Enzyme pyruvate oxidase

Structure of the thiamine- and flavin-dependent enzyme pyruvate oxidase. Science 259, 965-967. 

Laval and coworkers similarly prepared a monolayer of a lipid on top of the octadecanethiol film on gold. The enzyme pyruvate oxidase was then incorporated in the lipid layer in a fashion similar to that of lipid vesicles, and the activity of the immobilized enzyme was measured431. 

Crystallography has also resulted in the identification of several unique intermediates such as the penta-covalent phosphorus intermediate of /3-phosphoglucomutase reaction and the thiamin intermediates in the thiamin diphosphate and flavin-dependent enzyme, pyruvate oxidase. " ... 

Muller, Y.A., and Schulz, G.E., 1993. Structure of the thiamine- and flavin-dependent enzyme pyruvate oxidase. Science. 259 965-967. 

The pyruvate oxidase system (p. 139) should now be interpreted as pyruvate dihydrogenase together with the enzymes of the tricarboxylic acid cycle. The fact that sodium fluoroacetate itself did not poison the tricarboxylic acid cycle enzymes in... 

Oxidative decarboxylations of a-keto acids are mediated by either enzymes having more than one cofactor or complex multienzyme systems utilizing a number of cofactors. For example, pyruvate oxidase uses TPP and FAD as coenzymes, the function of the latter being to oxidize the intermediate (41). Conversion of pyruvate to acetyl-CoA requires a multienzyme complex with the involvement of no less than five coenzymes, TPP, CoA, dihydrolipoate, FAD and NAD+ (74ACR40). 

By 1998, X-ray structures had been determined for four thiamin diphosphate-dependent enzymes (1) a bacterial pyruvate oxidase,119120 (2) yeast and bacterial pyruvate decarboxylases,121 122c (3) transketolase,110123124 and (4) benzoylformate decarboxylase.1243 Tire reactions catalyzed by these enzymes are all quite different, as are the sequences of the proteins. However, the thiamin diphosphate is bound in a similar way in all of them. 

A related reaction that is known to proceed through acetyl-TDP is the previously mentioned bacterial pyruvate oxidase. As seen in Fig. 14-2, this enzyme has its own oxidant, FAD, which is ready to accept the two electrons of Eq. 14-22 to produce bound acetyl-TDP. The electrons may be able to jump directly to the FAD, with thiamin and flavin radicals being formed at an intermediate stage.1353 The electron transfers as well as other aspects of oxidative decarboxylation are discussed in Chapter 15, Section C. 

Pyruvate oxidase. The soluble flavoprotein pyruvate oxidase, which was discussed briefly in Chapter 14 (Fig. 14-2, Eq. 14-22), acts together with a membrane-bound electron transport system to convert pyruvate to acetyl phosphate and C02.319 Thiamin diphosphate is needed by this enzyme but lipoic acid is not. The flavin probably dehydrogenates the thiamin-bound intermediate to 2-acetylthiamin as shown in Eq. 15-34. The electron acceptor is the bound FAD and the reaction may occur in two steps as shown with a thiamin diphosphate radical intermediate.3193 Reaction with inorganic phosphate generates the energy storage metabolite acetyl phosphate. 

Tire enzyme does not require lipoic acid. It seems likely that a thiamin-bound enamine is oxidized by an iron-sulfide center in the oxidoreductase to 2-acetyl-thiamin which then reacts with CoA. A free radical intermediate has been detected318 321 and the proposed sequence for oxidation of the enamine intermediate is that in Eq. 15-34 but with the Fe-S center as the electron acceptor. Like pyruvate oxidase, this enzyme transfers the acetyl group from acetylthiamin to coenzyme A. Cleavage of the resulting acetyl-CoA is used to generate ATR An indolepyruvate ferredoxin oxidoreductase has similar properties 322... 

Mechanism of thiamine pyrophosphate action. Intermediate (a) is represented as a resonance-stabilized species. It arises from the decarboxylation of the pyruvate-thiamine pyrophosphate addition compound shown at the left of (a) and in equation (2). It can react as a carbanion with acetaldehyde, pyruvate, or H+ to form (b), (c), or (d), depending on the specificity of the enzyme. It can also be oxidized to acetyl-thiamine pyrophosphate (TPP) (e) by other enzymes, such as pyruvate oxidase. The intermediates (b) through (e) are further transformed to the products shown by the actions of specific enzymes. 

The -SH groups of dimercaptopropanol react with heavy metal ions including arsenic, to form very stable five-membered chelate rings, displacing heavy metal ions that would otherwise bind to essential -SH groups of enzymes such as succinoxidase and pyruvic oxidase. In this way, most of the enzyme activity can be restored if therapy is commenced soon after exposure. 

Coupling of a second enzyme reaction NAD(P)H+02->NADH oxidase NAD(P)++H20a->H202 monitoring [26] (For the particular case of lactate dehydrogenase, where pyruvate is one of the reaction products, pyruvate oxidase or pyruvate transaminase has been coupled to the main enzyme reaction) [23,27]... 

