Structural modifications induced by the switch from an endogenous bis-histidyl to an exogenous cyanomet hexa-coordination in a tetrameric haemoglobin

Two EPR- and structurally-distinct bis-histidyl conformers of the ferric haemoglobin from Trematomus bernacchii react differently with CN⁻

Structural basis of the lactate-dependent allosteric regulation of oxygen binding in arthropod hemocyanin

Hemocyanin (Hc) is an oxygen carrier protein in which oxygen binding is regulated by allosteric effectors such as H(+) and L-lactate. Isothermal titration calorimetric measurements showed that L-lactate binds to dodecameric and heterohexameric Hc and to the CaeSS3 homohexamer but not to the CaeSS2 monomer. The binding of lactate caused no change in the optical absorption and x-ray absorption spectra of either oxy- or deoxy-Hc, suggesting that no structural rearrangement of the active site occurred. At pH 6.5, the oxygen binding rate constant k(obs) obtained by flash photolysis showed a significant increase upon addition of L-lactate, whereas L-lactate addition had little effect at pH 8.3. Lactate binding caused a concentration-dependent shift in the interhexameric distances at pH 6.5 based on small angle x-ray scattering measurements. These results show that L-lactate affects oxygen affinity at pH 6.5 by modulating the global structure of Hc without affecting its binuclear copper center (the active site). In contrast to this, the active site structure of deoxy-Hc is affected by changes in pH (Hirota, S., Kawahara, T., Beltramini, M., Di Muro, P., Magliozzo, R. S., Peisach, J., Powers, L. S., Tanaka, N., Nagao, S., and Bubacco, L. (2008) J. Biol. Chem. 283, 31941-31948). Upon addiction of lactate, the kinetic behavior of oxygen rebinding for Hc was heterogeneous under low oxygen concentrations at pH 6.5 due to changes in the T and R state populations, and the equilibrium was found to shift from the T toward the R state with addition of lactate.

Correlation between hemichrome stability and the root effect in tetrameric hemoglobins

Oxidation of Hbs leads to the formation of different forms of Fe(III) that are relevant to a range of biochemical and physiological functions. Here we report a combined EPR/x-ray crystallography study performed at acidic pH on six ferric tetrameric Hbs. Five of the Hbs were isolated from the high-Antarctic notothenioid fishes Trematomus bernacchii, Trematomus newnesi, and Gymnodraco acuticeps, and one was isolated from the sub-Antarctic notothenioid Cottoperca gobio. Our EPR analysis reveals that 1), in all of these Hbs, at acidic pH the aquomet form and two hemichromes coexist; and 2), only in the three Hbs that exhibit the Root effect is a significant amount of the pentacoordinate (5C) high-spin Fe(III) form found. The crystal structure at acidic pH of the ferric form of the Root-effect Hb from T. bernacchii is also reported at 1.7 A resolution. This structure reveals a 5C state of the heme iron for both the alpha- and beta-chains within a T quaternary structure. Altogether, the spectroscopic and crystallographic results indicate that the Root effect and hemichrome stability at acidic pH are correlated in tetrameric Hbs. Furthermore, Antarctic fish Hbs exhibit higher peroxidase activity than mammalian and temperate fish Hbs, suggesting that a partial hemichrome state in tetrameric Hbs, unlike in monomeric Hbs, does not remove the need for protection from peroxide attack, in contrast to previous results from monomeric Hbs.

Aspects of structure and bonding in copper-amino acid complexes revealed by single-crystal EPR/ENDOR spectroscopy and density functional calculations

