Resonance Raman spectra of photodissociated hemoglobins: implications on cooperative mechanisms

Contribution of protein conformation to heme stereochemistry and reactivity. Low-temperature magnetic circular dichroism data

Visible and near infrared magnetic circular dichroism (MCD) spectra of heme proteins and enzymes as well as those of a protein-free heme bound to 2-methylimidazole were recorded and compared at 4.2 K in unrelaxed metastable and relaxed equilibrium heme stereochemistry. The relaxed and unrelaxed stereochemistries of a 5-coordinate ferrous heme were generated by chemical reduction of iron at room temperature before freezing the sample and by photolysis of CO or O2 complexes at 4.2 K, respectively. The results are discussed in terms of a protein contribution into energies of the Fe-N epsilon(His) and Fe-N(pyrrols) bonds and their change on a ligand binding. We observed and analyzed cases of weak (myoglobin, hemoglobin) and strong (leghemoglobin, peroxidases) constraints imposed by the protein conformation on the proximal heme stereochemistry by comparing the bond energies in proteins with those in the protoheme-(2-methylimidazole) model compound. The role of a protein moiety in modulating the ligand binding properties of leghemoglobin and the heme reactivity of horseradish peroxidase is discussed.

XANES spectroscopy of carp hemoglobin-iron in correlation with the affinity changes of the protein for ligand

The strong variation of ligand-binding properties with pH for carp hemoglobin is not reflected in the electronic distribution of the heme-iron. Thus, we can suppose that hemoglobin affinity is directly controlled by the protein and by some particular changes of the iron atom.

Local Fe site structure in the tense-to-relaxed transition in carp deoxyhemoglobin: a XANES (x-ray absorption near edge structure) study

The Fe-site structure variation in the transition from the low-affinity tense (T) quaternary structure to the high-affinity relaxed (R) structure in carp deoxyhemoglobin was studied by analysis of multiple scattering resonances in the XANES (x-ray absorption near edge structure) spectra. High signal-to-noise XANES spectra were measured at the Frascati "wiggler" synchrotron radiation facility. We find that the forces on the Fe active site due to the change of quaternary protein conformation do not induce variations greater than 0.01 A in interatomic Fe-N distances, variations greater than 0.1 A in the Fe displacement toward the heme plane, or the "doming" of the heme. The relevance of these results to the mechanism of protein control of ligand binding is discussed.

Picosecond time-resolved resonance Raman studies of hemoglobin: implications for reactivity

Picosecond time-resolved Raman spectra of hemoglobin generated with blue pulses (20 to 30 picoseconds) that were resonant with the Soret band and of sufficient intensity to completely photodissociate the starting liganded sample are reported. For both R- and T-state liganded hemoglobins, the peak frequencies in the spectrum of the deoxy transient were the same at approximately 25 picoseconds as those observed at 10 nanoseconds subsequent to photodissociation. In particular, the large R-T differences in the frequency of the stretching mode for the iron-proximal histidine bond (VFe-His) detected in previously reported nanosecond-resolved spectra were also evident in the picosecond-resolved spectra. The implications of this finding with respect to the distribution of strain energy in the liganded protein and the origin of the time course for geminate recombination are discussed. On the basis of these results, a microscopic model is proposed in which delocalization of strain energy is strongly coupled to the coordinate of the iron. The model is used to explain the origin of the R-T differences in the rates of ligand dissociation.