.beta.-Elimination of indole from L-tryptophan catalyzed by bacterial tryptophan synthase: a comparison between reactions catalyzed by tryptophanase and tryptophan synthase

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Abstract
Although tryptophan synthase catalyzes a number of pyridoxal phosphate dependent .beta.-elimination and .beta.-replacement reactions that are also catalyzed by tryptophanase, a principal and puzzling difference between the two enzymes lies in the apparent inability of tryptophan synthase to catalyze .beta.-elimination of indole from L-tryptophan. We now demonstrate for the first time that the .beta.2 subunit and the .alpha.2.beta.2 complex of tryptophan synthase from Escherichia coli and from Salmonella typhimurium do catalyze a slow .beta.-elimination reaction with L-tryptophan to produce indole, pyruvate, and ammonia. The rate of the reaction is about 10-fold higher in the presence of the .alpha. subunit. The rate of indole production is increased about 4-fold when the aminoacrylate produced is converted to S-(hydroxyethyl)-L-cysteine by a coupled .beta.-replacement reaction with .beta.-mercaptoethanol. The rate of L-tryptophan cleavage is also increased when the indole produced is removed by extraction with toluene or by condensation with D-glyceraldehyde 3-phosphate to form indole-3-glycerol phosphate in a reaction catalyzed by the .alpha. subunit of tryptophan synthase. The amount of L-tryptophan cleavage is greatest in the presence of both .beta.-mercaptoethanol and D-glyceraldehyde 3-phosphate, which cause the removal of both products of cleavage. The cleavage reaction is not due to contaminating tryptophanase since the activity is not inhibited by (3R)-2,3-dihydro-L-tryptophan, a specific inhibitor of tryptophanase, but is inhibited by (3S)-2,3-dihydro-L-tryptophan, a specific inhibitor of tryptophan synthase. The cleavage reaction is also inhibited by D-tryptophan, the product of a slow racemization reaction. Tryptophan synthase also catalyzes the .beta.-elimination of o-nitrothiophenol from S-(o-nitrophenyl)-L-cysteine but at a much lower rate than that catalyzed by tryptophanase. Our results show that tryptophan synthase is similar to tryptophanase in its reaction mechanism and specificity but catalyzes several of the reactions investigated at very different rates.

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