This work deduces from a series of well-defined copper-doped amino acid crystals, relationships between structural features of the copper complexes, and ligand-bound proton hyperfine parameters. These were established by combining results from electron paramagnetic resonance (EPR)/electron-nuclear double resonance (ENDOR) studies, crystallography, and were further assessed by quantum mechanical (QM) calculations. A detailed evaluation of previous studies on Cu(2+) doped into alpha-glycine, triglycine sulfate, alpha-glycylglycine, and L-alanine crystals reveal correlations between geometric features of the copper sites and proton hyperfine couplings from amino-bound and carbon-bound hydrogens. Experimental variations in proton isotropic hyperfine coupling values (a(iso)) could be fit to cosine-square dependences on dihedral angles, namely, for C(alpha)-bound hydrogens, a(iso) = -1.09 + 8.21 cos(2) theta MHz, and for amino hydrogens, a(iso) = -6.16 + 4.15 cos(2) phi MHz. For the C(alpha) hydrogens, this dependency suggests a hyperconjugative-like mechanism for transfer of spin density into the hydrogen 1s orbital. In the course of this work, it was also necessary to reanalyze the ENDOR measurements from Cu(2+)-doped alpha-glycine because the initial study determined the (14)N coupling parameters without holding its nuclear quadrupole tensor traceless. This new treatment of the data was needed to correctly align the (14)N hyperfine tensor principal directions in the molecular complex. To provide a theoretical basis for the coupling variations, QM calculations performed at the DFT level were used to compute the proton hyperfine tensors in the four crystal complexes as well as in a geometry-optimized Cu(2+)(glycine)(2) model. These theoretical calculations confirmed systematic changes in couplings with dihedral angles but greatly overestimated the experimental geometric sensitivity to the amino hydrogen isotropic coupling.

Molecular basis of the Bohr effect in arthropod hemocyanin

Flash photolysis and K-edge x-ray absorption spectroscopy (XAS) were used to investigate the functional and structural effects of pH on the oxygen affinity of three homologous arthropod hemocyanins (Hcs). Flash photolysis measurements showed that the well-characterized pH dependence of oxygen affinity (Bohr effect) is attributable to changes in the oxygen binding rate constant, k(on), rather than changes in k(off). In parallel, coordination geometry of copper in Hc was evaluated as a function of pH by XAS. It was found that the geometry of copper in the oxygenated protein is unchanged at all pH values investigated, while significant changes were observed for the deoxygenated protein as a function of pH. The interpretation of these changes was based on previously described correlations between spectral lineshape and coordination geometry obtained for model compounds of known structure (Blackburn, N. J., Strange, R. W., Reedijk, J., Volbeda, A., Farooq, A., Karlin, K. D., and Zubieta, J. (1989) Inorg. Chem., 28, 1349-1357). A pH-dependent change in the geometry of cuprous copper in the active site of deoxyHc, from pseudotetrahedral toward trigonal was assigned from the observed intensity dependence of the 1s --> 4p(z) transition in x-ray absorption near edge structure (XANES) spectra. The structural alteration correlated well with increase in oxygen affinity at alkaline pH determined in flash photolysis experiments. These results suggest that the oxygen binding rate in deoxyHc depends on the coordination geometry of Cu(I) and suggest a structural origin for the Bohr effect in arthropod Hcs.

Hemoglobin structure/function and globin-gene evolution in the Arctic fish Liparis tunicatus

The importance of the Arctic, in contributing to the knowledge of the overall ensemble of adaptive processes influencing the evolution of marine organisms, calls for investigations on molecular adaptations in Arctic fish. Unlike the vast majority of Antarctic Notothenioidei, several Arctic species display high hemoglobin multiplicity. The blood of four species, the spotted wolffish of the family Anarhichadidae and three Gadidae, contains three functionally distinct major components. Similar to many Antarctic notothenioids, Arctic Liparis tunicatus (suborder Cottoidei, family Liparidae) has one major hemoglobin (Hb 1) accompanied by a minor component (Hb 2). This paper reports the structural and functional characterisation of Hb 1 of L. tunicatus. This hemoglobin shows low oxygen affinity, and pronounced Bohr and Root effects. The amino-acid sequence of the beta chain displays an unusual substitution in NA2 (beta2) at the phosphate-binding site, and the replacement of Val E11 (beta67) with Ile. Similar to some Antarctic fish Hbs, electron paramagnetic resonance spectra reveal the formation of a ferric penta-coordinated species even at physiological pH. The amino-acid sequences have also been used to gain insight into the evolutionary history of globins of polar fish. L. tunicatus globins appear close to the notothenioid clades as predicted by teleostean phylogenies. Close phylogenetic relationships between Cottoidei and Notothenioidei, together with their life style, seem to be the main factor driving the globin-sequence evolution.

