A Novel Tryptophan Synthase -Subunit from the Hyperthermophile Thermotoga maritima

Tryptophan synthase catalyzes the last two steps in the biosynthesis of the amino acid tryptophan. The enzyme is an complex in mesophilic microorganisms. The -subunit (TrpA) catalyzes the cleavage of indoleglycerol phosphate to glyceraldehyde 3-phosphate and indole, which is channeled to the active site of the associated -subunit (TrpB1), where it reacts with serine to yield tryptophan. The TrpA and TrpB1 proteins are encoded by the adjacent trpA and trpB1 genes in the trp operon. The genomes of many hyperthermophilic microorganisms, however, contain an additional trpB2 gene located outside of the trp operon. To reveal the properties and potential physiological role of TrpB2, the trpA, trpB1, and trpB2 genes of Thermotoga maritima were expressed heterologously in Escherichia coli, and the resulting proteins were purified and characterized. TrpA and TrpB1 form the familiar complex, in which the two different subunits strongly activate each other. In contrast, TrpB2 forms a 2-homodimer that has a high catalytic efficiency kcat/Km indole because of a very low Km indole but does not bind to TrpA. These results suggest that TrpB2 acts as an indole rescue protein, which prevents the escape of this costly hydrophobic metabolite from the cell at the high growth temperatures of hyperthermophiles.