Stereochemistry of cooperativity effects in the prosthetic group of coboglobin

An electron paramagnetic resonance crystallographic study was made on oxycobaltomyoglobin with the dioxygen ligand enriched to 19.1% in 170. There are two srectroscopically distinct cobalt dioxygen species. The less abundant species, II (40%), has nonequivalent oxygen atoms with su- Although the direction cosines for this molecule cannot be precisely determined, the projection of its 0-0 axis ar proximately bisects N2-Fe-N3 and is parallel to the imidazo e ring of His-F8. Increase of temperature changes g, C-A, and OA values, with the largest effect seen with OA. This temperature dependence indicates averaging of the two bond structures which are stabilized at 77 K. The bonding of dioxygen to heme proteins has long been a topic of interest, extensive research activity, and debate. Various techniques and approaches have been brought to bear on the problem of seeking an accurate geometric and electronic description of the metal-dioxygen bond. Because of high electron density near the heme and ease of autoxidation of the ferrous ion, x-ray crystallography has yet to yield a definitive structure of oxymyoglobin or oxyhemoglobin. Recent development of low-temperature x-ray diffraction techniques in several laboratories may overcome these difficulties. Model oxyheme complexes have been synthesized and found by x-ray crystallography to have an Fe-O-O bond angle of t120' (1). One helpful development was the preparation of the Co analogs of hemoglobin (CoHb) and myoglobin (c'Mb) (2); these bind dioxygen reversibly. In the case of COHb the binding of dioxygen is cooperative (2-4). We have performed electron paramagnetic resonance (EPR) crystallography on C°Mb and CoMbO2 and have reported the g and COA tensors (5). At -195°C, two oxy species having the same g but different C6A tensors were observed (5), with one 0-0 axis turned approximately 90°with respect to the other in the crystal. By assuming the principal axes of the g tensor to be coincident with the molecular axes of the dioxygen ligand, the results were inThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "ad- MgO, ZnO, and KCI matrices (7,8). However, a preliminary report on the x-ray structure of CoMbO2 has appeared (9). The data were taken at -16'C and a Co-O-O angle of near 1200 is reported as contrast to the possible ir-bond structure (5). The discrepancy in the CoO-O bond angle must be resolved. Finally, the central purpose of this work was to determine all the hyperfine tensors and relevant eigenvectors in order to arrive at the detailed stereoelectronic structure of C°MbO2. MATERIALS AND METHODSCoMb was prepared and crystallized as described (5) with the addition of a purification step of adsorption onto a CM-cellulose column at pH 6.6 in 0.01 M phosphate and desorption with 0.1 M phosphate buffer at pH 7.5. There was approximately a 30% loss of non-native material at this step. The protein was crystallized over a pH range of 5.98-6.76 and an (NH4)2SO4 concentration ran...

Section: Resultsmentioning

confidence: 99%

Electron paramagnetic resonance crystallography of ¹⁷ O-enriched oxycobaltomyoglobin: Stereoelectronic structure of the cobalt dioxygen system

Dickinson

Chien

1980

“…3 shows that the transition from a domed to a planar heme can produce large changes in the relative positions of the porphyrin atoms; if all the pyrrole atoms are planar centers, the peripheral and substituent carbons move nearly 1 A. Thus a small change in the effective Fe-N bond lengths (about 0.18 A for Fe-NE2 and about 0.08 A for FeNpyrrole) is greatly amplified by the requirement of a planar, six-coordinate heme (15).…”

Section: Resultsmentioning

confidence: 99%

“…Details of the pathway and the supporting theoretical and experimental evidence are given in the following sections. Many of the data come from the work of Perutz and Ten Eyck (2,3), Hoard (10,11), Makinen and Eaton (12,13), Anderson (14), and Ibers et al (15), all of whom have made suggestions concerning some of the efements of the reaction path.…”

mentioning

confidence: 99%

Mechanism of tertiary structural change in hemoglobin.