Structural characterization of ferric hemoglobins from three antarctic fish species of the suborder notothenioidei

Spontaneous autoxidation of tetrameric Hbs leads to the formation of Fe (III) forms, whose physiological role is not fully understood. Here we report structural characterization by EPR of the oxidized states of tetrameric Hbs isolated from the Antarctic fish species Trematomus bernacchii, Trematomus newnesi, and Gymnodraco acuticeps, as well as the x-ray crystal structure of oxidized Trematomus bernacchii Hb, redetermined at high resolution. The oxidation of these Hbs leads to formation of states that were not usually detected in previous analyses of tetrameric Hbs. In addition to the commonly found aquo-met and hydroxy-met species, EPR analyses show that two distinct hemichromes coexist at physiological pH, referred to as hemichromes I and II, respectively. Together with the high-resolution crystal structure (1.5 A) of T. bernacchii and a survey of data available for other heme proteins, hemichrome I was assigned by x-ray crystallography and by EPR as a bis-His complex with a distorted geometry, whereas hemichrome II is a less constrained (cytochrome b5-like) bis-His complex. In four of the five Antartic fish Hbs examined, hemichrome I is the major form. EPR shows that for HbCTn, the amount of hemichrome I is substantially reduced. In addition, the concomitant presence of a penta-coordinated high-spin Fe (III) species, to our knowledge never reported before for a wild-type tetrameric Hb, was detected. A molecular modeling investigation demonstrates that the presence of the bulkier Ile in position 67beta in HbCTn in place of Val as in the other four Hbs impairs the formation of hemichrome I, thus favoring the formation of the ferric penta-coordinated species. Altogether the data show that ferric states commonly associated with monomeric and dimeric Hbs are also found in tetrameric Hbs.

The octarepeat domain of the prion protein binds Cu(II) with three distinct coordination modes at pH 7.4

The prion protein (PrP) binds Cu2+ in its N-terminal octarepeat domain. This unusual domain is comprised of four or more tandem repeats of the fundamental sequence PHGGGWGQ. Previous work from our laboratories demonstrates that at full copper occupancy, each HGGGW segment binds a single Cu2+. However, several recent studies suggest that low copper occupancy favors different coordination modes, possibly involving imidazoles from histidines in adjacent octapeptide segments. This is investigated here using a combination of X-band EPR, S-band EPR, and ESEEM, along with a library of modified peptides designed to favor different coordination interactions. At pH 7.4, three distinct coordination modes are identified. Each mode is fully characterized to reveal a series of copper-dependent octarepeat domain structures. Multiple His coordination is clearly identified at low copper stoichiometry. In addition, EPR detected copper-copper interactions at full occupancy suggest that the octarepeat domain partially collapses, perhaps stabilizing this specific binding mode and facilitating cooperative copper uptake. This work provides the first complete characterization of all dominant copper coordination modes at pH 7.4.

Copper coordination in the full-length, recombinant prion protein

The prion protein (PrP) binds divalent copper at physiologically relevant conditions and is believed to participate in copper regulation or act as a copper-dependent enzyme. Ongoing studies aim at determining the molecular features of the copper binding sites. The emerging consensus is that most copper binds in the octarepeat domain, which is composed of four or more copies of the fundamental sequence PHGGGWGQ. Previous work from our laboratory using PrP-derived peptides, in conjunction with EPR and X-ray crystallography, demonstrated that the HGGGW segment constitutes the fundamental binding unit in the octarepeat domain [Burns et al. (2002) Biochemistry 41, 3991-4001; Aronoff-Spencer et al. (2000) Biochemistry 39, 13760-13771]. Copper coordination arises from the His imidazole and sequential deprotonated glycine amides. In this present work, recombinant, full-length Syrian hamster PrP is investigated using EPR methodologies. Four copper ions are taken up in the octarepeat domain, which supports previous findings. However, quantification studies reveal a fifth binding site in the flexible region between the octarepeats and the PrP globular C-terminal domain. A series of PrP peptide constructs show that this site involves His96 in the PrP(92-96) segment GGGTH. Further examination by X-band EPR, S-band EPR, and electron spin-echo envelope spectroscopy, demonstrates coordination by the His96 imidazole and the glycine preceding the threonine. The copper affinity for this type of binding site is highly pH dependent, and EPR studies here show that recombinant PrP loses its affinity for copper below pH 6.0. These studies seem to provide a complete profile of the copper binding sites in PrP and support the hypothesis that PrP function is related to its ability to bind copper in a pH-dependent fashion.

