Fig. 4.

HusA binds a range of porphyrins, including metallated and unmetallated forms. a–d UV-visible spectra of 10−20 μM of ZnPPIX, PPIX, DPIX and haemin, alone and in the presence of 1–1.5 molar equivalents of HusA. e Scaled and corrected fluorescence intensities at 335 nm during titration of 1 μM of HusA with increasing concentrations of haemin and other porphyrins. Fitting to a 1:1 binding model yielded the dissociation equilibrium constants for haemin (7.3 ± 1.1 μM), PPIX (1.9 ± 0.6 μM), ZnPPIX (1.3 ± 0.4 μM), DPIX (0.36 ± 0.09 μM) and Fe(III)DPIX (0.22 ± 0.05 μM), in order of increasing affinity