INDOLEGLYCEROL PHOSPHATE SYNTHASE

The biosynthesis of tryptophan from the branchpoint compound, chorismate in E. coli The first step involves the conversion of chorismate to the aromatic compound anthranilate. The anthranilate is transferred to a ribose phosphate chain. The product is cyclized to indoleglycerol phosphate by the removal of water and loss of the ring carboxyl by indoleglycerol phosphate synthase. Finally, in a... 

Among different organisms the five enzyme activities required for tryptophan synthesis are distributed on different proteins (table 21.1). For example, in E. coli, indoleglycerol phosphate synthase catalyzes both the isomerization of phosphoribosylanthranilate and the cyclization step. Of particular interest is the occurrence of a single protein of the catalytic activities for nonconsecutive reactions in some cases. If in such cases the proteins were separate from each other in the cell, this arrangement, for example, in Neurospora, would necessitate the product of one reaction leaving the product site of one enzyme to be acted on by another... 

Figure 4.7 Two of the enzymatic activities involved in the biosynthesis of tryptophan in E. coli, phosphoribosyl anthranilate (PRA) isomerase and indoleglycerol phosphate (IGP) synthase, are performed by two separate domains in the polypeptide chain of a bifunctional enzyme. Both these domains are a/p-barrel structures, oriented such that their active sites are on opposite sides of the molecule. The two catalytic reactions are therefore independent of each other. The diagram shows the IGP-synthase domain (residues 48-254) with dark colors and the PRA-isomerase domain with light colors. The a helices are sequentially labeled a-h in both barrel domains. Residue 255 (arrow) is the first residue of the second domain. (Adapted from J.P. Priestle et al., Proc.

A (3 replacement reaction catalyzed by the PLP-dependent tryptophan synthase converts indoleglycerol phosphate and serine to tryptophan. Tryptophan synthase from E. coli consists of two subunits associated as an a2P2 tetramer (Fig. 25-3). The a subunit catalyzes the cleavage (essentially a reverse aldol) of indoleglycerol phosphate to glyceraldehyde 3-phosphate and free indole (Fig. 25-2, step s).67 The P subunit contains PLP. It presumably generates, from serine, the Schiff base of aminoacrylate, as indicated in Fig. 25-2 (step f). The enzyme catalyzes the addition of the free indole to the Schiff base to form tryptophan. The indole must diffuse for a distance of 2.5 ran... 

Fig. 11. Channeling in the tryptophan synthase reaction. Indoleglycerol 3-phosphate is cleaved to indole and glyceraldehyde 3-phosphate at the a-site. Indole is then passed through the hydrophobic channel to the /3-subunit where it reacts with serine to form tryptophan. This schematic was drawn from data presented in (55).

Indoleglycerol 3-phosphate (13) is converted into tryptophan (14) by the action of L-tryptophan synthase. The mechanism of this enzymatic reaction involves formation of a Schiff base with an enzyme-bound pyridoxal phosphate. The a-aminoacrylate Schiff base formed undergoes the addition of a p-substituent to produce tryptophan (Floss, 1986) (Fig. 7.5). 

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