An appreciation of William H. Orme-Johnson III

This short memoir of personal and professional interactions with Bill Orme-Johnson covers a period of more than 35 years. Included are reflections on the early history of copper oxidases, iron-sulfur proteins and nitrogenase.

Kinetic and magnetic resonance studies of the role of metal ions in the mechanism of Escherichia coli GDP-mannose mannosyl hydrolase, an unusual nudix enzyme

Escherichia coli GDP-mannose mannosyl hydrolase (GDPMH), a homodimer, catalyzes the hydrolysis of GDP-alpha-D-sugars to yield the beta-D-sugar and GDP by nucleophilic substitution with inversion at the C1' carbon of the sugar [Legler, P. M., Massiah, M. A., Bessman, M. J., and Mildvan, A. S. (2000) Biochemistry 39, 8603-8608]. GDPMH requires a divalent cation for activity such as Mn2+ or Mg2+, which yield similar kcat values of 0.15 and 0.13 s(-1), respectively, at 22 degrees C and pH 7.5. Kinetic analysis of the Mn2+-activated enzyme yielded a K(m) of free Mn2+ of 3.9 +/- 1.3 mM when extrapolated to zero substrate concentration (K(a)Mn2+), which tightened to 0.32 +/- 0.18 mM when extrapolated to infinite substrate concentration (K(m)Mn2+). Similarly, the K(m) of the substrate extrapolated to zero Mn2+ concentration (K(S)(GDPmann) = 1.9 +/- 0.5 mM) and to infinite Mn2+ concentration (K(m)(GDPmann) = 0.16 +/- 0.09 mM) showed an order of magnitude decrease at saturating Mn2+. Such mutual tightening of metal and substrate binding suggests the formation of an enzyme-metal-substrate bridge complex. Direct Mn2+ binding studies, monitoring the concentration of free Mn2+ by EPR and of bound Mn2+ by its enhanced paramagnetic effect on the longitudinal relaxation rate of water protons (PRR), detected three Mn2+ binding sites per enzyme monomer with an average dissociation constant (K(D)) of 3.2 +/- 1.0 mM, in agreement with the kinetically determined K(a)Mn2+. The enhancement factor (epsilon(b)) of 11.5 +/- 1.2 indicates solvent access to the enzyme-bound Mn2+ ions. No cross relaxation was detected among the three bound Mn2+ ions, suggesting them to be separated by at least 10 A. Such studies also yielded a weak dissociation constant for the binary Mn2+-GDP-mannose complex (K1 = 6.5 +/- 1.0 mM) which significantly exceeded the kinetically determined K(m) values of Mn2+, indicating the true substrate to be GDP-mannose rather than its Mn2+ complex. Substrate binding monitored by changes in 1H-15N HSQC spectra yielded a dissociation constant for the binary E-GDP-mannose complex (K(S)(GDPmann)) of 4.0 +/- 0.5 mM, comparable to the kinetically determined K(S) value (1.9 +/- 0.5 mM). To clarify the metal stoichiometry at the active site, product inhibition by GDP, a potent competitive inhibitor (K(I) = 46 +/- 27 microM), was studied. Binding studies revealed a weak, binary E-GDP complex (K(D)(GDP) = 9.4 +/- 3.2 mM) which tightened approximately 500-fold in the presence of Mn2+ to yield a ternary E-Mn2+-GDP complex with a dissociation constant, K3(GDP) = 18 +/- 9 microM, which overlaps with the K(I)(GDP). The tight binding of Mn2+ to 0.7 +/- 0.2 site per enzyme subunit in the ternary E-Mn2+-GDP complex (K(A)' = 15 microM) and the tight binding of GDP to 0.8 +/- 0.1 site per enzyme subunit in the ternary E-Mg2+-GDP complex (K3 < 0.5 mM) indicate a stoichiometry close to 1:1:1 at the active site. The decrease in the enhancement factor of the ternary E-Mn2+-GDP complex (epsilon(T) = 4.9 +/- 0.4) indicates decreased solvent access to the active site Mn2+, consistent with an E-Mn2+-GDP bridge complex. Fermi contact splitting (4.3 +/- 0.2 MHz) of the phosphorus signal in the ESEEM spectrum established the formation of an inner sphere E-Mn2+-GDP complex. The number of water molecules coordinated to Mn2+ in this ternary complex was determined by ESEEM studies in D2O to be two fewer than on the average Mn2+ in the binary E-Mn2+ complexes, consistent with bidentate coordination of enzyme-bound Mn2+ by GDP. Kinetic, metal binding, and GDP binding studies with Mg2+ yielded dissociation constants similar to those found with Mn2+. Hence, GDPMH requires one divalent cation per active site to promote catalysis by facilitating the departure of the GDP leaving group, unlike its homologues the MutT pyrophosphohydrolase, which requires two, or Ap4A pyrophosphatase, which requires three.

