Decarboxylase synthetic

A classical approach to driving the unfavorable equilibrium of an enzymatic process is to couple it to another, irreversible enzymatic process. Griengl and coworkers have applied this concept to asymmetric synthesis of 1,2-amino alcohols with a threonine aldolase [24] (Figure 6.7). While the equilibrium in threonine aldolase reactions typically does not favor the synthetic direction, and the bond formation leads to nearly equal amounts of two diastereomers, coupling the aldolase reaction with a selective tyrosine decarboxylase leads to irreversible formation of aryl amino alcohols in reasonable enantiomeric excess via a dynamic kinetic asymmetric transformation. A one-pot, two-enzyme asymmetric synthesis of amino alcohols, including noradrenaline and octopamine, from readily available starting materials was developed [25]. 

Pretreatment of rats with difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, prior to exposure to a tremorigenic dose of chlordecone, also resulted in inhibition of the tremor (Tilson et al. 1986b). DFMO was more effective if given 5 hours prior to the chlordecone than if given 24 hours prior to exposure. The DFMO was ineffective if given 19 hours after chlordecone exposure. These results suggest an interaction of the polyamine synthetic pathway with tremors produced by chlordecone. The mechanism of the interaction is unclear but may involve effects of polyamines on intracellular calcium homeostasis. Persons being treated with DFMO for cancer or protozoal infections would be likely to have reduced tremor severity after exposure to chlordecone. 

Figure 1. Synthetic pathway for PS and PE in mammalian cells. The major steps occuring in the synthesis and interconversion of PS and PE are shown. The PS synthases condense serine with a phosphatidyl moiety derived from PC and PE. The nascent PS can be converted to PE by decarboxylation. PE can also be formed by transfer of a phosphoethanolamine moiety from CDP-ethanolamine to diacylglycerol via the Kennedy pathway. The abbreviations used are PC, phosphatidylcholine PS, phosphatidylserine PE, phosphatidylethanolamine DG, diacylglycerol PSD, phosphatidylserine decarboxylase PSS, PS synthase.

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. 

Histamine occurs in the brain, particularly in certain hypothalamic neurons, and evidence is strong that histamine is a neurotransmitter. Distribution of histamine, its synthetic enzyme (histidine decarboxylase), and methyl histamine (the major brain metabolite) is not uniform. Possible roles for histamine in the regulation of food and water intake, thermoregulation, hormone release, and sleep have been suggested. Additional information on histamine can be found in Chapter 38. 

Mechanism of Action A topical antiprotozoal that inhibits ornithine decarboxylase cell division and synthetic function in the skin. Therapeutic Effect Reduces rate of hair growth. 

Fig. 30. The development of a synthetic decarboxylase (81) [74], An a-helical peptide framework orients correctly several amino groups for the catalysis of the decarboxylation of 82. (Reproduced with the permission of Ref. 1)...

Noradrenergic neurons. The noradrenergic neuron uses NE for its neurotransmitter. Monoamine neurotransmitters are synthesized by means of enzymes, which assemble neurotransmitters in the cell body or nerve terminal. For the noradrenergic neuron, this process starts with tyrosine, the amino acid precursor of NE, which is transported into the nervous system from the blood by means of an active transport pump (Fig. 5 — 17). Once inside the neuron, the tyrosine is acted on by three enzymes in sequence, the first of which is tyrosine hydroxylase (TOH), the rate-limiting and most important enzyme in the regulation of NE synthesis. Tyrosine hydroxylase converts the amino acid tyrosine into dihydroxyphenylalanine (DOPA). The second enzyme DOPA decarboxylase (DDC), then acts, converting DOPA into dopamine (DA), which itself is a neurotransmitter in some neurons. However, for NE neurons, DA is just a precursor of NE. In fact, the third and final NE synthetic enzyme, dopamine beta-hydroxylase (DBH), converts DA into NE. The NE is then stored in synaptic packages called vesicles until released by a nerve impulse (Fig. 5—17). 

There are significant differences in the reactivity of synthetic intermediate analogues for these reactions and the corresponding intermediates in the enzymic system. Lienhard and coworkers42,43 reported that the rate of decarboxylation of 2-(l-carboxy-l-hydroxyethyl)-3,4-dimethylthiazolium chloride is very fast relative to pyruvate (whose reaction is too slow to observe) but slower than the enzymic decarboxylation of pyruvate decarboxylase (PDC) by a factor of 105. Similar observations of a catalytic gap were seen for the rate of decarboxylation of lactylthiamin compared to PDC and the... 

Because of the difficulties of stabilizing the pH at the pH optimum of 6.0 owing to liberation of C02, a loop reactor was developed which keeps the C02 dissolved at lObar and thus helps to stabilize the pH. Co-immobilization of E. coli and Ps. dacunhae cells for direct production of L-alanine from fumaric acid was not successful because E. coli cells work best at a pH of 8.5, in comparison with a pH of 6.0 for Ps. dacunhae cells and the decarboxylase. The sequential process has been run since 1982. The high enantioselectivity of L-aspartate-/kdecarboxylase (ADC) led to a process, in 1989, in which inexpensive DL-aspartate was converted to L-alanine and D-aspartate [Eq. (7.1)] the latter commands interest for synthetic penicillins ... 

