Table 3. Heme affinity of HO-2.

Ferric and ferrous heme titrations were performed with HO-2 proteins in oxidized and reduced states under aerobic and anaerobic conditions, respectively. A freshly prepared heme solution was added to 8 μm HO-2 solution, pH 7.4, and the absorbance spectra were recorded. The absorbance value at the Soret peak (406 nm for ferric heme titration and 430 nm for ferrous heme titration) was plotted versus heme concentration. K_d was calculated using the quadratic equation with a one-site binding model (see “Experimental Procedures”). All of the measurements were done in triplicate.