Decarboxylation pyruvate

For preparative purposes fermenting baker s yeast (Saccharomyces cerevisiae) is commonly used instead of a purified enzyme preparation. However, isolated pyruvate decarboxylates can also be used30. In this context, the most important substrate is benzaldehyde31 which is converted by n-glucosc fermenting yeast to (7 )-l-hydroxy-l-phenyl-2-propanone. This conversion has gained considerable industrial importance because ( )-l-hydroxy-1-phenyl-2-propanonc is an important precursor for the synthesis of (-)-cphedrin. 

By the late 1930s it was widely accepted that active acetate arose from pyruvate decarboxylation and fatty acid oxidation. Acetate itself... 

CpPNO in carbohydrate metabolism is not yet known, a Nar/-like hydroge-nase has been identified in both C. parvum (Stejskal et al. 2003 Abrahamsen et al. 2004) and C. hominis (Xu et al. 2004), which may function to oxidize the NADPH produced by PNO during pyruvate decarboxylation. Unlike other amitochondriate protists (Entamoeba, Giardia, Trichomonas), neither of these Cryptosporidia possesses an [FeFe]-hydrogenase capable of transferring electrons produced during the oxidation of PFO (Horner et al. 2000). It is proposed that the acetyl-CoA resulting from the decarboxylation of pyruvate in C. parvum may then be converted to malonyl-CoA (Templeton et al. 

In the first step of the conversion catalyzed by pyruvate decarboxylase, a carbon atom from thiamine pyrophosphate adds to the carbonyl carbon of pyruvate. Decarboxylation produces the key reactive intermediate, hydroxyethyl thiamine pyrophosphate (HETPP). As shown in figure 13.5, the ionized ylid form of HETPP is resonance-stabilized by the existence of a form without charge separation. The next enzyme, dihydrolipoyltransacetylase, catalyzes the transfer of the two-carbon moiety to lipoic acid. A nucleophilic attack by HETPP on the sulfur atom attached to carbon 8 of oxidized lipoic acid displaces the electrons of the disulfide bond to the sulfur atom attached to carbon 6. The sulfur then picks up a proton from the environment as shown in figure 13.5. This simple displacement reaction is also an oxidation-reduction reaction, in which the attacking carbon atom is oxidized from the aldehyde level in HETPP to the carboxyl level in the lipoic acid derivative. The oxidized (disulfide) form of lipoic acid is converted to the reduced (mer-capto) form. The fact that the two-carbon moiety has become an acyl group is shown more clearly after dissocia-... 

Scheme 3. Reaction path of enzymatic pyruvate decarboxylation and formation of a-hydroxy ketones...

In yet another study the thiazolium ring was attached to a macrotricyclic quaternary ammonium ion 9, bearing several positive charges to determine if rate accelerations of pyruvate decarboxylation could be observed46. Such rate accelerations could indeed be observed, especially for phenylpyruvic acid as a substrate. In addition, lumiflavin-3-acetic acid as a potential oxidant of the intermediate (see the oxidative decarboxylation pathway in Scheme 1) was shown to be reduced by the pyruvic acid analog in the presence of DBU in ethanol and the macrotricyclic quaternary ammonium salt. 

Unfortunately diacetyl formation is still not well understood. Acetoin formation occurs either by nonspecific interaction of acetaldehyde with the a-hydroxyethyl thiamine pyrophosphate intermediate in pyruvate decarboxylation (209) or by decarboxylation of a-acetolactate (210), which in turn arises either from interaction of pyruvate with a-hydroxyethyl thiamine pyrophosphate (211) or as a specific intermediate in valine biosynthesis (212, 213). Diacetyl does not appear to be formed directly from acetoin (208, 214). It is formed from a-acetolactate, in absence of cells, by O2 oxidation (215), and even under N2 (216), although an oxidation must occur. It is also formed from acetyl CoA (217, 218), probably by interaction with a-hydroxyethyl thiamine pyrophosphate [cf. stimulation by acetyl CoA addition to a solution of pyruvate and pyruvate decarboxylase (2i5)]. It is not known whether this involves a specific enzyme or is a mere side reaction. 

