Time-resolved resonance Raman study of intermediates generated after photodissociation of wild-type and mutant co-myoglobins

A role of heme side-chains of human hemoglobin in its function revealed by circular dichroism and resonance Raman spectroscopy

Structural changes of heme side-chains of human adult hemoglobin (Hb A) upon ligand (O₂ or CO) dissociation have been studied by circular dichroism (CD) and resonance Raman (RR) spectroscopies. We point out the occurrence of appreciable deformation of heme side-chains like vinyl and propionate groups prior to the out-of-plane displacement of heme iron. Referring to the recent fine resolved crystal structure of Hb A, the deformations of heme side-chains take place only in the β subunits. However, these changes are not observed in the isolated β chain (β₄ homotetramer) and, therefore, are associated with the α-β inter-subunit interactions. For the communications between α and β subunits in Hb A regarding signals of ligand dissociation, possible routes are proposed on the basis of the time-resolved absorption, CD, MCD (magnetic CD), and RR spectroscopies. Our finding of the movements of heme side-chains would serve as one of the clues to solve the cooperative O₂ binding mechanism of Hb A.

Crystallographic and Spectroscopic Studies of Peroxide-derived Myoglobin Compound II and Occurrence of Protonated FeIV–O

High resolution crystal structures of myoglobin in the pH range 5.2-8.7 have been used as models for the peroxide-derived compound II intermediates in heme peroxidases and oxygenases. The observed Fe-O bond length (1.86-1.90 A) is consistent with that of a single bond. The compound II state of myoglobin in crystals was controlled by single-crystal microspectrophotometry before and after synchrotron data collection. We observe some radiation-induced changes in both compound II (resulting in intermediate H) and in the resting ferric state of myoglobin. These radiation-induced states are quite unstable, and compound II and ferric myoglobin are immediately regenerated through a short heating above the glass transition temperature (<1 s) of the crystals. It is unclear how this influences our compound II structures compared with the unaffected compound II, but some crystallographic data suggest that the influence on the Fe-O bond distance is minimal. Based on our crystallographic and spectroscopic data we suggest that for myoglobin the compound II intermediate consists of an Fe(IV)-O species with a single bond. The presence of Fe(IV) is indicated by a small isomer shift of delta = 0.07 mm/s from Mössbauer spectroscopy. Earlier quantum refinements (crystallographic refinement where the molecular-mechanics potential is replaced by a quantum chemical calculation) and density functional theory calculations suggest that this intermediate H species is protonated.

The contribution of heme propionate groups to the conformational dynamics associated with CO photodissociation from horse heart myoglobin

Photoacoustic calorimetry and transient absorption spectroscopy were used to study conformational dynamics associated with CO photodissociation from horse heart myoglobin (Mb) reconstituted with either Fe protoporphyrin IX dimethylester (FePPDME), Fe octaethylporphyrin (FeOEP), or with native Fe protoporphyrin IX (FePPIX). The volume and enthalpy changes associated with the Fe-CO bond dissociation and formation of a transient deoxyMb intermediate for the reconstituted Mbs were found to be similar to those determined for native Mb (DeltaV1 = -2.5+/-0.6 ml mol(-1) and DeltaH1 = 8.1+/-3.0 kcal mol(-1)). The replacement of FePPIX by FeOEP significantly alters the conformational dynamics associated with CO release from protein. Ligand escape from FeOEP reconstituted Mb was determined to be roughly a factor of two faster (tau=330 ns) relative to native protein (tau=700 ns) and accompanying reaction volume and enthalpy changes were also found to be smaller (DeltaV2 = 5.4+/-2.5 ml mol(-1) and DeltaH2 = 0.7+/-2.2 kcal mol(-1)) than those for native Mb (DeltaV2 = 14.3+/-0.8 ml mol(-1) and DeltaH2 = 7.8+/-3.5 kcal mol(-1)). On the other hand, volume and enthalpy changes for CO release from FePPIX or FePPDME reconstituted Mb were nearly identical to those of the native protein. These results suggest that the hydrogen bonding network between heme propionate groups and nearby amino acid residues likely play an important role in regulating ligand diffusion through protein matrix. Disruption of this network leads to a partially open conformation of protein with less restricted ligand access to the heme binding pocket.

Multiple active site conformers in the carbon monoxide complexes of trematode hemoglobins

Sequence alignment of hemoglobins of the trematodes Paramphistomum epiclitum and Gastrothylax crumenifer with myoglobin suggests the presence of an unusual active site structure in which two tyrosine residues occupy the E7 and B10 helical positions. In the crystal structure of P. epiclitum hemoglobin, such an E7-B10 tyrosine pair at the putative helical positions has been observed, although the E7 Tyr is displaced toward CD region of the polypeptide. Resonance Raman data on both P. epiclitum and G. crumenifer hemoglobins show that interactions of heme-bound ligands with neighboring amino acid residues are unusual. Multiple conformers in the CO complex, termed the C, O, and N conformers, are observed. The conformers are separated by a large difference (approximately 60 cm(-1)) in the frequencies of their Fe-CO stretching modes. In the C conformer the Fe-CO stretching frequency is very high, 539 and 535 cm(-1), for the P. epiclitum and G. crumenifer hemoglobins, respectively. The Fe-CO stretching of the N conformer appears at an unusually low frequency, 479 and 476 cm(-1), respectively, for the two globins. A population of an O conformer is seen in both hemoglobins, at 496 and 492 cm(-1), respectively. The C conformer is stabilized by a strong polar interaction of the CO with the distal B10 tyrosine residue. The O conformer is similar to the ones typically seen in mutant myoglobins in which there are no strong interactions between the CO and residues in the distal pocket. The N conformer possesses an unusual configuration in which a negatively charged group, assigned as the oxygen atom of the B10 Tyr side chain, interacts with the CO. In this conformer, the B10 Tyr assumes an alternative conformation consistent with one of the conformers seen the crystal structure. Implications of the multiple configurations on the ligand kinetics are discussed.

