Heme orientation affects holo-myoglobin folding and unfolding kinetics

Native myoglobin (Mb) consists of two populations which differ in the orien tation of the heme by 180 degrees rotation (as verified by nuclear magnetic resonance) but have identical absorption spectra and equilibrium-thermodyn amic stability. Here, we report that these two fractions of native oxidized Mb (from horse) both unfold and refold (chemical denaturant, pH 7, 20 degr ees C) in two parallel kinetic reactions with rate constants differing 10-f old, In accord, the oxidized heme remains coordinated to unfolded horse Mb in up to 4 M guanidine hydrochloride (pH 7, 20 degrees C). (C) 2000 Federat ion of European Biochemical Societies.