Molecular features of the copper binding sites in the octarepeat domain of the prion protein

Recent evidence suggests that the prion protein (PrP) is a copper binding protein. The N-terminal region of human PrP contains four sequential copies of the highly conserved octarepeat sequence PHGGGWGQ spanning residues 60-91. This region selectively binds Cu2+ in vivo. In a previous study using peptide design, EPR, and CD spectroscopy, we showed that the HGGGW segment within each octarepeat comprises the fundamental Cu2+ binding unit [Aronoff-Spencer et al. (2000) Biochemistry 40, 13760-13771]. Here we present the first atomic resolution view of the copper binding site within an octarepeat. The crystal structure of HGGGW in a complex with Cu2+ reveals equatorial coordination by the histidine imidazole, two deprotonated glycine amides, and a glycine carbonyl, along with an axial water bridging to the Trp indole. Companion S-band EPR, X-band ESEEM, and HYSCORE experiments performed on a library of 15N-labeled peptides indicate that the structure of the copper binding site in HGGGW and PHGGGWGQ in solution is consistent with that of the crystal structure. Moreover, EPR performed on PrP(23-28, 57-91) and an 15N-labeled analogue demonstrates that the identified structure is maintained in the full PrP octarepeat domain. It has been shown that copper stimulates PrP endocytosis. The identified Gly-Cu linkage is unstable below pH approximately 6.5 and thus suggests a pH-dependent molecular mechanism by which PrP detects Cu2+ in the extracellular matrix or releases PrP-bound Cu2+ within the endosome. The structure also reveals an unusual complementary interaction between copper-structured HGGGW units that may facilitate molecular recognition between prion proteins, thereby suggesting a mechanism for transmembrane signaling and perhaps conversion to the pathogenic form.

Identification of the Cu2+ binding sites in the N-terminal domain of the prion protein by EPR and CD spectroscopy

