Product Binding to the .alpha.-Carboxyl Subsite Results in a Conformational Change at the Active Site of O-Acetylserine Sulfhydrylase-A: Evidence from Fluorescence Spectroscopy

The intrinsic fluorescence of the pyridoxal 5'-phosphate (PLP) enzyme O-acetylserine sulfhydrylase-A (OASS-A) was studied in order to gain insight into the structural basis for binding of substrates and products and for catalysis. Excitation of OASS-A with 298-nm light gives an emission spectrum with two maxima, 337 and 498 nm. OASS-A has two tryptophan residues, and the 337-nm maximum indicates that at least one of these is exposed somewhat to...