Pyruvate decarboxylase properties

In order to further characterize the key role of the 4 -amino group of ThDP for cofactor activation, the influence of the chemical environment at the active site of pyruvate decarboxylase from Zymomonas mobilis on the electronic properties of the 4 -amino group was studied by two-dimensional proton-nitrogen correlated NMR spectroscopy (Tittmann et al., 2005a). Chemical shift analysis and its pH dependence indicate that the acceleration of C2 deprotonation by 5 orders of magnitude is not mainly of thermodynamic nature caused by a significant increase in basicity... 

Before the molecular mechanism of the pyruvic decarboxylase reaction is discussed, it might be appropriate to summarize briefly modern concepts on the mechanism of action of coenzymes. Modern research in that field suggests that the coenzyme functions in biochemical reactions by virtue of its unique molecular properties, and that the apoenzyme acts mainly as a basic or acidic catalyst and provides a structural skeleton that brings substrate and coenzyme in close contact. 

Candy, J.M., and Duggleby, R.G., 1998. Structure and properties of pyruvate decarboxylase and site-directed mutagenesis of the Zymomonas mobilis enzyme. Biochimica et Biophysica Acta. 1385 323-338. 

Pyruvate decarboxylase is by far the best characterized enzyme among thiamine pyrophosphate-linked a-keto acid decarboxylases. Properties of this enzyme, its structure, and its catalytic mechanism have been described in a recent review [8]. Two other enzjmies belonging to the class of a-keto acid decarboxylases, benzoylformate decarboxylase and phenylpyruvate decarboxylase, are much less studied. 

The properties of pyruvate decarboxylase have been extensively studied particularly with respect to its catal3dic mechanism involving the enzyme-bound reaction intermediate 2a-hydroxyethylthiamine pyrophosphate (HETPP) [24-26]. The ability of the coenzyme (TPP) and more particularly its thiazolium ring to bind to the carbonyl group and act as an electronic sink makes the decarboxylation of an a-keto acid possible. This is because decarboxylation of an a-keto acid requires the buildup of negative charge on the carbonyl atom in the transition state. The reaction mechanism of pyruvate decarboxylase is described by Voet and Voet [27] and is based on isolation and identification of die HETPP intermediate. 

Benzoylformate decarboxylases purified from Acinetobacter calcoaceticus [81] and Pseudomonas putida [85] were shown to have similar properties. Like pyruvate decarboxylase this enzyme is thiamine pyrophosphate- and magnesium-dependent and has a tetrameric structure. The molecular weight of a subunit is 58,000 for A. calcoaceticus [81] and 57,000-57,500 for P. putida [86,87], as estimated from sodium dodecyl sulfate-polyamide gel electophoresis (SDS-PAGE). 

Cacciapuoti G, M Porcelli, M de Rosa, A Gambacorta, C Bertoldo, V Zappia (1991) 5 -adenosylmethionine decarboxylase from the thermophilic archaebacterium Sulfolobus solfataricus. Putification, molecular properties and studies on the covalently bound pyruvate. Eur J Biochem 199 395-400. 

In the case of histidine decarboxylase, both PLP-dependent and pyruvate-dependent enzymes exist. Comparison of properties of these two enzymes provides useful information about the comparative catalytic properties of pyruvate and PLP. 

AU of the pyravate decarboxylases investigated with respect to their kinetic properties, with the exception of die enzyme from Zymomonas mobilis, exhibit sigmoidal v/s plots and a lag ph e during product formation. The for pyruvate has been determined as 0.25—1.1 mM. The enzyme from the yeast demonstrates specific activity in the range of 45-60 U/mg. The pH optimum is 6.0-6.8. 

---