Structure of carboxymyoglobin in crystals and in solution.

Abstract

The configuration of the heme-carbonyl group upon binding of carbon monoxide to sperm whale myoglobin (Mb) in crystals is evaluated on the basis of infrared spectroscopic methods. Multiplets of the totally symmetric C-O stretching mode are observed for the heme-bound ligand near 1933, 1944, and 1967 cm-', corresponding to three different heme-carbonyl conformers. Variations in the relative proportions of these conformers can be induced by incorporation of small fractions of metMb or deoxyMb into MbCO crystals. The configuration of the iron-carbonyl with respect to the immediate coordination environment of the heme iron is assigned for each x(CO) stretching frequency on the basis of a detailed comparison of the three-dimensional structures of the heme environments of MbCO, metMb, and deoxyMb defined by crystallographic methods. The structures of the three hemecarbonyl conformers account for the s(CO) infrared absorption bands that can be observed for MbCO in solution.The three-dimensional structures of carbon monoxide (CO)-liganded hemoglobins (Hbs) and myoglobins (Mbs) (1-5) exhibit a bent or tilted configuration of the CO ligand with respect to the porphyrin ring, whereas small-molecule model CO-liganded heme complexes (6, 7) exhibit a perpendicular linear (Fe-C-O) bonding structure. The origin of the different configuration in heme proteins is attributed primarily to nonbonding steric interactions of the axial ligand with nearby amino acid residues. The assumption is generally made that the heme-carbonyl configuration of heme proteins in crystals is identical to that in solution. It is well known, nonetheless, that the infrared (IR) absorption spectrum of sperm whale carboxymyoglobin (MbCO) in solution is indicative of two structurally distinguishable components (8, 9), although only one configuration is identified in the crystal by neutron diffraction dataIn recent studies of the polarized single crystal absorption spectra of Mb complexes (10, 11), we have detected differences in the configuration of the heme-carbonyl group induced by crystal environment and have demonstrated that the singlecrystal spectrum of MbCO formed under conditions comparable to those in neutron diffraction studies (3) is not quantitatively compatible with the spectrum of MbCO in solution. In this communication we report the results of investigations in which we have extended these observations by application of IR spectroscopic methods. We provide evidence on the basis of the IR absorption of MbCO in crystals that three structurally distinct heme-carbonyl conformers arising from changes in nearest neighbor intermolecular interactions in the crystal can be identified on the basis of the v(CO) stretching frequency.The structural origin of the v(CO) stretching frequencies indicative of each conformer is explained on the basis of the three-dimensional structure of sperm whale Mb (3, 12, 13) determined by crystallographic methods. The results suggest that structural changes in the immediate environment of the iron-carbonyl group ...

A Photolysis-Triggered Heme Ligand Switch in H93G Myoglobin

“…The CO stretching frequency at equilibrium is shown in the top spectrum. Difference spectra are shown at 125 µs, 250 µs, 375 µs, 500 µs, 1 ms, and 4 ms. quency of 507 cm -1 and corresponding CO stretching frequency of 1945 cm -1 are consistent with a histidine-ligated species (8,15,(29)(30)(31). Since the proximal histidine H93 is absent, there are two strong candidates for histidine ligation: H64 on the distal side and H97 on the proximal side.…”

Ultrafast Dynamics of Myoglobin without the Distal Histidine: Stimulated Vibrational Echo Experiments and Molecular Dynamics Simulations

“…In contrast, the spectrum of wtMbCO shows at least three distinct absorption lines, centered at 1934 (A 3 ), 1945 (A 1 ), and 1965 cm -1 (A 0 ). 5,26,27 The A 0 peak, the smallest of the three features at room temperature and neutral pH, is believed to arise from configurations in which the ligand interacts most weakly with H64 because the distal histidine is swung out of the pocket. [36][37][38][39]56 As has been discussed in detail previously 5,8,[57][58][59] Figure 1 suggests that the substitution of the distal histidine (H64) by a valine spectroscopically mimics the A 0 state of wtMbCO.…”

Probing the electronic and geometric structure of ferric and ferrous myoglobins in physiological solutions by Fe K-edge absorption spectroscopy

“…9 The strong IR absorption of the CO bond [70][71][72] performed iron K-edge polarized XANES investigations of MbCO at 20 and 100 K respectively. Their results largely agree with other previous studies (see Table 1) and only a slight increase in the Fe-C and C-O distances were reported.…”