Sulfate as a synergistic anion facilitating iron binding by the bacterial transferrin FbpA: the origins and effects of anion promiscuity.

The ferric binding protein, FbpA, has been demonstrated to facilitate the transport of naked Fe3+ across the periplasmic space of several Gram-negative bacteria. The sequestration of iron by FbpA is facilitated by the presence of a synergistic anion, such as phosphate or sulfate. Here we report the sequestration of Fe3+ by FbpA in the presence of sulfate, at an assumed periplasmic pH of 6.5 to form FeFbpA-SO4 with K'(eff) = 1.7 x 10(16) M(-1) (at 20 degrees C, 50 mM MES, 200 mM KCl). The iron affinity of the FeFbpA-SO4 protein assembly is 2 orders of magnitude lower than when bound with phosphate and is the lowest of any of the FeFbpA-X assemblies yet reported. Iron reduction at the cytosolic membrane receptor may be an essential aspect of the periplasmic iron-transport process, and with an E(1/2) of -158 mV (NHE), FeFbpA-SO4 is the most easily reduced of all FeFbpA-X assemblies yet studied. The variation of FeFbpA-X assembly stability (K'(eff)) and ease of reduction (E(1/2)) with differing synergistic anions X(n-) are correlated over a range of 14 kJ, suggesting that the variations in redox potentials are due to stabilization of Fe3+ in FeFbpA-X by X(n-). Anion promiscuity of FbpA in the diverse composition of the periplasmic space is illustrated by the ex vivo exchange kinetics of FeFbpA-SO4 with phosphate and arsenate, where first-order kinetics with respect to FeFbpA-SO4 (k = 30 s(-1)) are observed at pH 6.5, independent of entering anion concentration and identity. Anion lability and influence on the iron affinity and reduction potential for FeFbpA-X support the hypothesis that synergistic anion exchange may be an important regulator in iron delivery to the cytosol. This structural and thermodynamic analysis of anion binding in FeFbpA-X provides additional insight into anion promiscuity and importance.

New insights into the proton-dependent oxygen affinity of Root effect haemoglobins.

A long-standing puzzle with regard to protein structure/function relationships is the proton-dependent modification of haemoglobin (Hb) structure that causes oxygen to be unloaded from Root effect Hbs into the swim bladders and eyes of fish even against high oxygen pressure gradients. Although oxygen unloading in Root effect Hbs has generally been attributed to proton-dependent stabilization of the T-state, protonation of Root effect Hbs can alter their ligand affinities in both R- and T-state conformations and either stabilize the T-state or destabilize the R-state. The C-terminal residues that are so important in the Bohr effect of human Hb appear to be involved in the Root effects of some fish Hbs and not in others, indicating that several evolutionary pathways have resulted in expression of highly pH-dependent Hbs. New data are presented that show surprising similarities in the pH- and anion-dependence of sulfhydryl group reactivity and anaerobic oxidation of human and fish Hbs. The available evidence supports the concept that in both Bohr effect and Root effect Hbs a large steric component acts in addition to quaternary shifts between R and T conformations to regulate ligand affinity. Allosteric effectors moderate these steric effects within both R- and T-state conformations and allow for an elegant match between Hb function and the wide-ranging physiological needs of diverse organisms.

31P NMR probes of chemical dynamics: paramagnetic relaxation enhancement of the (1)H and (31)P NMR resonances of methyl phosphite and methylethyl phosphate anions by selected metal complexes.

Methyl phosphite ((CH(3)O)P(H)(O)(2)(-); MeOPH) and methylethyl phosphate ((CH(3)O)P(OCH(2)CH(3))(O)(2)(-); MEP) are two members of a class of anionic ligands whose (31)P T(2) relaxation rates are remarkably sensitive to paramagnetic metal ions. The temperature dependence of the (31)P NMR line broadenings caused by the Mn(H(2)O)(6)(2+) ion and a water-soluble manganese(III) porphyrin (Mn(III)TMPyP(5+)) indicates that the extent of paramagnetic relaxation enhancement is a measure of the rate at which the anionic probes come into physical contact with the paramagnetic center (i.e., enter the inner coordination shell); that is, piDeltanu(par) = k(assn)[M], where Deltanu(par) is the difference between the line widths of the resonance in paramagnetic and diamagnetic solutions, and k(assn) is the second-order rate constant for association of the phosphorus ligand with the metal, M. Comparison of the (31)P T(1) and T(2) relaxation enhancements shows that rapid T(2) relaxation by the metal ion is caused by scalar interaction with the electronic spin. Relaxation of the phosphorus-bound proton of MeOPH ((1)H-P) by Mn(III)TMPyP(5+) displayed intermediate exchange kinetics over much of the observable temperature range. The field strength dependence of (1)H-P T(2) enhancement and the independence of the (31)P T(2) support these assertions. As in the case of the (31)P T(2), the (1)H-P T(2) relaxation enhancement results from scalar interaction with the electronic spin. The scalar coupling interpretation of the NMR data is supported by a pulsed EPR study of the interactions of Mn(H(2)O)(6)(2+) with the P-deuterated analogue of methyl phosphite, CH(3)OP((2)H)(O)(2)(-). The electron to (31)P and (2)H nuclear scalar coupling constants were found to be 4.6 and 0.10 MHz, respectively. In contrast, the effects of paramagnetic ions on the methoxy and ethoxy (1)H resonances of MeOPH and MEP are weak, and the evidence suggests that relaxation of these nuclei occurs by a dipolar mechanism. The wide variation in the relaxation sensitivities of the (1)H and (31)P nuclei of MeOPH and MEP permits us to study how differences in the strengths of the interactions between an observed nucleus and a paramagnetic center affect NMR T(2) relaxations. We propose that these anion ligand probes may be used to study ligand-exchange reactivities of manganese complexes without requiring variable temperature studies. The (31)P T(2) is determined by chemical association kinetics when the following condition is met: (T(2M,P)/T(2M,H))(Deltanu(P)/Deltanu(HP) - 1) < 0.2 where T(2M,P) and T(2M,H) are the transverse relaxation times of the (31)P and (1)H nuclei when the probe is bound to the metal, and Deltanu(P) and Deltanu(HP) are the paramagnetic line broadenings of the (31)P and (1)H-P nuclei, respectively. We assert that the ratio T(2M,P)/T(2M,H) can be estimated for a general metal complex using the results of EPR and NMR experiments.

