Mechanism of the Physiological Reaction Catalyzed by Tryptophan Synthase from Escherichia coli

Andrew N. Lane, Kasper Kirschner

Research output: Contribution to journalArticlepeer-review

73 Scopus citations

Abstract

The physiological synthesis of L-tryptophan from indoleglycerol phosphate and L-serine catalyzed by the α2β2 bienzyme complex of tryptophan synthase requires spatial and dynamic cooperation between the two distant α and β active sites. The carbanion of the adduct of L-tryptophan to pyridoxal phosphate accumulated during the steady state of the catalyzed reaction. Moreover, it was formed transiently and without a lag in single turnovers, and glyceraldehyde 3-phosphate was released only after formation of the carbanion. These and further data prove first that the affinity for indoleglycerol phosphate and its cleavage to indole in the α subunit are enhanced substantially by aminoacrylate bound to the β subunit. This indirect activation explains why the turnover number of the physiological reaction is larger than that of the indoleglycerol phosphate cleavage reaction. Second, reprotonation of nascent tryptophan carbanion is rate limiting for overall tryptophan synthesis. Third, most of the indole generated in the active site of the α subunit is transferred directly to the active site of the β subunit and only insignificant amounts pass through the solvent. Comparison of the single turnover rate constants with the known elementary rate constants of the partial reactions catalyzed by the α and β active sites suggests that the cleavage reaction rather than the transfer of indole or its condensation with aminoacrylate is rate limiting for the formation of nascent tryptophan.

Original languageEnglish
Pages (from-to)479-484
Number of pages6
JournalBiochemistry
Volume30
Issue number2
DOIs
StatePublished - Jan 1 1991

ASJC Scopus subject areas

  • Biochemistry

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