Fluorescence‐quenching studies on a conformational transition within a domain of the β2 subunit of Escherichia coli tryptophan synthase

Abstract

The fluorescence quenching by acrylamide of the single tryptophan residue in the B2 subunit of tryptophan synthase from E. coli K12 is studied for different state of the protein: the native apo-enzyme and holoenzyme, the nicked apo-protein and holo-protein and the isolated proteolytic fragment F1 corresponding to the N-terminal two thirds of .beta.2. The quenching constants measured are used to estimate the accessibility of the tryptophan in these different forms. The results are discussed in terms of conformational transition within the F1 domain, occurring in the presence of cofactor, pyridoxal 5''-phosphate, in the native enzyme. The proteolytic cleavage of the native enzyme renders the nicked protein unable to undergo this conformational change.