Paper electrophoretic determination of the stability constants of binary and ternary complexes of copper(II) and cobalt(II) with nitrilotriacetate and cysteine

A paper ionophoretic method is described for the study of equilibria in mixed ligand (nitrilotriacetate-cysteine) complex system in solution. The proportion of ionic species of nitrilotriacetate (NTA) and cysteine were varied by changing the pH of background electrolyte. The stability constants of Cu(II)-NTA-cysteine and Co(II)-NTA-cysteine complexes were found to be 6.35+/-0.05 and 5.45+/-0.02 (logarithm stability constant values), respectively, at ionic strength 0.1 M and a temperature of 35 degrees C.

The Escape Process of Carbon Monoxide from Myoglobin to Solution at Physiological Temperature

The carbon monoxide (CO) docking sites involved in the ligand escape process from the iron atom in hem of myoglobin (Mb) to solution at physiological temperature were studied on the basis of the effect of xenon (Xe) on the ligand escape rate by the transient grating (TG) technique. The TG method provides a direct measurement of the changes in molecular volume. The apparent CO escaping rate and the volume contraction increase with increasing Xe pressure. The pressure dependence of the rate is consistent with that of the Xe population at the Xe(1) site. This result clearly shows that CO is trapped at the Xe(1) site before escaping to solvent in a Xe-free solution at room temperature. It is shown that only CO but not the trapped Xe is released by the photoexcitation of the Xe-trapped MbCO. A dissociation scheme is proposed to explain the enhancement of the escaping rate by the presence of Xe(1). There are two branches for the CO escaping pathway. The dominant part of the dissociated CO escapes to the solvent through the Xe(1) trapping site under the Xe-free condition, and there are at least three intermediate states along this pathway. When a Xe atom blocks the Xe(1) site, the CO escapes through another route.

Mutual control of axial and equatorial ligands: model studies with [Ni]-bacteriochlorophyll-a

Modification of the metal's electronic environment by ligand association and dissociation in metalloenzymes is considered cardinal to their catalytic activity. We have recently presented a novel system that utilizes the bacteriochlorophyll (BChl) macrocycle as a ligand and reporter. This system allows for charge mobilization in the equatorial plane and experimental estimate of changes in the electronic charge density around the metal with no modification of the metal's chemical environment. The unique spectroscopy, electrochemistry and coordination chemistry of [Ni]-bacteriochlorophyll ([Ni]-BChl) enable us to follow directly fine details and steps involved in the function of the metal redox center. This approach is utilized here whereby electro-chemical reduction of [Ni]-BChl to the monoanion [Ni]-BChl(-) results in reversible dissociation of biologically relevant axial ligands. Similar ligand dissociation was previously detected upon photoexcitation of [Ni]-BChl (Musewald, C.; Hartwich, G.; Lossau, H.; Gilch, P.; Pollinger-Dammer, F.; Scheer, H.; Michel-Beyerle, M. E. J. Phys. Chem. B 1999, 103, 7055-7060 and Noy, D.; Yerushalmi, R.; Brumfeld, V.; Ashur, I.; Baldridge, K. K.; Scheer, H.; Scherz, A. J. Am. Chem. Soc. 2000, 122, 3937-3944). The electrochemical measurements and quantum mechanical (QM) calculations performed here for the neutral, singly reduced, monoligated, and singly reduced, monoligated [Ni]-BChl suggest the following: (a) Electroreduction, although resulting in a pi anion [Ni]-BChl(-) radical, causes electron density migration to the [Ni]-BChl core. (b) Reduction of nonligated [Ni]-BChl does not change the macrocycle conformation, whereas axial ligation results in a dramatic expansion of the metal core and a flattening of the highly ruffled macrocycle conformation. (c) In both the monoanion and singly excited [Ni]-BChl ([Ni]-BChl*), the frontier singly occupied molecular orbital (SOMO) has a small but nonnegligible metal character. Finally, (d) computationally, we found that a reduction of [Ni]-BChl*imidazole results in a weaker metal-axial ligand bond. Yet, it remains weakly bound in the gas phase. The experimentally observed ligand dissociation is accounted for computationally when solvation is considered. On the basis of the experimental observations and QM calculations, we propose a mechanism whereby alterations in the equatorial pi system and modulation of sigma bonding between the axial ligands and the metal core are mutually correlated. Such a mechanism highlights the dynamic role of axial ligands in regulating the activity of metal centers such as factor F430 (F430), a nickel-based coenzyme that is essential in methanogenic archea.

Carbon monoxide complex of cytochrome b(5) at acidic pH

CO complex of cyt b(5) generated at acidic pH is investigated by absorption, resonance Raman (RR), and far UV CD measurements. The Soret maximum wavelength blue-shifted to 420 nm with other absorption bands observed around 540 and 570 nm for reduced cyt b(5) upon interaction with CO at acidic pH (pH 3.1-3.5). Under this condition, the iron-carbon stretching RR band was observed at 529 cm(-1) (520 cm(-1) for C(18)O), which indicated formation of a heme&bond;CO adduct with a histidine as an axial ligand. Heme dissociated from the reduced cyt b(5) protein at pH approximately 3.5, whereas its rate decreased under CO atmosphere compared with N(2) atmosphere, due to formation of a heme&bond;CO adduct with a histidine as an axial ligand.