Decarboxylation, nonoxidative

MC Scrutton, MR Young, AW Alberts, PR Vagelos, HG Wood, MF Utter, HM Kolenbrander, MI Siegel, M Wishnick, MD Lane, BB Buchman, EA Boecker, EE Snell, I Fridovich. In PD Boyer, ed. The Enzymes, 3rd ed. Carboxylation and Decarboxylation (Nonoxidative). New... 

A pure culture of the organism was inoculated into a basal medium with the addition of 0.025% caffeic acid. After 7 days incubation at 25°C under conditions of reduced oxygen tension, the caffeic acid was completely metabolized. Metabolites of caffeic acid are identified as dihydrocaffeic acid and ethyl catechol, respectively. In the 1960s, it has been reported that a constitutive enzyme present in strains of Aerobacter decarboxylates caffeic acid to 4-vinylcatechol nonoxidatively [20], Several cinnamic acids have been tested and the decarboxylation product from /7-coumaric acid has been identified as 4-vinylphenol. Thus, the bacterial enzyme activity requires a relatively unhindered 4-hydroxy group on the aromatic ring and an acrylic acid side chain. 

Thiamine, biotin and pyridoxine (vitamin B) coenzymes are grouped together because they catalyze similar phenomena, i.e., the removal of a carboxyl group, COOH, from a metabolite. However, each requires different specific circumstances. Thiamine coenzyme decarboxylates only alpha-keto acids, is frequently accompanied by dehydrogenation, and is mainly associated with carbohydrate metabolism. Biotin enzymes do not require the alpha-keto configuration, are readily reversible, and are concerned primarily with lipid metabolism. Pyridoxine coenzymes perform nonoxidative decarboxylation and are closely allied with amino acid metabolism. 

Pyridoxal phosphate enzymes mediate the nonoxidative decarboxylation of amino acids. This mechanism is of primary importance in bacteria, but it may be essential to proper function of the nervous system in humans... 

The a-keto acid decarboxylases such as pyruvate (E.C. 4.1.1.1) and benzoyl formate (E.C. 4.1.1.7) decarboxylases are a thiamine pyrophosphate (TPP)-dependent group of enzymes, which in addition to nonoxidatively decarboxylating their substrates, catalyze a carboligation reaction forming a C-C bond leading to the formation of a-hydroxy ketones.269-270 The hydroxy ketone (R)-phenylacetylcarbinol (55), a precursor to L-ephedrine (56), has been synthesized with pyruvate decarboxylase (Scheme 19.35). BASF scientists have made mutations in the pyruvate decarboxylase from Zymomonas mobilis to make the enzyme more resistant than the wild-type enzyme to inactivation by acetaldehyde for the preparation of chiral phenylacetylcarbinols.271... 

Pyruvate decarboxylase catalyzes the nonoxidative decarboxylation of pyruvate to acetaldehyde and carbon dioxide. When an aldehyde is present with pyruvate, the enzyme promotes an acyloin condensation reaction. The mechanistic reason for this fortuitous reaction is well understood and involves the aldehyde outcompeting a proton for bond formation with a reactive thiamine pyrophosphate-bound intermediate (90,91). When acetaldehyde is present, the product formed is acetoin. Benzalde-hyde results in the production of phenylacetylcarbinol (Fig. 26). Both of these condensations are enantioselective, forming the R enantiomer preferentially in both cases. 

After biochemical conversion of glucose to pyruvic acid intermediate, the next step in ethanol synthesis is nonoxidative decarboxylation and acetaldehyde formation catalyzed by a native decarboxylase, and then acetaldehyde reduction to ethanol catalyzed by a native dehydrogenase. 

Figure 4. Possible routes for the formation of 3-mercaptoproplonate and methanethlol. I = Oxidative deamination or transamination II. Oxidative decarboxylation III. Nonoxidative decarboxylation IV. Oxidation V. Demethylation VI. Demethlolatlon VII. Aerobic/anaerobic catabolism VIII. Michael addition.

The simplest example of such reactions is the decarboxylation of pyruvate. Both model and enzyme studies have shown the intermediacy of covalent complexes formed between the cofactor and the substrate. Kluger and coworkers have studied extensively the chemical and enzymatic behavior of the pyruvate and acetaldehyde complexes of ThDP (2-lactyl or LThDP, and 2-hydroxyethylThDP or HEThDP, respectively) . As Scheme 1 indicates, the coenzyme catalyzes both nonoxidative and oxidative pathways of pyruvate decarboxylation. The latter reactions are of immense consequence in human physiology. While the oxidation is a complex process, requiring an oxidizing agent (lipoic acid in the a-keto acid dehydrogenases , or flavin adenine dinucleotide, FAD or nicotinamide adenine dinucleotide , NAD " in the a-keto acid oxidases and Fe4.S4 in the pyruvate-ferredoxin oxidoreductase ) in addition to ThDP, it is generally accepted that the enamine is the substrate for the oxidation reactions. 

Although benzoate is generally metabolized by oxidative decarboxylation to catechol followed by ring cleavage, nonoxidative decarboxylation may also occur (1) strains of Bacillus megaterium transform vanillate to guaiacol by decarboxylation (Crawford and Olson 1978) and (2) a number of decarboxylations of aromatic carboxylic acids by facultatively anaerobic Enterobacteri-aceae have been noted in Chapter 4, Section 4.3.2. 

The coenzyme thiamin diphosphate (ThDP, I in Scheme 16.1), the biologically active form of vitamin Bi, is used by different enzymes that perform a vide range of catalytic functions, such as the oxidative and nonoxidative decarboxylation of a-ketoacids, the formation of acetohydroxyacids and ketol transfer bet veen sugars. 

