The Tryptophanase from Escherichia coli K12

A chemically modified protein, reduced holotryptophanase, was obtained by reduction of the Schiff base between the protein and its coenzyme, pyridoxal 5'-phosphate. The renaturation from 8 m urea of mixtures originally containing the native and the modified enzymes is described. It is shown that during this process hybrid molecules are formed and that, after resolution of the unmodified coenzyme sites, the hybrid molecules remain tetrameric at low temperatures whereas the apoenzyme dissociates into dimers. This observation strongly supports previous results concerning the existence of a tight coupling between the tertiary structures of the protomers within tryptophanase.