Application of rapid-scanning, stopped-flow spectroscopy to the characterization of intermediates formed in the reactions of L- and D-tryptophan and beta-mercaptoethanol with Escherichia coli tryptophan synthase.

The reactions of the alpha 2 beta 2 complex of Escherichia coli tryptophan synthase with D- and L-Trp and the presteady-state reaction of L-Ser and beta-mercaptoethanol under different premixing conditions have been investigated by rapid-scanning stopped-flow (RSSF) UV-visible spectroscopy. The reaction of alpha 2 beta 2 with L-Ser and beta-mercaptoethanol occurs in 3 detectable relaxations with rates similar to the 3 relaxations seen in the partial reaction with L-Ser and in the reaction with L-Ser and indole. The presteady-state phase of the reaction of beta-mercaptoethanol with the alpha-aminoacrylate intermediate is characterized by 2 relaxations. The RSSF spectra for this reaction show that the spectral changes that take place in these 2 phases are essentially identical. The L-Trp reaction is biphasic, and the spectral changes occurring in each phase of the reaction also are identical. The 2 new spectral bands formed (lambda max congruent to 420 nm and congruent to 476 nm) are assigned as the L-Trp external aldimine (Schiff's base) and L-Trp quinonoid intermediates, respectively. The reaction of D-Trp also is biphasic. Analysis of first and second derivatives of the RSSF spectral changes give evidence for the formation of spectral bands with lambda max of approximately 423 nm, approximately 450 nm, and approximately 478 nm. The positions and shapes of these bands suggest a D-Trp external aldimine structure (423 nm) and a quinonoidal species (450 and 478 nm). However, product studies do not support this latter assignment. The behavior of the D- and L-Trp reactions and the reaction of beta-mercaptoethanol with the alpha-aminoacrylate strongly indicate the pre-existence of 2 slowly equilibrating forms of the internal aldimine and of the alpha-aminoacrylate.