UV spectroscopic studies of human erythrocyte superoxide dismutase

Using UV absorption spectroscopy, first derivative spectroscopy, and UV difference spectroscopy, the active site of human superoxide dismutase is probed. First derivative spectra (dA/dλ versus λ) show the HESOD spectrum to be a composite of Phe and Trp absorbance. The 278 and 288 nm Trp absorbance peaks are sensitive to solvent polarity. A 5–10% decrease in these peaks accompanies copper removal from the active site indicating greater solvent access to Trp in the apoenzyme than the holoenzyme. A Trp UV difference peak at 305–310 nm documents the presence or absence of copper at the active site, and documents also the movement of a nonbridging copper-binding His (His 46 or 120) when HESOD is inhibited by azide or when the copper moiety is reduced. Trp absorbances indicate that neither cyanide nor KCl inhibition affects the Cu(II)-His bonds. Phe UV absorbance is increased by the presence of copper at the active site and increased further by the addition of cyanide or azide. Neither Trp nor Phe responds to the presence of zinc in the active site. A molecular graphics program, FRODO, shows Trp and the four Phe residues lying in an approximate ring around the active site of HESOD and thus excellently placed to report on active site perturbations.