Under physiological conditions, alcohol may be formed in intermediary metabolism as an intermediate product, e. g. in pyruvate decarboxylation and threonine degradation. Here minimal amounts of alcohol may arise (blood alcohol concentration of about 0.0009-... 

The metabolic functions of pantothenic acid in human biochemistry are mediated through the synthesis of CoA. Pantothenic acid is a structural component of CoA. which is necessary for many important metabolic processes. Pantothenic acid is incorporated into CoA by a. series of five enzyme-catalyzed reactions. CoA is involved in the activation of fatty acids before oxidation, which requires ATP to form the respective fatty ocyl-CoA derivatives. Pantothenic acid aI.so participates in fatty acid oxidation in the final step, forming acetyl-CoA. Acetyl-CoA is also formed from pyruvate decarboxylation, in which CoA participates with thiamine pyrophosphate and lipoic acid, two other important coenzymes. Thiamine pyrophosphate is the actual decarboxylating coenzyme that functions with lipoic acid to form acetyidihydrolipoic acid from pyruvate decarboxylation. CoA then accepts the acetyl group from acetyidihydrolipoic acid to form acetyl-CoA. Acetyl-CoA is an acetyl donor in many processes and is the precursor in important biosyntheses (e.g.. those of fatty acids, steroids, porphyrins, and acetylcholine). 

Ethanol accumulated in maturing citrus fruit as the end product of pyruvate decarboxylation. Conditions that promote this reaction include low and high CC, and ethylene levels. Maturation increased the levels of PDC and ADH and increased the NADH to NAD ratio. The higher redox ratio could slow the PDH reaction which competes with PDC for pyruvate. Development of the alternative oxidase activity when ethanol began to accumulate suggests that membrane function was modified which affected rates of various metabolic pathways. The lower phosphorylation efficiency of the alternative oxidase compared to the cytochrome pathway (22) could affect numerous metabolic activities including decarboxylation of pyruvate. Also, membrane transport of pyruvate and cofactors could be altered in mitochondria containing fewer phosphorylation sites (26). 

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. 

Yano and coworkers synthesized the 18-crown-6-thiazolium conjugate 7. It was found that the conjugate accelerated the flavin-dependent oxidative trapping of the enamine produced from pyruvate decarboxylation in the presence of alkali metal cations such as and Na" ", but not Li, indicating the specificity of the particular crown ether for the larger cations. The authors suggested that the pyruvate anion is held by the crown-ether-bound metal ion in the proximity of the thiazolium ion, thereby accelerating the nucleophilic attack. 

Starting with the work of Lienhard and his coworkers there have been several contributions in the literature that indicate that the thiazolium catalyzed decarboxylation is significantly accelerated by the use of a low dielectric medium, leading to the suggestion that a major source of rate acceleration in enzymes that perform this reaction is provided by a medium effect. As estimated recently by Alvarez and coworkers , the enzyme accelerates the overall pyruvate decarboxylation reaction by a factor of 10, over and above the corresponding thiamin catalyzed rate. It was also estimated that the enzyme accelerates the decarboxylation step by a factor of 10. ... 

The behavior of lipoamide dehydrogenase in the intact pyruvate dehydrogenase complex has been investigated. The multienzyme complex consists of 24 pyruvate decarboxylase subunits, 24 dihydrolipoyl transacetylase subunits, and 12 lipoamide dehydrogenase subunits. The complex catalyzes a series of reactions beginning with pyruvate decarboxylation, followed by transfer of an acetyl moiety to CoA, and finally the oxidation of dihydrolipoamide by NAD. Stopped-flow smdies showed that lipoamide dehydrogenase in the complex behaves largely the same as it does when studied alone. 

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