Reversibility oftheTryptophanase Reaction: Synthesis ofTryptophan fromIndole, Pyruvate, andAmmonia

Degradation of tryptophan to indole, pyruvate, andammoniabytryptophanase (EC4....) from Escherichia coli, previously thoughttobeanirreversible reaction, isreadily reversible athighconcentrations of pyruvate andammonia.Tryptophanandcertain ofits analogues, e.g., 5-hydroxytryptophan, canbesynthesized bythisreaction frompyruvate, ammonia,andindole oran appropriate derivative atmaximumvelocities approaching thoseofthedegradative reactions. Concentrations of ammonia required forthesynthetic reactions produce specific changesinthespectrumoftryptophanase that differ fromthoseproduced byK+ andindicate thatam- moniainteracts withboundpyridoxal 5'-phosphate to forman imine.Kinetic results indicate thatpyruvate is thesecondsubstrate bound,henceindolemustbethe third. Theseresults favor amodified mechanismforthe multitude oftryptophanase-catalyzed reactions inwhich a-aminoacrylate, whichfunctions asacommon enzyme- boundintermediate inbothsynthetic anddegradative reactions, isnotreleased intothemedium duringthe latter reactions, butisdegraded topyruvate andammonia bysequential reversible stepsviaenzyme-bound inter- mediates.