Decarboxylation catalyzing

Phenazines — The phenazines are biosynthesized by the shikimic acid pathway, through the intermediate chorismic acid. The process was studied using different strains of Pseudomonas species, the major producers of phenazines. The best-known phenazine, pyocyanine, seems to be produced from the intermediate phenazine-1-carboxylic acid (PCA). Although intensive biochemical studies were done, not all the details and the intermediates of conversion of chorismic acid to PCA are known. In the first step, PCA is N-methylated by a SAM-dependent methyltransferase. The second step is a hydroxylative decarboxylation catalyzed by a flavoprotein monooxygenase dependent on NADH. PCA is also the precursor of phenazine-1-carboxamide and 1-hydroxyphenazine from Pseudomonas species. - - ... 

The ability to grow at the expense of 4-hydroxy- and 3,4-dihydroxybenzoate has been nsed for the classification of medically important yeasts inclnding Candida parapsilosis (Cooper and Land 1979). This organism degrades these snbstrates by oxidative decarboxylation, catalyzed by a flavoprotein monooxygenase (Eppink et al. 1997). 

This enzyme [EC 1.14.12.1], also known as anthranilate hydroxylase, decarboxylating, catalyzes the reaction of anthranilate with NAD(P)H, dioxygen, and two water molecules to produce catechol, carbon dioxide, NAD(P)+, and ammonia. The enzyme requires an iron ion as a cofactor. 

Researchers at the University of Graz, in collaboration with scientists from DSM, have developed an elegant and novel approach to the synthesis of P-amino alcohols using two different enzymes in one pot (Scheme 2.35). For example, a threonine aldolase-catalyzed reaction was initially used, under reversible conditions, to prepare L-70 from glycine 69 and benzaldehyde 68. L-70 was then converted to (R)-71 by an irreversible decarboxylation catalyzed by L-tyrosine decarboxylase. In a second example, D/L-syn-70 was converted to (R)-71 using the two enzymes shown combined with a D-threonine aldolase in greater than 99% e. e. and 67% yield ]37, 38]. 

Since aluminum ion is a strong catalyst, no valence change of the metal ion is involved in the reaction. The following observations of the decarboxylations catalyzed by ferric ion are strongly indicative of the above process. 

Phenol can also be prepared by the decomposition of benzoic acid prepared by the oxidation of toluene.927,978 The process is an oxidative decarboxylation catalyzed by copper(II). An interesting feature of this reaction is that the phenolic hydroxyl group enters into the position ortho to the carboxyl group as was proved by 14C labeling.979 In the Dow process980 molten benzoic acid is transformed with steam and air in the presence of Cu(II) and Mg(II) salts at 230-240°C. A copper oxide catalyst is used in a vapor-phase oxidation developed by Lummus.981... 

The fluoride-catalyzed, allylic C = C bond migration in a perfluoroalk-1-enc was first reported in 1953.27 Whereas pyrolysis of sodium perfluoropentanoate at 290-300 C gives per-fluorobut-1-enc (la) in high yield, the potassium salt decarboxylates at 165-200 C to yield principally perfluorobut-2-ene (2a). Apparently, the potassium fluoride formed in the decarboxylation catalyzes the isomerization. That sodium perfluoropentanoate yields no rearranged product demonstrates that sodium fluoride is ineffective in catalyzing the isomerization. 

As shown in Figure 8.4, the synthesis of NAD from tryptophan involves the nonenzymic cyclization of aminocarhoxymuconic semialdehyde to quinolinic acid. The alternative metahoUc fate of aminocarhoxymuconic semialdehyde is decarboxylation, catalyzed hy picolinate carboxylase, leading into the oxidative branch of the pathway, and catabolism via acetyl coenzyme A. There is thus competition between an enzyme-catalyzed reaction that has hyperbolic, saturable kinetics, and a nonenzymic reaction thathas linear, first-order kinetics. 

Two carbon atoms enter the cycle in the condensation of an acetyl unit (from acetyl CoA) with oxaloacetate. Two carbon atoms leave the cycle in the form of CO2 in the successive decarboxylations catalyzed by isocitrate... 

