Decarboxylases, coenzymes

By increasing the activity of the kynurenine pathway, estrogens could also increase the requirements for PLP and make less available to act as the coenzyme for 5-hydroxytryptophan decarboxylase, or estrogen conjugates could displace PLP from the decarboxylase coenzyme directly. Even though in nonhuman mammals tryptophan hydroxylase, not the decarboxylase, is thought to be rate-limiting in serotonin synthesis (Jl), levels of decarboxylase are said to be so low in human brain... 

Fears have been expressed [510, 511] that long-term administration of L-dopa may induce a state of pyridoxine deficiency, since excess dietary pyridoxine, which is rapidly converted in vivo to the decarboxylase coenzyme pyridoxine-5 -phosphate [512], can nullify the beneficial effects of the amino acid [513-515]. Pyridoxine apparently both complexes with L-dopa and produces an accelerated decarboxylation of the amino acid in extracerebral tissues, both processes effectively reducing the amount of available dopamine in the striatum [512, 516]. The decarboxylase inhibitor MK-485 (37) prevents this reversal of the therapeutic effect by pyridoxine [517] and, more significantly, pyridoxine actually enhances the effects of L-dopa when given in conjunction with such an inhibitor [518]. The mechanism involved in this potentiation reflects enhancement by pyridoxine of dopa decarboxylase activity within the striatum in the presence of complete inhibition of extracerebral decarboxylase. The use of combinations of L-dopa, pyridoxine, and inhibitors of aromatic L-amino-acid decarboxylase, may lead to a more... 

The neurotransmitter must be present in presynaptic nerve terminals and the precursors and enzymes necessary for its synthesis must be present in the neuron. For example, ACh is stored in vesicles specifically in cholinergic nerve terminals. It is synthesized from choline and acetyl-coenzyme A (acetyl-CoA) by the enzyme, choline acetyltransferase. Choline is taken up by a high affinity transporter specific to cholinergic nerve terminals. Choline uptake appears to be the rate-limiting step in ACh synthesis, and is regulated to keep pace with demands for the neurotransmitter. Dopamine [51 -61-6] (2) is synthesized from tyrosine by tyrosine hydroxylase, which converts tyrosine to L-dopa (3,4-dihydroxy-L-phenylalanine) (3), and dopa decarboxylase, which converts L-dopa to dopamine. 

Pyridoxamine phosphate serves as a coenzyme of transaminases, e.g., lysyl oxidase (collagen biosynthesis), serine hydroxymethyl transferase (Cl-metabolism), S-aminolevulinate synthase (porphyrin biosynthesis), glycogen phosphoiylase (mobilization of glycogen), aspartate aminotransferase (transamination), alanine aminotransferase (transamination), kynureninase (biosynthesis of niacin), glutamate decarboxylase (biosynthesis of GABA), tyrosine decarboxylase (biosynthesis of tyramine), serine dehydratase ((3-elimination), cystathionine 3-synthase (metabolism of methionine), and cystathionine y-lyase (y-elimination). 

To clarify the characteristics of AMDase, the effects of some additives were examined using phenylmalonic acid as the representative substrate. The addihon of ATP and coenzyme A did not enhance the rate of the reaction, different from the case of malonyl-CoA decarboxylase and others in those, ATP and substrate acid form a mixed anhydride, which in turn reacts with coenzyme A to form a thiol ester of the substrate. In the present case, as both ATP and CoA-SH had no effect, the mechanism of the reaction will be totally different from the ordinary one described above. It is well estabhshed that avidin is a potent inhibitor of the formation of the biotin-enzyme complex. In the case of AMDase, addition of avidin has no influence on the enzyme activity, indicating that AMDase is not a biotin enzyme. 

Eederici E, B Vitali, R Gotti, MR Pasca, S Gobbi, AB Peck, P Brigidi (2004) Characterization and heterologous expression of the oxalyl coenzyme A decarboxylase gene from Bifidobacterium lactis. Appl Environ Microbiol 70 5066-5073. 

