Axial Coordination in Myoglobin Transients at Low pH
10 September 1989
Low pH transients (pH ~2.5) of deoxy, CO-bound and met myoglobins were generated in a rapid mixing apparatus. The transients were examined with resonance Raman and optical absorption spectroscopies. The optical absorption spectra confirmed those reported previously on various myoglobins at low pH. The resonance Raman spectra reveal new structural information on these transients. First, at low pH, deoxy myoglobin has a spectrum consistent with a five-coordinate ferrous heme. Second, at low pH, carbonmonoxy bound myoglobin has a spectrum of a six-coordinate unhindered ferrous heme. Third, at low pH, met myoglobin takes on a spectrum consistent with a four-coordinate ferric heme. From these data we are able to conclude that in the derivatives of ferrous myoglobin which we studied, the iron proximal histidine bond is not cleaved at low pH. However, it may be substantially weakened in the deoxy heme. Furthermore, in all cases, in the low pH transient the distal pocket adopts an open conformation. A molecular description of the increase in CO on-rates emerges from these results.