It is important to note that animals are unable to effect the net synthesis of glucose from fatty acids. Specifically, acetyl CoA cannot be converted into pyruvate or oxaloacetate in animals. The two carbon atoms of the acetyl group of acetyl CoA enter the citric acid cycle, but two carbon atoms leave the cycle in the decarboxylations catalyzed by isocitrate dehydrogenase and a-ketoglutarate dehydrogenase. Consequently, oxaloacetate is regenerated, but it is not formed de novo when the acetyl unit of acetyl CoA is oxidized by the citric acid cycle. In contrast, plants have two additional enzymes enabling them to convert the carbon atoms of acetyl CoA into oxaloacetate (Section 17.4.). 

Degradation of all three branched-chain amino acids begins with a transamination followed by an oxidative decarboxylation catalyzed by the branched-chain a-keto acid dehydrogenase complex. This enzyme, like a-ketoglutarate dehydrogenase, requires thiamine pyrophosphate, lipoic acid, coenzyme A, FAD, and NAD+ (Figure 7-11). 

In humans, TPP is a coenzyme for transketolation, an important reaction in the pentose-phosphate pathway, and for the oxidative decarboxylations catalyzed by pyruvate dehydrogenase, branched-chain a-ketoacid decarboxylase, and a-ketoglutarate dehydrogenase complexes. In lower organisms, TPP is also a cofactor for nonoxida-tive decarboxylations such as the conversion of pyruvate to acetaldehyde that occurs in yeast. 

Tryptophan hydroxylase uses 02 and the electron donor BH4 to hydroxylate C-5 of tryptophan. The product, called 5-hyroxy-tryptophan, then undergoes a decarboxylation catalyzed by 5-hydroxy tryptophan decarboxylase, a pyridoxal phosphate-requiring enzyme. Serotonin, often referred to as 5-hydroxytryptamine, is the product of this reacdon. 

The hydroxylation of tryptophan produces 5-hydroxytryptophan, which can then be decarboxylated, catalyzed by tryptophan decarboxylase, a PALP-requiring enzyme, to 5-hydroxy tryptamine, also known as serotonin. Serotonin is an important compound in normal brain function and tranquility. Therefore, any disturbance of tryptophan metabolism via this pathway can lead to mental disturbances. Serotonin can be destroyed by the enzyme monoamine oxidase (a flavo protein), which catalyzes the formation of ammonia and 5-hydroxyindole acetaldehyde in an irreversible reaction. The aldehyde is rapidly oxidized enzymatically, utilizing NAD+ to form 5-hydroxy indoleacetate, which is then usually excreted. The formation and turnover of serotonin can be estimated by 5-hydroxy indoleacetate output in the urine. 

Pyruvate decarboxylase is an enzyme that requires thiamine pyrophosphate. Pyruvate decarboxylate catalyzes the decarboxylation of pyruvate and transfers the resulting two-carbon fragment to a proton, resulting in the formation of acetaldehyde. 

The stereochemistry of the overall process has recently been studied. When [S-a- Hjhistidine is used as substrate, the product is [5-a- H]histamine. This observation requires retention of configuration, and is therefore analogous to the stereochemistry of decarboxylations catalyzed by pyridoxal, or by imine formation. 

Figure 11.20 Decarboxylation catalyzed by antibody with nonpolar microenvironment In the decarboxylation of 3-carboxybenzisoxazole by a tailored antibody, the anionic substrate is destabilized relative to the charge dispersed transition state in the nonpolar microenvironment of the antibody binding site. The hapten used to elicit the antibody is shown.

The racemic imidates 35 and carbamates 36, which were obtained from an MBH adduct, can be transformed into enantioenriched amides 37 and amines 38 via the regio- and enantioselective [l,3]-sigmatropic O- to A-rearrangement directly or through a decarboxylation catalyzed by cinchona alkaloids. 

Removal of the coproduct via an additional enzymatic step is usually more effective For instance, decarboxylation of an a-ketoacid (e.g., pyruvate or phenylpymvate, formed from alanine or phenylalanine, respectively) using pyruvate or phenylpymvate decarboxylase, yields an aldehyde and CO2 [1735, 1736]. In a similar fashion, pymvate may be removed by condensation to acetoin going in hand with decarboxylation catalyzed by acetolactate synthase [1737]. 

T. Furukl, F. Hosokawa, M. Sakurai, Y. Inoue and R. Chujo, Microscopic medium effects on a chemical reaction. A theoretical study of decarboxylation catalyzed by cyclodextrins as an enzyme model, J. Am. Chem. Soc., 115 (1993) 2903. 

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