Other types of enzymes When no oxidase or dehydrogenase is available for a target analyte, other types of enzymes have been used for biospedfic recognition e.g. for citric acid detection, citrate lyase, and amperometric detection was possible by coupling to two more enzymatic reactions oxaloacetate decarboxylase and pyruvate oxidase, which convert citric add into H2O2 with the latter being monitored amperometrically with an H202 probe. For detection of acetic add, acetate kinase is used, coupled to pyruvate kinase and pyruvate oxidase [34,35]. 

An important parameter in a number of fields is the study of inorganic phosphate. Recently, Kwan et al. [206,207] have reported on a screen-printed phosphate biosensor based on immobilised pyruvate oxidase (PyOD) for monitoring phosphate concentrations in a sequencing batch reactor system [206] and in human saliva [207]. The enzyme was immobilised by drop-coating a Nation solution onto the working electrode surface this was then covered by a poly(carbamoyl) sulfonate (PCS) hydrogel membrane. 

Berberine inhibits oxidative decarboxylation of yeast pyruvic acid (310) the same dose has, however, no effect upon aerobic glycolysis, Warburg s respiratory enzymes, indophenol oxidase, etc. Berberine and tetrahydroberberine have an inhibitory effect on oxidation of (+ )-alanine in rat kidney homogenates (498). Berberine and palmatine show a specific inhibitory effect upon cholinesterase in rabbit spleen and on pseudocholinesterase in horse serum (499). Berberine inhibits cellular respiration in ascitic tumors and even in tissue cultures (500-502). The specific toxic effect of berberine on the respiration of cells of ascitic tumors in mice was described (310). The glycolysis was not found to be affected, but the uptake of oxygen was smaller. Fluorescence was used in order to demonstrate berberine in cellular granules. Hirsch (503) assumed that respiration is inhibited by the effect of berberine on the yellow respiratory enzymes. Since the tumorous tissue contains a smaller number of yellow respiratory enzymes than normal tissue it is more readily affected by berberine. Subcutaneous injections of berberine, palmatine, or tetrahydropalmatine significantly reduce the content of ascorbic acid in the suprarenals, which is not affected by hypophysectomy (504). 

Recently, Schloss et al (33) showed that IM and TP were able to quantitatively displace a radiolabelled SU herbicide from ALS, indicating competitive binding. Curiously, the SU ligand was also displaced by the quinone, Qo. It was proposed that SU, TP, and IM bind to ALS in a vestigial quinone binding site associated with the evolution of ALS from pyruvate oxidase. This enzyme is an FAD-protein that catalyzes the oxidation of pyruvate to acetate. 

Several aaRS-like proteins are involved in metabobc pathways (1). For example, E. coli asparagine synthase, an aspartyl-tRNA synthetase (AspRS)-like enzyme, catalyzes the synthesis of asparagine from aspartate and ATP. A paralog of LysRS-II, called PoxA/GenX, is important for pyruvate oxidase activity in E. coli and Salmonella typhimurium and for virulence in S. typhimurium. The E. coli biotin synthetase/repressor protein (BirA), which has a domain that resembles structurally the seryl-tRNA synthetase (SerRS) catalytic domain, activates biotin to modify posttranslationaUy various metabolic proteins involved in carboxylation and decarboxylation. BirA can also bind DNA and regulate its own transcription using biotin as a corepressor. A histidyl-tRNA synthetase (HisRS)-hke protein from Lactococcus lactis, HisZ is involved in the allosteric activation of the phosphoribosyl-transferase reaction. 

The deprotonation and addition of a base to thiazolium salts are combined to produce an acyl carbanion equivalent (an active aldehyde) [363, 364], which is known to play an essential role in catalysis of the thiamine diphosphate (ThDP) coenzyme [365, 366]. The active aldehyde in ThDP dependent enzymes has the ability to mediate an efScient electron transfer to various physiological electron acceptors, such as lipoamide in pyruvate dehydrogenase multienzyme complex [367], flavin adenine dinucleotide (FAD) in pyruvate oxidase [368] and Fc4S4 cluster in pyruvate ferredoxin oxidoreductase [369]. 

Sensors have also been constructed from some oxidases directly contacted to electrodes to give bioelectrocatalytic systems. These enzymes utilize molecular oxygen as the electron acceptor for the oxidation of their substrates. Enzymes such as catechol oxidase, amino acid oxidase, glucose oxidase, lactate oxidase, pyruvate oxidase, alcohol oxidase, xanthine oxidase and cholesterol oxidase catalyze the oxidation of their respective substrates with the concomitant reduction of O2 to H2O2 ... 

We have in the present chapter shown results from theoretical model system studies of the catalytic reaction mechanisms of three radical enzymes Galatose oxidase. Pyruvate formate-lyase and Ribonucleotide reductase. It is concluded that small models of the key parts of the active sites in combination with the DPT hybrid functional B3LYP and large basis sets provides a good description of the catalytic machineries, with low barriers for the rate determining steps and moderate overall exothermicity. The models employed are furthermore able to reproduce all the observed features in terms of spin distributions and reactive intermediates. 

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