Spectroscopic studies of myoglobin at low pH: heme ligation kinetics.

摘要: On the basis of characterization heme structure and ligation in equilibrium, we explore both proximal distal kinetics myoglobin below pH 4. Upon photolysis MbCO, a significant five-coordinate population is observed, with an intact iron-histidine bond that persists on time scale CO rebinding. Incomplete attributed to rapidly exchanging minority four-coordinate hemes, which leads fast (greater than 10(10) s-1) geminate recombination. The possible relevance such mechanism at 7 also noted. Using novel experimental protocol, observe resonance Raman spectrum partially photolyzed MbCO as function continuous wave illumination (tau). Under extended (tau approximately 35 ms 3.4), there loss intensity nu 4 region mode bleached from photoproduct. In Fe-CO stretching CO-bound fraction, 526-cm-1 increases tau expense 491-cm-1 mode. These changes are interpreted being due replacement histidine ligand under illumination. Complete relaxation pure deoxy observed equilibrium not even tau----infinity, presumably since rebinding acidification iron its complexation histidine. We propose kinetic model account for our results discuss implications previous low-pH measurements.