Indole pyruvate decarboxylase

Similar results were obtained in our studies on the thiamin-dependent indole-pyruvate decarboxylase, where protonation and deprotonation reactions are also catalyzed by a Glu-cofactor proton shuttle and a His-Asp-Glu relay (Schiitz et al., 2005). 

Indoleacetic acid (lAA) (17) is involved in many aspects of plant growth and development (Bonner and Varner, 1976 Kosuge and Sanger, 1986). This hormonal substance is derived in most plants by conversion of tryptophan to indole 3-pyruvic acid (15) (tryptophan amino transferase), decarboxylation to the indole 3-acetaldehyde (16) (indole pyruvate decarboxylase), and oxidation to indole 3-acetic acid (17) (indole acetaldehyde oxidase) (Fig. 7.6) (Goodwin and Mercer, 1983). 

Most known thiamin diphosphate-dependent reactions (Table 14-2) can be derived from the five halfreactions, a through e, shown in Fig. 14-3. Each half-reaction is an a cleavage which leads to a thiamin- bound enamine (center. Fig. 14-3) The decarboxylation of an a-oxo acid to an aldehyde is represented by step h followed by fl in reverse. The most studied enzyme catalyzing a reaction of this type is yeast pyruvate decarboxylase, an enzyme essential to alcoholic fermentation (Fig. 10-3). There are two 250-kDa isoenzyme forms, one an tetramer and one with an (aP)2 quaternary structure. The isolation of a-hydroxyethylthiamin diphosphate from reaction mixtures of this enzyme with pyruvate provided important verification of the mechanisms of Eqs. 14-14,14-15. Other decarboxylases produce aldehydes in specialized metabolic pathways indolepyruvate decarboxylase in the biosynthesis of the plant hormone indole-3-acetate and ben-zoylformate decarboxylase in the mandelate pathway of bacterial metabolism (Chapter 25). Formation of a-ketols from a-oxo acids also starts with step h of Fig. 14-3 but is followed by condensation with another carbonyl compound in step c, in reverse. An example is decarboxylation of pyruvate and condensation of the resulting active acetaldehyde with a second pyruvate molecule to give l -a-acetolactate, a reaction catalyzed by acetohydroxy acid synthase (acetolactate synthase). Acetolactate is the precursor to valine and leucine. A similar ketol condensation, which is catalyzed by the same S5mthase, is... 

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