In addition to its role in the action of pyruvate dehydrogenase, thiamine pyrophosphate (TPP) serves as a cofactor for other enzymes, such as pyruvate decarboxylase, which catalyzes the nonoxidative decarboxylation of pyruvate. Propose a mechanism for the reaction catalyzed by pyruvate decarboxylase. What product would you expect Why, in contrast to pyruvate dehydrogenase, are lipoamide and FAD not needed as cofactors for pyruvate decarboxylase ... 

In vivo, pyruvate decarboxylase [EC 4.1.1.1] catalyzes the nonoxidative decarboxylation of pyruvate to acetaldehyde and is thus a key enzyme in the fermentative production of ethanol. The most well-studied PE)Cs are obtained from baker s yeast [1477, 1482, 1483] and from Zymomonas mobilis [1484]. 

BED [EC 4.1.1.7] is derived from mandelate catabolism, where it catalyzes the nonoxidative decarboxylation of benzoyl formate to yield benzaldehyde. Again, the reverse carboUgatiMi reaction is more important [1488-1490]. As may be deduced from its natural substrate, is exhibits a strong preference for large aldehydes as donor substrates encompassing a broad range of aromatic, heteroaromatic, cyclic aliphatic and olefinic aldehydes [1480]. With acetaldehyde as acceptor, it yields the complementary regio-isomeric product to PDC (Scheme 2.200). 

Anaerobic P. d., anaerobic xartthine degradation. In certain microorganisms Micrococcus and Clostridium) the substrate of nonoxidative P.d. is xanthine. Hydrolysis between C6 and N1 of the 6-membered ling of xanthine produces ureidoimidazolyl-carboxyl-ic add. Further hydrolytic removal of ammonia and CO2 produces aminoimidazolecarboxylic add. This is decarboxylated to aminoimidazole. The ring of ami-... 

Most of ThDP-dependent enzymes catalyse the processing of 2-keto acids such as pyruvate, branched-chain keto acids and ketoglutarate. Among them, pyruvate is processed by various ThDP-dependent enzymes (Figure 4.2). Its carbonyl group is attacked by the ThDP ylide, yielding a tetrahedral 2-(2-lactyl)-ThDP adduct. The reactions followed by decarboxylation can be roughly classified into oxidative or nonoxidative ones. 

PDC is a representative of ThDP-enzymes that catalyses a simple nonoxidative conversion of pyruvate—the decarboxylation of pyruvate into acetaldehyde and carbon dioxide. PDC is widely distributed in plants and fungi, but it is rarely found in bacteria and not found in animals. PDC catalyses a key reaction in alcohol fermentation by yeast. The structure and function of PDCs from... 

The DC family contains enzymes catalysing the decarboxylation of a 2-keto acid and it is the largest family including at least 16 enzymes. Of these, POX is atypical member of the DC family, because it catalyses a redox reaction, whereas others do not. The TK family consists of nonoxidizing enzymes such as TK and PK. 

The structure of cocarboxylase (see Fig. 1 for formula) was established by Lohmann and Schuster. A long-established function of cocarboxylase (thiamine pyrophosphate, aneurin) is as the coenzyme of a-ketoaeid carboxylase. Mg++ is also required. The reaction involved is the nonoxidative decarboxylation of an a-keto acid to CO2 and an aldehyde with one less carbon atom. The most important example is the splitting of pyruvic acid to acetaldehyde and CO2 (equation 9). This... 

The enzyme was found in Arthrobacter nicotianae FI1612 and several molds. Its activity was induced specifically by indole-3-carboxylate, but not by indole. The indole-3-carboxylate decarboxylase of A. nicotianae FI1612 catalyzed the nonoxidative decarboxylation of indole-3-carboxylate into indole, and efficiently carboxylated indole and 2-methyhndole by the reverse reactirai. 

Yoshida T, Fujita K, Nagasawa T (2002) Novel reversible indole-3-carboxylate decarboxylase catalyzing nonoxidative decarboxylation. Biosci Biotechnol Biochem 66(11) 2388-2394... 

The coenzyme participates in reactions involving formation and breaking of carbon-carbon bonds immediately adjacent to a carbonyl group. Examples include nonoxidative and oxidative decarboxylations and aldol condensations. For instance it is involved in the nonoxidative decarboxylation of pyruvic acid to acetaldelyde ... 

Decarboxylative Sulfonation. When a solution of an 0-acyl thiohydroxamate in CH2CI2 containing excess SO2 is photolyzed at —10 °C, the initially formed radicals are intercepted by the latter before decarboxylative rearrangement can take place. The resulting radicals (RS02 ) then react with a second molecule of the O-acyl thiohydroxamate to give an 5-pyridyl alkylthiosulfonate (38-91%) and the radical R (eq 9). The products are useful as precursors to unsymmetrical sulfones and sulfonamides, significantly by a nonoxidative procedure. 

A third pathway of protocatechuic acid metabolism occurs in a Rhodo-pseudomonas. Proctor and Scher (1960) have reported that this organism forms protocatechuate from benzoate, then decarboxylates the product to catechol. (This is the second example of a nonoxidative aromatic decarboxylase.) Subsequent metabolism of catechol yields a keto acid not yet identified, but possibly that produced in other catechol-utilizing systems described below. A unique feature of the Rhodopseudomonas system is its reported dependence on hydrogen peroxide for the oxidation of catechol. It will be of interest to learn whether peroxide is consumed stoichiometrically in the reaction, or whether it is an activator, as has been found for tryptophan pyrrolase. 

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