The precise mechanism of dimethylhydrazine toxicity is uncertain. In addition to the contact irritant effects, the acute effects of dimethylhydrazine exposure may involve the central nervous system as exemplified by tremors and convulsions (Shaffer and Wands 1973) and behavioral changes at sublethal doses (Streman et al. 1969). Back and Thomas (1963) noted that the deaths probably involve respiratory arrest and cardiovascular collapse. The central nervous system as a target is consistent with the delayed latency in response reported for dimethylhydrazine (Back and Thomas 1963). There is some evidence that 1,1-dimethylhydrazine may act as an inhibitor of glutamic acid decarboxylase, thereby adversely affecting the aminobutyric acid shunt, and could explain the latency of central-nervous-system effects (Back and Thomas 1963). Furthermore, vitamin B6 analogues that act as coenzymes in the aminobutyric acid shunt have been shown to be effective antagonists to 1,1-dimethylhydrazine toxicity (reviewed in Back and Thomas 1963). 

Dopamine synthesis in dopaminergic terminals (Fig. 46-3) requires tyrosine hydroxylase (TH) which, in the presence of iron and tetrahydropteridine, oxidizes tyrosine to 3,4-dihydroxyphenylalanine (levodopa.l-DOPA). Levodopa is decarboxylated to dopamine by aromatic amino acid decarboxylase (AADC), an enzyme which requires pyri-doxyl phosphate as a coenzyme (see also in Ch. 12). 

The leucocyte method estimates pyridoxal phosphate in isolated leucocytes it is based on a coenzyme-catalyzed tyrosine decarboxylase system from S. faecalis (B32). Enough data are not yet on hand to evaluate this method. The determination of circulating or available vitamin Ba should offer a more direct approach. 

Pyruvate carboxylase is a mitochondrial enzyme and like other carboxylase or decarboxylase enzymes requires biotin as coenzyme. The biotin is firmly attached to the enzyme protein (i.e. a prosthetic group) via a lysine residue. The role of biotin is to hold the C02 in the correct orientation to allow its incorporation into the pyruvate. 

Figure 6.1 Pathways involved in glucose oxidation by plant cells (a) glycolysis, (b) Krebs cycle, (c) mitochondrial cytochrome chain. Under anoxic conditions. Reactions 1, 2 and 3 of glycolysis are catalysed by lactate dehydrogenase, pyruvate decarboxylase and alcohol dehydrogenase, respectively. ATP and ADP, adenosine tri- and diphosphate NAD and NADHa, oxidized and reduced forms of nicotinamide adenine dinucleotide PGA, phosphoglyceraldehyde PEP, phosphoenolpyruvate Acetyl-CoA, acetyl coenzyme A FP, flavoprotein cyt, cytochrome e, electron. (Modified from Fitter and Hay, 2002). Reprinted with permission from Elsevier...

Serine hydroxymethyl transferase catalyzes the decarboxylation reaction of a-amino-a-methylmalonic acid to give (J )-a-aminopropionic acid with retention of configuration [1]. The reaction of methylmalonyl-CoA catalyzed by malonyl-coenzyme A decarboxylase also proceeds with perfect retention of configuration, but the notation of the absolute configuration is reversed in accordance with the CIP-priority rule [2]. Of course, water is a good proton source and, if it comes in contact with these reactants, the product of decarboxylation should be a one-to-one mixture of the two enantiomers. Thus, the stereoselectivity of the reaction indicates that the reaction environment is highly hydro-phobic, so that no free water molecule attacks the intermediate. Even if some water molecules are present in the active site of the enzyme, they are entirely under the control of the enzyme. If this type of reaction can be realized using synthetic substrates, a new method will be developed for the preparation of optically active carboxylic acids that have a chiral center at the a-position. 

In the first step, pyruvate is decarboxylated in an irreversible reaction catalyzed by pyruvate decarboxylase. This reaction is a simple decarboxylation and does not involve the net oxidation of pyruvate. Pyruvate decarboxylase requires Mg24" and has a tightly bound coenzyme, thiamine pyrophosphate, discussed below. In the second step, acetaldehyde is reduced to ethanol through the action of alcohol dehydrogenase, with... 

The pyruvate decarboxylase reaction provides our first encounter with thiamine pyrophosphate (TPP) (Fig. 14-13), a coenzyme derived from vitamin B Lack of vitamin Bi in the human diet leads to the condition known as beriberi, characterized by an accumulation of body fluids (swelling), pain, paralysis, and ultimately death. 