Crystal structure of ferrioxamine B: a comparative analysis and implications for molecular recognition.

Ferrioxamine B was successfully co-crystallized with ethanolpentaaquomagnesium(II) and perchlorate ions as counter ions, C27H62Cl3FeMgN6O26, and the crystal structure has been determined by single-crystal X-ray diffraction. The crystals are monoclinic, space group P2(1)/n, four molecules per unit cell with dimensions a=21.1945(7) A, b=10.0034(3) A, c=106.560(1) A, and beta=106.560(1) degrees. The crystal structure contains a racemic mixture of Lambda-N-cis,cis and Delta-N-cis,cis coordination isomers. The structural parameters and the conformational features of ferrioxamine B compare very well with those of ferrioxamines D1 and E, with an exception of the orientation of the pendant protonated amine, which is pointing away from the connecting amide chains and towards the carbonyl face of the inner coordination shell distorted octahedron. This pendant protonated amine, in conjunction with the carbonyl face of the Fe(III) coordination shell, is proposed to play an important role in the recognition and membrane transport processes.

Use of SDS micelles to stabilize a ternary intermediate in the reaction of ferrioxamine B and 1,10-phenanthroline.

Spectrophotometric measurements of the reaction of ferrioxamine B (FeHDFB(+)) with 1,10-phenanthroline (phen) reveal the presence of a ternary intermediate complex in both aqueous solution and an aqueous solution of 0.16 M sodium dodecyl sulfate (SDS). The stoichiometry of the intermediate is Fe(H(2)DFB)(phen)(2+) on the basis of a Schwarzenbach analysis of spectrophotometric data obtained at variable pH and phen concentrations. The ternary complex formation constant for the reaction FeHDFB(+) + H(+) + phen right arrow over left arrow Fe(H(2)DFB)(phen)(2+) is log K = 6.96 in aqueous solution and log K = 8.64 in aqueous 0.16 M SDS. The enhanced stability of Fe(H(2)DFB)(phen)(2+) in micellar solution was analyzed in terms of the pseudophase ion-exchange (PPIE) model of micellar reactions. The association constants for the binding of each reactant to the micellar pseudophase were measured by ultrafiltration. According to PPIE model calculations, the enhanced stability of Fe(H(2)DFB)(phen)(2+) in micellar SDS arises from a proximity effect created by the high local concentrations of reactants in the micellar pseudophase. The calculations also indicate that an inhibitory medium or compartmentalization effect is operative since the observed micellar enhancement is much smaller than predicted by the PPIE model. The micellar stabilization of the Fe(H(2)DFB)(phen)(2+) intermediate and the overall conversion of FeHDFB(+) to Fe(phen)(3)(2+) are discussed as a possible model system for siderophore iron release in microbial organisms.

Optimization of the lariat ether carboxylic acid host structure for ferrioxamine B: demonstration of a second coordination shell chelate effect.

Host-guest supramolecular assembly formation constants involving the second-sphere complexation of the siderophore ferrioxamine B (FeHDFB(+)) by a lariat ether carboxylic acid host (L(n+2)COOH) in wet chloroform were obtained from liquid-liquid extractions at pH values above and below the host pK(a) (approximately 5.3). The host-guest formation constants, K(a), determined at pH = 3.2 for the assemblies [FeHDFB(+),L(n+2)COOH,ClO(4)(-)] (n = 4, 7, 10, 15) in wet chloroform are similar to those of the parent crown ether, benzo-18-crown-6. At pH = 9.3, the lariat ethers are ionized, and this results in a more stable assembly, [FeHDFB(+),L(n+2)COO(-)], as measured by the host-guest formation constant, K(app). This enhanced stability is shown to be a function of the lariat ether sidearm chain length (n = 4, 7, 10, 15) and is corroborated by molecular modeling calculations. Additionally, molecular modeling and extraction data demonstrate that there is an optimum lariat ether sidearm chain length with respect to host-guest assembly stability as measured by K(app). We attribute the enhanced stability effect of the ionized lariat ether in the host-guest assembly [FeHDFB(+),L(n+2)COO(-)], relative to [FeHDFB(+),L(n)+2)COOH,ClO(4)(-)] or to [FeHDFB(+),B18C6,ClO(4)(-)], to a second coordination shell chelate effect.

