Amino acid decarboxylases phosphate-dependent

Histamine is synthesised by decarboxylation of histidine, its amino-acid precursor, by the specific enzyme histidine decarboxylase, which like glutaminic acid decarboxylase requires pyridoxal phosphate as co-factor. Histidine is a poor substrate for the L-amino-acid decarboxylase responsible for DA and NA synthesis. The synthesis of histamine in the brain can be increased by the administration of histidine, so its decarboxylase is presumably not saturated normally, but it can be inhibited by a fluoromethylhistidine. No high-affinity neuronal uptake has been demonstrated for histamine although after initial metabolism by histamine A-methyl transferase to 3-methylhistamine, it is deaminated by intraneuronal MAOb to 3-methylimidazole acetic acid (Fig. 13.4). A Ca +-dependent KCl-induced release of histamine has been demonstrated by microdialysis in the rat hypothalamus (Russell et al. 1990) but its overflow in some areas, such as the striatum, is neither increased by KCl nor reduced by tetradotoxin and probably comes from mast cells. 

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. 

A number of decarboxylase enzymes have been described as catalysts for the preparation of chiral synthons, which are difficult to access chemically (see Chapter 2).264 The amino acid decarboxylases catalyze the pyridoxal phosphate (PLP)-dependent removal of C02 from their respective substrates. This reaction has found great industrial utility with one specific enzyme in particular, L-aspartate-P-decarboxylase (E.C. 4.1.1.12) from Pseudomonas dacunhae. This biocatalyst, most often used in immobilized whole cells, has been utilized by Tanabe to synthesize L-alanine on an industrial scale (multi-tons) since the mid-1960s (Scheme 19.33).242-265 Another use for this biocatalyst has been the resolution of racemic aspartic acid to produce L-alanine and D-aspartic acid (Scheme 19.34). The cloning of the L-aspartate-P-decarboxylase from Alcaligenes faecalis into E. coli offers additional potential to produce both of these amino acids.266... 

Amino acids NAD(P)H Pyridoxamine 5 -phosphate Pyruvate Amines Pyridoxal 5-phosphate dependent enzymes Dehydrogenases Transaminases Pyridoxal 5-phosphate dependent enzymes Amino acid decarboxylases... 

The enzyme, i.e. lysine decarboxylase, that is required for the conversion of lysine into cadaverine, and thus the first step of alkaloid biosynthesis, has been isolated from chloroplasts of L. polyphyllus,28 Like the majority of amino-acid decarboxylases, this enzyme is dependent on pyridoxal 5 phosphate. Its activity was found not to be affected by the presence or absence of quinolizidine alkaloids. Control of the enzyme by simple product feedback inhibition therefore seems unlikely. The operational parameters of this enzyme resemble those of the 17-oxosparteine synthase. Co-operation between the two enzymes would explain why cadaverine is almost undetectable in vivo. 

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. 

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). 

Aspartate undergoes /3-decarboxylation to /S-alanine unlike most amino acid decarboxylases, aspartate decarboxylase is not pyridoxal phosphate-dependent, but has a catalytic pyruvate residue, derived by postsynthetic modification of a serine residue (Section 9.8.1). Pantothenic acid results from the formation of a peptide bond between /3-alanine and pantoic acid. 

The enzymes, amino acid decarboxylases are pyridoxal phosphate- dependent enzymes. Pyridoxal phosphate forms a Schiff s base with e amino acid so as to stabilise the a-carbanion formed by the cleavage of bond between carboxyl and a-carbon atom. The physiologically active amines epinephrine, nor-epinephrine, dopamine, serotonin, y-amino butyrate and histamine are formed through decarboxylation of the corresponding precursor amino acids... 

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). 

Pyridoxal phosphate is a co-enzyme for numerous enzymes, notably amino acid decarboxylases, amino acid transaminases, histaminase and probably diamine oxidase Ais.iw. As most of the evidence on which the mechanism of action of pyridoxal-dependent enzymes is based has been obtained from studies of the non-enzymic interaction of pyridoxal with amino acids, these non-enzymic reactions will be considered first in some detail. 

Pyridoxal Phosphate, Codecarboxylase. An independent approach to the nature of the amino acid decarboxylases was made by Gunsalus, Umbreit, and collaborators. They found that the production of tyrosine decarboxylase by Streptococcus faecalis depended on the vitamin, pyri-doxine. In the absence of pyridoxine the cells grew but had little decarboxylase. However, addition of the vitamin permitted deficient cells to decarboxylate tyrosine, and dried cells exhibited active enzyme in the presence of pyridoxal (a derivative of pyridoxine) and ATP, implying the formation of an active cofactor from these substances. Pyridoxal is a more active growth factor for a strain of Streptococcus faecalis than pyridoxine both synthetic pyridoxal and pyridoxamine exhibit 5000 to 9000 times the activity of the hydroxy compound. ... 

