Active-site iron dynamics in heme proteins and model compounds
Active-site iron dynamics in heme proteins and model compounds are studied via nuclear resonance vibrational spectroscopy (NRVS) and compared with other experimental vibrational probes and theoretical calculations, yielding new insight into the vibrational dynamics of biologically significant proteins. NRVS is a novel technique that selectively probes only 57Fe; experiments provide quantitative information on the amplitude and frequency of all normal modes having significant iron vibrational motion. Data from other vibrational probes including traditional Mössbauer, resonance Raman spectroscopy, and inelastic neutron scattering provide complementary information on the vibrational dynamics of the heme-protein system as a whole. Specific iron participation is identified in resonance Raman and inelastic neutron modes when these spectroscopic modes are seen at the same frequency as NRVS vibrations.^ NRVS data is examined for 57Fe-containing myoglobin, cytochrome f, hemoglobin, and several heme model compounds. A template of heme normal modes is obtained from analyses of several model compound systems: phenyl-like modes or modes associated with the protein peak at very low-energies ∼30--60 cm-1, out-of-plane modes in the ∼70--130 cm-1 region, imidazole or histidine modes near 220 cm-1, in-plane modes from ∼200--500 cm-1, and ligand modes at the highest frequencies ∼460--600 cm-1. A qualitative understanding of the more complicated heme-protein dynamics is obtained by applying this template to the protein vibrational density of states (VDOS). Differences in heme-to-protein binding configurations in myoglobin and cytochrome f are reflected in the iron VDOS, suggesting a structural and dynamical correlation with their different biological functions (i.e. ligand binding versus electron transport). Hemoglobin hybrids containing 57Fe and 56Fe have been prepared for the first time and NRVS measurements show data for the selected alpha or beta subunits within a relatively unperturbed hemoglobin sample. Myoglobin and hybrid hemoglobin data show a broad range of modes with doming character, but modes near 100 cm-1 appear to have importance in the communication pathway of the hemoglobin allosteric effect.^
Stephen M. Durbin, Purdue University.
Physics, Condensed Matter|Biophysics, General