarboxylase enzyme. 

Among the numerous enzymes that utilize pyridoxal phosphate (PLP) as cofactor, the amino acid racemases, amino acid decarboxylases (e.g., aromatic amino acids, ornithine, glutamic acid), aminotransferases (y-aminobutyrate transaminase), and a-oxamine synthases, have been the main targets in the search for fluorinated mechanism-based inhibitors. Pharmaceutical companies have played a very active role in this promising research (control of the metabolism of amino acids and neuroamines is very important at the physiological level). 

Table 6.2.2 Typical CSF profiles of HVA, 5HIAA and 3-methyldopa (3-MD) for the inborn errors of metabolism associated with a disruption of biogenic amine metabolism. A downward-pointing arrow indicates that a particular metabolite is below the established reference range. An upward pointing arrow is indicative that a metabolite is above the established reference range. WR indicates that the concentration of the metabolite is likely to be within the reference range. AADC Aromatic amino acid decarboxylase, PNPO pyridox(am)ine-5 -phosphate oxidase...

Tyrosine is converted to dopa by the rate-limiting enzyme tyrosine hydroxylase, which requires tetrahydrobiopterin, and is inhibited by a-methyltyrosine. Dopa is decarboxylated to dopamine by L-aromatic amino acid decarboxylase, which requires pyridoxal phosphate (vitamin B6) as a coenzyme. Carbidopa, which is used with levodopa in the treatment of parkinsonism, inhibits this enzyme. Dopamine is converted to norepinephrine by dopamine P-hydroxylase, which requires ascorbic acid (vitamin C), and is inhibited by diethyldithiocarbamate. Norepinephrine is converted to epinephrine by phenylethanolamine A -methyltransferase (PNMT), requiring S-adeno-sylmethionine. The activity of PNMT is stimulated by corticosteroids. 

Tissue also contains some endogenous species that exhibit fluorescence, such as aromatic amino acids present in proteins (phenylalanine, tyrosine, and tryptophan), pyridine nucleotide enzyme cofactors (e.g., oxidized nicotinamide adenine dinucleotide, NADH pyridoxal phosphate flavin adenine dinucleotide, FAD), and cross-links between the collagen and the elastin in extracellular matrix.100 These typically possess excitation maxima in the ultraviolet, short natural lifetimes, and low quantum yields (see Table 10.1 for examples), but their characteristics strongly depend on whether they are bound to proteins. Excitation of these molecules would elicit background emission that would contaminate the emission due to implanted sensors, resulting in baseline offsets or even major spectral shifts in extreme cases therefore, it is necessary to carefully select fluorophores for implants. It is also noteworthy that the lifetimes are fairly short, such that use of longer lifetime emitters in sensors would allow lifetime-resolved measurements to extract sensor emission from overriding tissue fluorescence. 

Histamine, serotonin and the catecholamines (dopamine, epinephrine and norepinephrine) are synthesized from the aromatic amino acids histidine, tryptophan and phenylalanine, respectively. The biosynthesis of catecholamines in adrenal medulla cells and catecholamine-secreting neurons can be simply summarized as follows [the enzyme catalysing the reaction and the key additional reagents are in square brackets] phenylalanine — tyrosine [via liver phenylalanine hydroxylase + tetrahydrobiopterin] —> i.-dopa (l.-dihydroxyphenylalanine) [via tyrosine hydroxylase + tetrahydrobiopterin] —> dopamine (dihydroxyphenylethylamine) [via dopa decarboxylase + pyridoxal phosphate] — norepinephrine (2-hydroxydopamine) [via dopamine [J-hydroxylasc + ascorbate] —> epinephrine (jV-methyl norepinephrine) [via phenylethanolamine jV-methyltransferase + S-adenosylmethionine]. 

There is a great deal of evidence that deficiency of serotonin (5-hydroxytryptamine) is a factor in depressive illness, and many antidepressant drugs act to decrease its catabolism or enhance its interaction with receptors. A key enzyme involved in the synthesis of serotonin (and the catecholamines) is aromatic amino acid decarboxylase, which is pyridoxal phosphate-dependent. Therefore, it has been suggested that vitamin Be deficiency may result in reduced formation of the neurotransmitters and thus be a factor in the etiology of depression. Conversely, it has been suggested that supplements of vitamin Be may increase aromatic amino acid decarboxylase activity, and increase amine synthesis and have a mood-elevating or antidepressant effect. There is little evidence that vitamin Be deficiency affects the activity of aromatic amino acid decarboxylase. In patients with kidney failure, undergoing renal dialysis, the brain concentration of pyridoxal phosphate falls to about 50% of normal, with no effect on serotonin, catecholamines, or their metabolites (Perry etal., 1985). 

Pyridoxal phosphate is the coenzyme in a large number of amino acid reactions. At this point it is convenient to consider together 1,he mechanism of those pyridoxal-dependent reactions concerned with aromatic amino acids. The reactions concerned are (1) keto acid formation (e.g., from kynurenine, above), 2) decarboxylation (e.g., of 5-hydroxytrypto-phan to 5-hydroxytryptamine, p. 106), (3) scission of the side claain (e.g., 3-tyrosinase, p. 78 tryptophanase, p. 110 and kynureninase, above), and 4) synthesis (e.g., of tryptophan from indole and serine, p. 40). Many workers have considered the mechanism of one or more of these reactions (e.g., 24, 216, 361, 595), but a unified theory is primarily due to Snell and his colleagues (summarized in 593). Snell s experiments have been carried out largely in vitro, and it should be emphasized that in vivo it is the enzyme protein which probably directs the electromeric changes. 