Most alkaloids are derived from amino acids (Fig. 3) and the first reaction in the otherwise independent pathways is the decarboxylation of the respective amino acid by an amino acid decarboxylase (AADC) (Figs. 1 and 3) this step is often under complex regulation. Plant and animal AADCs share high amino acid identity, with significant similarities in subunit structure and kinetic characteristics. In contrast to their mammahan and insect counterparts, plant AADCs exhibit high specificity for their respective substrates. The reaction is pyridoxal-5 -phosphate (PLP)-dependent. 

Allenic amino acids belong to the classical suicide substrates for the irreversible mechanism-based inhibition of enzymes [5], Among the different types of allenic substrates used for enzyme inhibition [128, 129], the deactivation of vitamin B6 (pyr-idoxal phosphate)-dependent decarboxylases by a-allenic a-amino acids plays an important role (Scheme 18.45). In analogy with the corresponding activity of other /3,y-unsaturated amino acids [102,130], it is assumed that the allenic amino acid 139 reacts with the decarboxylase 138 to furnish the imine 140, which is transformed into a Michael acceptor of type 141 by decarboxylation or deprotonation. Subsequent attack of a suitable nucleophilic group of the active site then leads to inhibition of the decarboxylase by irreversible formation of the adduct 142 [131,132]. 

Scheme 18.45 Postulated inhibition mechanism of pyridoxal phosphate-dependent decarboxylases by a-allenic a-amino acids.

Most people have heard of antihistamines, even if they have little concept of the nature of histamine. Histamine is the decarboxylation product from histidine, and is formed from the amino acid by the action of the enzyme histidine decarboxylase. The mechanism of this pyridoxal phosphate-dependent reaction will be studied in more detail later (see Section 15.7). 

Due to the absence of a hydrogen atom on the a-carbon, the a-fluoroalkyl amino acids (except, of course, the fluoroalanines, vide supra) cannot undergo an elimination of HR Consequently, they are more stable than fluoroalanines and other jS-fluoro amino acids previously described. On the other hand, similar to proteogenic amino acids, jS-fluoro amino acids and a-fluoroalkyl amino acids are generally substrates of pyridoxal phosphate depending on enzymes such as racemases and decarboxylases. When an amino acid is a substrate of such enzymes, the enzyme induces the development of a negative charge on the a-carbon, which can initiate a /(-elimination process. This reaction affords an electrophilic species (Michael acceptor type), which is able to add a nucleophilic residue of the enzyme. This notion of mechanism-based inhibitor is detailed in Chapter 7. 

Pyridoxal Phosphate-Catalyzed Enzyme Reactions of Amino Acids Amines Formed by Pyridoxal Phosphate-Dependent Decarboxylases... 

The conversion of tyrosine to 3,4-dihydroxyphenylalanine occurs both in vivo in man (590) and in vitro by the action of tissue tyrosinase (205, 688). Mammals can decarboxylate both tyrosine (402,407) and dihydroxyphenyl-alanine (406), tyrosine decarboxylase and dihydroxyphenylalanine (dopa) decarboxylases being quite distinct and separable (405), though both are dependent on pyridoxal phosphate (73, 758, and review 72). In mammals dihydroxyphenylalanine is the most readily decarboxylated of all amino acids, and it is therefore not unreasonable to assume that this is the substrate normally decarboxylated in adrenaline biosynthesis cf. 74, 75). Support for this concept derives from the fact that both the substrate and the product of the reaction (3,4-dihydroxyphenylethylamine diagram 11) can or do occur in the adrenal (298, 299, 802), and the amine is moreover, like adrenaline and noradrenaline, a normal urinary excretion product (245, 404). 

Inhibition by Inactivation or Displacement of the Co-enzyme The function of pyridoxal phosphate as the co-enzyme of histidine decarboxylase depends on the ability of the aldehyde group to react with the a-amino group of histidine. However, the aldehyde group also reacts with a wide variety of amino acids, amines and carbonyl reagents. The specificity of the holo-enzyme-substrate interaction is therefore due to the apo-enzyme. The co-enzyme attached to the active centre is presumably in equilibrium with free co-enzyme in the surrounding solution. 

These reactions involve the activities of transaminases and decarboxylases (see p. 210), and over 50 pyridoxal phosphate-dependent enzymes have been identified. In transamination, pyridoxal phosphate accepts the a-amino group of the amino acid to form pyridoxamine phosphate and a keto acid. The amino group of pyri-doxamine phosphate can be transferred to another keto acid, regenerating pyridoxal phosphate. The vitamin is believed to play a role in the absorption of amino acids from the intestine. 

One important subgroup of the lyases are the decarboxylases. The decarboxylation of amino acids is assisted by pyridoxal phosphate as a prosthetic group, whereas in the decarboxylation of pyruvate to acetaldehyde, thiamine pyrophosphate (TPP) plays that role. Oxidative decarboxylation, lastly, depends on the cooperation of no fewer than five cofactors thiamine pyrophosphate, lipoic acid, coenzyme A, flavin-adenine dinucleotide, and nicotinamide-adenine dinucleotide. 

---