The known properties of proteins and the known mechanism of amine-catalyzed decarboxylation of /3-oxo acids (Laursen, 1966) were combined to generate a synthetic decarboxylase in the form of two tetradecameric peptides, termed Oxaldie 1 and 2, with leucine and lysine as the main amino acids (Johnsson, 1993). 

Ornithine decarboxylase catalyzes the transformation of ornithine to the polycationic bases, putresine, spermine, and spermidine. These compounds exert regulatory effects on cell growth. It has been shown that quercetin (10 to 30 pmol/mouse) markedly suppressed the stimulatory effect of the transporters associated with antigen processing (TPA) on ornithine decarboxylase (ODC) activity and on skin tumor formation in mice initiated with dimethylbenzanthracene. Such inhibition may be related to the activation of the catalytic site, which is under nonconventional regulation by small molecules. Also, the synthetic flavonoid flavone acetic acid was shown to inhibit the activity of ODC in stimulated human peripheral blood lymphocytes and human colonic lamina propria lymphocytes. 

Enzymatic reactions forming new carbon-carbon bonds are a further important field of biotransformations in natural product synthesis. The construction of new, often complex carbon frameworks or their decomposition is performed by nature under catalysis of a set of enzymes. For organic chemists some of these enzymes, belonging to the enzyme class of lyases, such as aldolases, decarboxylases, hydro-xynitrile lyases (HNLs), or benzaldehyde lyases (BALs), have been proven to represent versatile amendments to their synthetic toolbox. 

Although the utility of transaminases has been widely examined, one such limitation is the fact that the equilibrium constant for the reaction is near unity. Therefore, a shift in this equilibrium is necessary for the reaction to be synthetically useful. A number of approaches to shift the equilibrium can be found in the literature.53 124135 Another method to shift the equilibrium is a modification of that previously described. Aspartate, when used as the amino donor, is converted into oxaloacetate (32) (Scheme 19.21). Because 32 is unstable, it decomposes to pyruvate (33) and thus favors product formation. However, because pyruvate is itself an a-keto acid, it must be removed, or it will serve as a substrate and be transaminated into alanine, which could potentially cause downstream processing problems. This is accomplished by including the alsS gene encoding for the enzyme acetolactate synthase (E.C. 4.1.3.18), which condenses two moles of pyruvate to form (S)-aceto-lactate (34). The (S)-acetolactate undergoes decarboxylation either spontaneously or by the enzyme acetolactate decarboxylase (E.C. 4.1.1.5) to the final by-product, UU-acetoin (35), which is meta-bolically inert. This process, for example, can be used for the production of both l- and d-2-aminobutyrate (36 and 37, respectively) (Scheme 19.21).8132 136 137... 

As an obvious extension of the benzoin reaction, the cross-coupling of aldehydes or of aldehydes and ketones was first achieved with the thiamine-dependent enzyme benzoylformate decarboxylase. This linked a variety of mostly aromatic aldehydes to acetaldehyde to form the corresponding a-hydroxy ketones, both chemo- and stereoselectively [31]. Synthetic thiazolium salts, developed by Stetter and co-workers and similar to thiamine itself [32], have been successfully used by Suzuki et al. for a diastereoselective intramolecular crossed aldehyde-ketone benzoin reaction during the course of an elegant natural product synthesis [33], Stereocontrol was exerted by pre-existing stereocenters in the specific substrates, the catalysts being achiral. 

Class 4. Lyases split a molecule by a nonhydrolytic process, leaving double bonds (or alternatively, by adding groups to double bonds). They include decarboxylases, aldolases, hydratases, dehydratases, and synthases (synthetic enzymes). 

Dekkera brwcellensis Spoilage yeast Active hydroxycinnamate decarboxylase and vinylphenol reductase producing ethylphenols in synthetic media, juices and wines Heresztyn (1986) Chatonnet et al. (1995) Shinohara et al. (2000) Rodrigues et al. (2001) Dias et al. (2003a, 2003b)... 

Pichia guilliermondi Contamination yeast Active hydroxycinnamate decarboxylase and vinylphenol reductase producing ethylphenols in synthetic media and grape juices Barata et al. (2006)... 

DOPA to dopamine, by the cytosolic enzyme, DOPA decarboxylase. In the central and peripheral nervous systems, dopamine is converted to noradrenaline by dopamine-P-hydroxylase (DBH), which, though a relatively non-specific enzyme, is restricted to catecholamine-synthesizing cells. It can be inhibited by many drugs, which brings the risk of complex drug interactions. In the peripheral sympathetic nervous system, noradrenaline, in turn, is converted to adrenaline, by phenylethylamine N-methyl transferase, so inhibition of DBH can therefore, in principle, slow production of both adrenaline and noradrenaline but normally tyrosine hydroxylase is the rate-limiting step in the synthetic pathway. 

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