L-aromatic amino add decarboxylase (EC 4.1.1.26) is a stereospedfic enzyme for several l-aromatic amino acids (phenylalanine, tyrosine, DOPA. S-HTP). It is widely distributed and is characterized by its pyridoxal-S -phosphate requirement as coenzyme. The general r61e of this enzyme in the biogenesis of many amines is well documented but its regulatory action seems doubtful in view of its large excess compared with the low levels of tryptophan-S-hydroxylase and tyrosine hydroxylase. Numerous substances belonging to various chemical classes inhibit this enzyme in vivo and have been reviewed in Section B. Chapter 5.2 and elsewhere... 

The other enzyme involved in the synthesis of 5-HT, aromatic L-amino acid decarboxylase (AADC) (EC 4.1.1.28), is a soluble pyridoxal-5 -phosphate-dependent enzyme, which converts 5-HTP to 5-HT (Fig. 13-5). It has been demonstrated that administration of pyridoxine increases the rate of synthesis of 5-HT in monkey brain, as revealed using position emission tomography (this technique is discussed in Ch. 58). This presumably reflects a regulatory effect of pyridoxine on AADC activity and raises the interesting issue of the use of pyridoxine supplementation in situations associated with 5-HT deficiency. 

This enzyme uses cysteine conjugates as substrates, releasing the thiol of the xenobiotic, pyruvic acid, and ammonia, with subsequent methylation giving rise to the methylthio derivative. The enzyme from the cytosolic fraction of rat liver is pyridoxal phosphate requiring protein of about 175,000 daltons. Cysteine conjugates of aromatic compounds are the best substrates, and it is necessary for the cysteine amino and carboxyl groups to be unsubstituted for enzyme activity. 

Finally, the decarboxylation of amino acids catalyzed by several pyridoxal phosphate-dependent enzymes has been shown to proceed by a retention of configuration at the Ca atom144. The stereochemical course of the decarboxylation of 5-hydroxy tryptophan to 5-hydroxytryptamine (serotonin) catalyzed by the pyridoxal phosphate-dependent aromatic L-amino acid decarboxylase (equation 15) exemplifies such studies145. 

A variety of amino acid racemases have been identified in bacteria, archaea, and eukaryotes. They are dassified into two groups pyridoxal 5 -phosphate (PLP) -dependent and -independent enzymes. Therefore, racemization can be achieved via two mechanisms through a chiraUy unstable Schiff base intermediate with an aromatic aldehyde serving as co-factor PLP (Scheme 13.22a) and by a two-base mechanism without co-factor (Scheme 13.22b). 

L-Amino acid transaminases are ubiquitous in nature and are involved, be it directly or indirectly, in the biosynthesis of most natural amino acids. All three common types of the enzyme, aspartate, aromatic, and branched chain transaminases require pyridoxal 5 -phosphate as cofactor, covalently bound to the enzyme through the formation of a Schiff base with the e-amino group of a lysine side chain. The reaction mechanism is well understood, with the enzyme shuttling between pyridoxal and pyridoxamine forms [39]. With broad substrate specificity and no requirement for external cofactor regeneration, transaminases have appropriate characteristics to function as commercial biocatalysts. The overall transformation is comprised of the transfer of an amino group from a donor, usually aspartic or glutamic acids, to an a-keto acid (Scheme 15). In most cases, the equilibrium constant is approximately 1. 

Dopa is decarboxylated to 2-(3,4-dihydroxyphenyl) ethylamine (dopamine) by aromatic L-amino acid decarboxylase, a nonspecific cytosolic pyridoxal phosphate-dependent enzyme also involved in formation of other amines (e.g., 5-hydroxytryptamine). 

Aromatic L-amino acid decarboxylase Pyridoxal phosphate... 

Schiff base formation between pyridoxal phosphate and amino acids are the basis for most enzymatic transformations of amino acids including transamination, decarboxylation, and racemization. Schiff bases formed between amino acids and pyridoxal phosphate or other heteroaromatic or aromatic aldehydes are, however, not only transformed enzymatically, but can, without enzymatic catalysis, undergo a large number of reactions, although at lower rate and/or higher temperatures than those for the corresponding enzymatic reactions. The enzymatic reactions require metal ions as cofactors and in analogy the nonenzymatic reaction are also catalyzed by metal ions, most effectively by cupric ions. 

In addition to the readily ionizable amino-acid side chains listed in Table 1, there are several amino-acid moieties in enzymes, which do not contain ionizable hydrogens. These include amino acids such as alanine, valine, and phenylalanine among others, that contain only carbon-hydrogen bonds in either aliphatic or aromatic side chains, and thus do not engage in proton transfer except in very unusual circumstances. Proton transfers in enzymatic reactions can also occur from co-enzymes (co-factors) which can be considered as tightly bound substrates. Typical examples of co-enzymes that are involved in proton transfer are pyridoxal phosphate and thiamine pyrophosphate. 

Metabolism of P-hydroxy-a-amino acids involves pyridoxal phosphate-dependent enzymes that catalyze a reversible cleavage to aldehydes (Fig, 31) and glycine (89). The distinction between L-threonine aldolase (ThrA EC 4.1.2.5), L-a//o-threonine aldolase (EC 4.1.2.6), or serine hydroxymethyltransferase (SHMT EC 2.1,2.1) has often been rather vague since many catalysts display only poor capacity for erythro/threo (i.e., 91/90) discrimination [22]. Many enzymes display a broad substrate tolerance for the aldehyde acceptor, notably including variously substituted aliphatic as well as aromatic aldehydes (Fig. 31) however, a,P-unsaturated aldehydes are not accepted. 

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