Structural studies of staphylococcal protease. III. Binding of anions to the spin-labeled enzyme

The staphylococcal protease was coupled at the active-site serine residue with a spin-labeled analog of diisopropyl fluorophosphonate and the interaction of competitive inhibitors such as chloride and acetate anions, as well as N-carbobenzoxy-L-glutamic acid (Z-L-Glu), was investigated by electron paramagnetic resonance spectroscopy. It was observed that the addition of chloride ions to the spin-labeled enzyme increased the freedom of motion of the spin label while the presence of acetate ions and Z-L-Glu resulted in an increase in the immobilization of the spin label. These results suggest that these ions bind to the active site region in different ways.