Native myoglobin (Mb) consists of two populations which differ in the orientation of the heme by 180° rotation (as verified by nuclear magnetic resonance) but have identical absorption spectra and equilibrium–thermodynamic stability. Here, we report that these two fractions of native oxidized Mb (from horse) both unfold and refold (chemical denaturant, pH 7, 20°C) in two parallel kinetic reactions with rate constants differing 10-fold. In accord, the oxidized heme remains coordinated to unfolded horse Mb in up to 4 M guanidine hydrochloride (pH 7, 20°C).