Recent evidence indicates that the prion protein (PrP) plays a role in copper metabolism in the central nervous system. The N-terminal region of human PrP contains four sequential copies of the highly conserved octarepeat sequence PHGGGWGQ spanning residues 60-91. This region selectively binds divalent copper ions (Cu(2+)) in vivo. To elucidate the specific mode and site of binding, we have studied a series of Cu(2+)-peptide complexes composed of 1-, 2-, and 4-octarepeats and several sub-octarepeat peptides, by electron paramagnetic resonance (EPR, conventional X-band and low-frequency S-band) and circular dichroism (CD) spectroscopy. At pH 7.45, two EPR active binding modes are observed where the dominant mode appears to involve coordination of three nitrogens and one oxygen to the copper ion, while in the minor mode two nitrogens and two oxygens coordinate. ESEEM spectra demonstrate that the histidine imidazole contributes one of these nitrogens. The truncated sequence HGGGW gives EPR and CD that are indistinguishable from the dominant binding mode observed for the multi-octarepeat sequences and may therefore comprise the fundamental Cu(2+) binding unit. Both EPR and CD titration experiments demonstrate rigorously a 1:1 Cu(2+)/octarepeat binding stoichiometry regardless of the number of octarepeats in a given peptide sequence. Detailed spin integration of the EPR signals demonstrates that all of the bound Cu(2+) is detected thereby ruling out strong exchange coupling that is often found when there is imidazolate bridging between paramagnetic metal centers. A model consistent with these data is proposed in which Cu(2+) is bound to the nitrogen of the histidine imidazole side chain and to two nitrogens from sequential glycine backbone amides.

Demonstration of a conserved histidine and two water ligands at the Mn2+ site in Diocleinae lectins by pulsed EPR spectroscopy

Lectins from the Diocleinae subtribe, including Canavalia brasiliensis, Canavalia bonariensis, Canavalia grandiflora, Cratylia floribunda, Dioclea grandiflora, Dioclea guianensis, Dioclea rostrata, Dioclea violacea, and Dioclea virgata, have been recently isolated and characterized in terms of their carbohydrate binding specificities. Although all of the lectins are Man/Glc specific, they possess different biological activities. In the present study, electron paramagnetic resonance (EPR) spectroscopy demonstrates that all nine Diocleinae lectins contain Mn2+. The spectra of C. floribunda and D. rostrata suggest Mn2+ site symmetry different from that of the other seven lectins. However, electron spin-echo envelope modulation (ESEEM) spectroscopy indicates that all nine lectins are coordinated to a histidyl imidazole, with similar electron-nuclear coupling to the Mn2+-bound imidazole nitrogen. ESEEM also demonstrates ligation of two water molecules to Mn2+ in all nine Diocleinae lectins. Thus, the EPR and ESEEM data indicate the presence of a Mn2+ binding site in the above Diocleinae lectins with a conserved histidine residue and two water ligands.

Electron spin-echo envelope modulation study of multicrystalline Cu(2+)-insulin: effects of Cd(2+) on the nuclear quadrupole interaction of the Cu(2+)-coordinated imidazole remote nitrogen

A comparison of electron spin-echo envelope modulation (ESEEM) spectra from multi-crystalline Cu(2+)-insulin with and without additional Cd(2+) show a dramatic change in the quadrupole coupling parameters of the remote nitrogens of the two histidine imidazoles that ligate to copper. Without Cd(2+), the quadrupole parameters are like those observed in blue copper proteins and in copper substituted lactoferrin. With Cd(2+) soaked into the Cu(2+)-insulin crystals, the quadrupole parameters are similar to those found in galactose oxidase. Theoretical simulations of ESEEM spectra guided by structure modeling suggest that these changes originate from differences in the hydrogen bonding environments of the imidazole remote nitrogen. In addition, a compilation of results from previous ESEEM studies of copper proteins reveals that the asymmetry parameter, eta, may be an indicator of type of hydrogen bond the imidazole remote nitrogen makes. When eta > or = 0.9, the nitrogen hydrogen bonds to water, whereas when eta < 0.9, the nitrogen hydrogen bonds to the protein.