The combined dehydrogenation and decarboxylation of pyruvate to the acetyl group of acetyl-CoA (Fig. 16-2) requires the sequential action of three different enzymes and five different coenzymes or prosthetic groups—thiamine pyrophosphate (TPP), flavin adenine dinucleotide (FAD), coenzyme A (CoA, sometimes denoted CoA-SH, to emphasize the role of the —SH group), nicotinamide adenine dinucleotide (NAD), and lipoate. Four different vitamins required in human nutrition are vital components of this system thiamine (in TPP), riboflavin (in FAD), niacin (in NAD), and pantothenate (in CoA). We have already described the roles of FAD and NAD as electron carriers (Chapter 13), and we have encountered TPP as the coenzyme of pyruvate decarboxylase (see Fig. 14-13). 

The conversion of pyruvate to ethanol occurs by the two reactions summarized in Figure 8.24. The decarboxylation of pyruvate by pyruvate decarboxylase occurs in yeast and certain microorganisms, but not in humans. The enzyme requires thiamine pyrophosphate as a coenzyme, and catalyzes a reaction similar to that described for pyruvate dehydrogenase (see p. 108). 

TPP-dependent enzymes catalyze either simple decarboxylation of a-keto acids to yield aldehydes (i.e. replacement of C02 with H+), or oxidative decarboxylation to yield acids or thioesters. The latter type of reaction requires a redox coenzyme as well (see below). The best known example of the former non-oxidative type of decarboxylation is the pyruvate decarboxylase-mediated conversion of pyruvate to acetaldehyde and C02. The accepted pathway for this reaction is shown in Scheme 10 (69MI11002, B-70MI11003, B-77MI11001>. 

At about the same time, Gunsalus and coworkers noticed that the activity of tyrosine decarboxylase produced by lactic acid bacteria was unusually low when the medium was deficient in pyridoxine. Addition of pyridoxal plus ATP increased the decarboxylase activity of cell extracts.146 PLP was synthesized and was found to be the essential coenzyme for this and a variety of other enzymes.147... 

Orotidine 5 -phosphate undergoes an unusual decarboxylation (Fig. 25-14, step/), which apparently is not assisted by any coenzyme or metal ion but is enhanced over the spontaneous decarboxylation rate 1017-fold. No covalent bond formation with the enzyme has been detected.268 It has been suggested that the enzyme stabilizes a dipolar ionic tautomer of the substrate. Decarboxylation to form an intermediate ylid would be assisted by the adjacent positive charge.269,270 Alternatively, a concerted mechanism may be assisted by a nearby lysine side chain.270a d Hereditary absence of this decarboxylase is one cause of orotic aciduria. Treatment with uridine is of some value.271... 

The conversion of pyruvate to acetyl-CoA. The reactions are catalyzed by the enzymes of the pyruvate dehydrogenase complex. This complex has three enzymes pyruvate decarboxylase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase. In addition, five coenzymes are required thiamine pyrophosphate, lipoic acid, CoASH, FAD, and NAD+. Lipoic acid is covalently attached to... 

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. 

Pyridoxine, pyridoxal, and pyridoxamine, which occur in foodstuffs, are collectively known as vitamin Bg. In the body, all three are converted to pyridoxal phosphate which is the coenzyme for amino-acid decarboxylase and for transaminase. The structures of the three active forms of vitamin Bg and the pyridoxal phosphate, are shown below (55). 

In contrast to mammals, /i-alanine 3 is generated in Escherichia coli [12] mainly by decarboxylation of L-aspartate 4 [13] (Scheme 1.6.3). The tetrameric enzyme, l-aspartate-a-decarboxylase (EC 4.1.1.15), was isolated from E. coli [13], cloned [14], and its crystal structure [15] was determined. In bacteria, pantothenate synthase converts /(-alanine to pantothenate, a constituent of coenzyme A [16],... 

Electrophilic action in enzymes is carried out by metal ion or coenzyme. The metal ion which is a component of the active center is bound to imidazole moiety in histidine by chelation. The typical enzymes are carboxypeptidase and decarboxylase, etc. A change in electronic structure of the metal ion by the chelation plays an important role in the catalysis. The catalytic activity of carboxypeptidase can be illustrated as follows (11),... 

Another example where mechanism and model have been developed is that for the decarboxylation of acetoacetic acid here no coenzyme is required, and the chemistry involves the enzyme itself. The mechanism for the enzymic decarboxylation with crystalline decarboxylase from Clostridium acetobutylicum has been worked out in some detail it is presented below (20, 21). The initial work, carried out in the author s laboratory by G. Hamilton (22) and I. Fridovich (23, 24) proved that the essential intermediate is a ketimine much of the subsequent development of the enzymic system resulted from the researches of W. Tagaki (25). 

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