Tryptophan synthase indole reaction mechanism

MECHANISM FIGURE 22-18 Tryptophan synthase reaction. This enzyme catalyzes a multistep reaction with several types of chemical rearrangements. An aldol cleavage produces indole and glyceraldehyde 3-phosphate this reaction does not require PLP. Dehydration of serine forms a PLP-aminoacrylate intermediate. In steps and this condenses with indole, and the product is hydrolyzed to release tryptophan. These PLP-facilitated transformations occur at the /3 carbon (C-3) of the amino acid, as opposed to the a-carbon reactions described in Figure 18-6. The /3 carbon of serine is attached to the indole ring system. Tryptophan Synthase Mechanism... 

The tryptophan synthase bienzyme complex from enteric bacteria provides an important example wherein RSSF has been used to good advantage for the study of both enzyme mechanism and protein structure-function relationships. This enzyme complex is composed of heterologous a- and P2-subunits arranged in a nearly linear a-(3-(l-a array (81). The a-subunit catalyzes the aldolytic cleavage of IGP to indole and G3P, while the P-subunit catalyzes the PLP-dependent condensation of i-Ser and indole to yield i-Trp. The aP-reaction is essentially the sum of the individual a- and P-reactions (scheme I). Indole, the common intermediate produced at the a-site, is direcdy channeled to the P-active site via a tunnel located in the interior of the protein complex which directly interconnects the a- and P-catalytic centers (81-84). Although the individual subunits may be isolated and are functional, formation of the bienzyme complex not only increases the catalytic activities of the separate subunits by nearly 100-fold, but also alters the thermodynamic stability of P-site reaction intermediates and introduces heterotropic allosteric interactions between sites. 

In /3-replacement reactions, the /3-substituent of an amino acid substrate is replaced by a new /3-substituent. For the three enzymes (TRPS, OASS, and cystathionine /3-synthase (CBS)) whose mechanisms are discussed in this section, the catalytic reaction is composed of two distinct half-reactions. The /3-elimination is followed by a /3-addition where nucleophilic agents, indole, sulfide, and homocysteine, respectively, react with the ci-aminoacrylate Schiff base to form the final product, L-tryptophan, L-cysteine, and L-cystathionine, respectively. 

Fig. 10. Mechanism of the reactions catalyzed by both the a and B subunits of the tryptophan synthase bienzyme complex. The a reaction involves cleavage of IGP. The g reaction involves two stages stage I is reaction of L-Ser with E(Ain) to give E(A-A) stage II is reaction of indole with E(A-A) to give L-Trp. Indole, the common intermediate, is directly channeled between the two catalytic centers via a 25-A-long tunnel that interconnects the a and B active sites through the interior of the protein. See text for details.

Biosynthesis of some classes of terpene indole alkaloids is well understood. In certain cases, many of the enzymes that are responsible for biosynthesis have been cloned and mechanistically studied. In other cases, biosynthesis pathway is only proposed based on the results of feeding studies with isotopically labeled substtates and from the structures of isolated biosynthetic intermediates. All terpene indole alkaloids are derived from tryptophan and the iridoid terpene secologanin (Fig. 14.11). Tryptophan decarboxylase, a pyridoxal-dependent enzyme [29], converts tryptophan to tryptamine [30]. The following strictosidine synthase-catalyzed Mannich reaction connects ttyptamine and secologanin to yield strictosidine [31]. The Apocynaceae, Loganiaceae, Rubiaceae, and Nyssaceae families of plants each produce terpene indole alkaloids with dramatically diverse structures [32-34]. The mechanisms and control of... 

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