Identification of the ligands to the ferric heme of Chlamydomonas chloroplast hemoglobin: evidence for ligation of tyrosine-63 (B10) to the heme

We have studied the unusual heme ligand structure of the ferric forms of a recombinant Chlamydomonas chloroplast hemoglobin and its several single-amino acid mutants by EPR, optical absorbance, and resonance Raman spectroscopy. The helical positions of glutamine-84, tyrosine-63, and lysine-87 are suggested to correspond to E7, B10, and E10, respectively, in the distal heme pocket on the basis of amino acid sequence comparison of mammalian globins. The protein undergoes a transition with a pK of 6.3 from a six-coordinate high-spin aquomet form at acidic pH to a six-coordinate low-spin form. The EPR signal of the low-spin form for the wild-type protein is absent for the Tyr63Leu mutant, suggesting that the B10 tyrosine in the wild-type protein ligates to the heme as tyrosinate. For the Tyr63Leu mutant, a new low-spin signal resembling that of alkaline cytochrome c (a His-heme-Lys species) is resolved, suggesting that the E10 lysine now coordinates to the heme. In the wild-type protein, the oxygen of the tyrosine-63 side chain is likely to share a proton with the side chain of lysine-87, suggested by the observation of a H/D sensitive resonance Raman line at 502 cm(-)(1) that is tentatively assigned as a vibrational mode of the Fe-O bond between the iron and the tyrosinate. We propose that the transition from the high-spin to the low-spin form of the protein occurs by deprotonation and ligation to the heme of the B10 tyrosine oxygen, facilitated by strong interaction with the E10 lysine side chain.

Chlamydomonas chloroplast ferrous hemoglobin. Heme pocket structure and reactions with ligands

We report the optical and resonance Raman spectral characterization of ferrous recombinant Chlamydomonas LI637 hemoglobin. We show that it is present in three pH-dependent equilibrium forms including a 4-coordinate species at acid pH, a 5-coordinate high spin species at neutral pH, and a 6-coordinate low spin species at alkaline pH. The proximal ligand to the heme is the imidazole group of a histidine. Kinetics of the reactions with ligands were determined by stopped-flow spectroscopy. At alkaline pH, combination with oxygen, nitric oxide, and carbon monoxide displays a kinetic behavior that is interpreted as being rate-limited by conversion of the 6-coordinate form to a reactive 5-coordinate form. At neutral pH, combination rates of the 5-coordinate form with oxygen and carbon monoxide were much faster (>10(7) microM-1 s-1). The dissociation rate constant measured for oxygen is among the slowest known, 0.014 s-1, and is independent of pH. Replacement of the tyrosine 63 (B10) by leucine or of the putative distal glutamine by glycine increases the dissociation rate constant 70- and 30-fold and increases the rate of autoxidation 20- and 90-fold, respectively. These results are consistent with at least two hydrogen bonds stabilizing the bound oxygen molecule, one from tyrosine B10 and the other from the distal glutamine. In addition, the high frequency (232 cm-1) of the iron-histidine bond suggests a structure that lacks any proximal strain thus contributing to high ligand affinity.

The heme environment in barley hemoglobin

To elucidate the environment and ligand structure of the heme in barley hemoglobin (Hb), resonance Raman and electron paramagnetic resonance spectroscopic studies have been carried out. The heme is shown to have bis-imidazole coordination, and neither of the histidines has imidazolate character. Barley Hb has a unique heme environment as judged from the Fe-CO and C-O stretching frequencies in the CO complex. Two Fe-CO stretching modes are observed with frequencies at 534 and 493 cm-1, with relative intensities that are pH sensitive. The 534 cm-1 conformer shows a deuterium shift, indicating that the iron-bound CO is hydrogen-bonded, presumably to the distal histidine. A C-O stretching mode at 1924 cm-1 is assigned as being associated with the 534 cm-1 conformer. Evidence is presented that the high Fe-CO and low C-O stretching frequencies (534 and 1924 cm-1, respectively) arise from a short hydrogen bond between the distal histidine and the CO. The 493 cm-1 conformer arises from an open conformation of the heme pocket and becomes the dominant population under acidic conditions when the distal histidine moves away from the CO. Strong hydrogen bonding between the bound ligand and the distal histidine in the CO complex of barley Hb implies that a similar structure may occur in the oxy derivative, imparting a high stability to the bound oxygen. This stabilization is confirmed by the dramatic decrease in the oxygen dissociation rate compared with sperm whale myoglobin.