Gelin¹,

Karplus²

1977

225

107

A reaction path is presented by which the effects of oxygen binding in hemoglobin are transmitted from a heme group to the surface of its subunit. Starting from the known deoxy geometry, it is shown by calculations with emirical energy functions and comparisons with available data Iow the change in heme geometry on ligation introduces a perturbation that leads to the tertiary structural alterations essential for cooperativity. It is found that there is little strain on the unliganded heme; instead, the reduced oxygen affinity of hemoglobin results from the strain on the liganded subunit in a tetramer with the deoxy quaternary structure. The cooperative nature of the binding of oxygen by hemoglobin is one of the most intensively studied phenomena in protein chemistry (1). From the classic x-ray work of Perutz and his collaborators (2, 3), supplemented by the physical and chemical studies of others (4, 5), the outlines of the cooperative mechanism have been determined. The essential elements are two quaternary structures (oxy and deoxy) for the hemoglobin tetramer (6), two tertiary structures (liganded and unliganded) for each subunit, and the presence of ionic, van der Waals, and hydrophobic interactions that couple the tertiary structural change of the subunits to the relative stabilities of the quaternary structures. These elements have been incorporated into a statistical mechanical model that describes ligand binding as a-function of solution conditions and accounts for the effects of mutations and chemical modifications (7,8).To complete the description of the cooperative mechanism, the statistical-mechanical model must be supplemented by an understanding of the origin of the important structural changes and their associated energies. It is necessary to know at the atomic level how ligand binding alters the tertiary structure of an individual subunit and how these alterations in subunit geometry affect and can be affected by the quaternary structure of the hemoglobin tetramer. In this paper, we focus on the first of these two problems. From calculations based on empirical energy functions (9) and comparisons with the available data, we are able to determine the properties of the heme group and of the surrounding globin chain that lead to the essential tertiary structural changes. A localized reaction path that involves directly only a relatively small number of globin atoms is found to transmit information concerning ligand binding from the heme group to the surface of the subunit. It is shown that there is little strain on the heme in unliganded hemoglobin, but that the flattening of the heme and shortening of the iron-histidine bond induced by oxygen binding produce steric repulsions between the heme and the globin that alter the subunit geometry. Nonbonded contacts between the asymmetrically positioned His F8(87) and the heme appear to initiate a rotation of the latter. This in turn produces a large displacement of Val FG5(93), which is the key residue in transmitting structural changes to the al...

“…In six-coordinate Co(II) porphyrins, the cobalt atom sits in the porphyrin plane, and is 0.15 i out of the plane in five-coordinate Co(II) porphyrins (10,30). A displacement of 0-0.15 A would seem unlikely to trigger a significant protein conformation change, but it is conceivable that the protein, imposing its own constraints, pulls the cobalt atom significantly out of the heme plane in the deoxy quaternary structure.…”

mentioning

confidence: 99%

“…A displacement of 0-0.15 A would seem unlikely to trigger a significant protein conformation change, but it is conceivable that the protein, imposing its own constraints, pulls the cobalt atom significantly out of the heme plane in the deoxy quaternary structure. Moreover, contraction of the cobalt-histidine bond on oxygenation, estimated at 0.22 A (10,30), could play a significant role.…”

mentioning

confidence: 99%

Resonance Raman Spectra of Cobalt-Substituted Hemoglobin: Cooperativity and Displacement of the Cobalt Atom upon Oxygenation

Woodruff

Spiro

Yonetani

1974

The resonance Raman spectra of oxy and deoxy cobalt-substituted hemoglobin (CoHb) are reported. Comparison of these spectra to those of hemoglobin, methemoglobin, cytochrome c, and model cobalt porphyrin complexes suggests that the displacement of the cobalt atom upon oxygenation of CoHb is no greater than the out-of-plane distance in five-coordinate Co(II) porphyrins, 0.15 A. Combining this distance with the expected contraction of the cobalt-histidine bond, Ibers has estimated a maximum displacement of 0.37 A for the proximal histidine with respect to the heme plane upon oxygenation, about one-third the corresponding distance estimated for iron hemoglobin. The free energy of cooperativity for cobalt hemoglobin is also estimated to be onethird that of iron hemoglobin. These results are therefore consistent with Hopfield's distributed energy model, which predicts proportionality between proximal histidine displacement and the free energy of cooperativity. By implication they support Perutz's trigger mechanism for cooperativity.