Proton Transfers in the β-Reaction Catalyzed by Tryptophan Synthase

Reactions catalyzed by the beta-subunits of the tryptophan synthase alpha(2)beta(2) complex involve multiple covalent transformations facilitated by proton transfers between the coenzyme, the reacting substrates, and acid-base catalytic groups of the enzyme. However, the UV/Vis absorbance spectra of covalent intermediates formed between the pyridoxal 5'-phosphate coenzyme (PLP) and the reacting substrate are remarkably pH-independent. Furthermore, the alpha-aminoacrylate Schiff base intermediate, E(A-A), formed between L-Ser and enzyme-bound PLP has an unusual spectrum with lambda(max) = 350 nm and a shoulder extending to greater than 500 nm. Other PLP enzymes that form E(A-A) species exhibit intense bands with lambda(max) approximately 460-470 nm. To further investigate this unusual tryptophan synthase E(A-A) species, these studies examine the kinetics of H(+) release in the reaction of L-Ser with the enzyme using rapid kinetics and the H(+) indicator phenol red in solutions weakly buffered by substrate L-serine. This work establishes that the reaction of L-Ser with tryptophan synthase gives an H(+) release when the external aldimine of L-Ser, E(Aex(1)), is converted to E(A-A). This same H(+) release occurs in the reaction of L-Ser plus the indole analogue, aniline, in a step that is rate-determining for the appearance of E(Q)(Aniline). We propose that the kinetic and spectroscopic properties of the L-Ser reaction with tryptophan synthase reflect a mechanism wherein the kinetically detected proton release arises from conversion of an E(Aex(1)) species protonated at the Schiff base nitrogen to an E(A-A) species with a neutral Schiff base nitrogen. The mechanistic and conformational implications of this transformation are discussed.