Synthesis, ligand pK(a), and Fe(III) complexation constants for a series of bipodal dihydroxamic acids.

The synthesis of four bipodal dihydroxamic acids containing an apical C atom and amide linkages is described, where Ia,b represent "normal" and "retro" hydroxamate isomers: (R)CH[C(=O)NH(CH(2))(2)NHC(=O)(CH(2))(n)()R'](2) (Ia, R = CH(3), R' = N(OH)(C=O)CH(3), n = 2; Ib, R = CH(3), R' = (C=O)N(OH)CH(3), n = 2; Ic, R = CH(3), R' = (C=O)N(OH)CH(3), n = 3; Id, R = C(4)H(9), R' = (C=O)N(OH)CH(3), n = 2.). The pK(a1) and pK(a2) values in aqueous solution are reported, and some degree of cooperativity is noted. Complexation equilibria with Fe(aq)(3+) are described, and values for stepwise and overall equilibrium constants are reported. log beta(230) values for Ia-d are 59.22, 59.45, 58.91, and 58.46, slightly lower than for rhodotorulic acid, although the pFe values for the synthetic siderophores are comparable to that for rhodotorulic acid.

Kinetics and mechanism of a catalytic chloride ion effect on the dissociation of model siderophore hydroxamate-iron(III) complexes.

Proton-driven ligand dissociation kinetics in the presence of chloride, bromide, and nitrate ions have been investigated for model siderophore complexes of Fe(III) with the mono- and dihydroxamic acid ligands R(1)C(=O)N(OH)R(2) (R(1) = CH(3), R(2) = H; R(1) = CH(3), R(2) = CH(3); R(1) = C(6)H(5), R(2) = H; R(1) = C(6)H(5), R(2) = C(6)H(5)) and CH(3)N(OH)C(=O)[CH(2)](n)C(=O)N(OH)CH(3) (H(2)L(n); n = 2, 4, 6). Significant rate acceleration in the presence of chloride ion is observed for ligand dissociation from the bis(hydroxamate)- and mono(hydroxamate)-bound complexes. Rate acceleration was also observed in the presence of bromide and nitrate ions but to a lesser extent. A mechanism for chloride ion catalysis of ligand dissociation is proposed which involves chloride ion dependent parallel paths with transient Cl(-) coordination to Fe(III). The labilizing effect of Cl(-) results in an increase in microscopic rate constants on the order of 10(2)-10(3). Second-order rate constants for the proton driven dissociation of dinuclear Fe(III) complexes formed with H(2)L(n)() were found to vary with Fe-Fe distance. An analysis of these data permits us to propose a reactive intermediate of the structure (H(2)O)(4)Fe(L(n)())Fe(HL(n))(Cl)(OH(2))(2+) for the chloride ion dependent ligand dissociation path. Environmental and biological implications of chloride ion enhancement of Fe(III)-ligand dissociation reactions are presented.

Aqueous solution speciation of Fe(III) complexes with dihydroxamate siderophores alcaligin and rhodotorulic acid and synthetic analogues using electrospray ionization mass spectrometry.

Aqueous solutions of Fe3+ complexes of cyclic (alcaligin) and linear (rhodotorulic acid) dihydroxamate siderophores and synthetic linear eight-carbon-chain and two-carbon-chain dihydroxamic acids ([CH3N(OH)C=O)]2(CH2)n; H2Ln; n = 2 and 8) were investigated by electrospray ionization mass spectrometry (ESI-MS). Information was obtained relevant to the structure and the speciation of various Fe(III)-dihydroxamate complexes present in aqueous solution by (1) comparing different ionization techniques (ESI and FAB), (2) altering the experimental parameters (Fe3+/ligand ratio, pH, cone voltage), (3) using high-stability hexacoordinated Fe(III) siderophore complex mixtures (ferrioxamine B/ferrioxamine E) as a calibrant to quantify intrinsically neutral (H+ clustered or protonated) and intrinsically charged complexes, and (4) using mixed-metal complexes containing Fe3+, Ga3+, and Al3+. These results illustrate that for all dihydroxamic acid ligands investigated multiple tris- and bis-chelated mono- and di-Fe(III) species are present in relative concentrations that depend on the pH and Fe/L ratio.

Fe3+ coordination and redox properties of a bacterial transferrin.

The Fe(3+) binding site of recombinant nFbp, a ferric-binding protein found in the periplasmic space of pathogenic Neisseria, has been characterized by physicochemical techniques. An effective Fe(3+) binding constant in the presence of 350 microm phosphate at pH 6.5 and 25 degrees C was determined as 2.4 x 10(18) m(-1). EPR spectra for the recombinant Fe(3+)nFbp gave g' = 4.3 and 9 signals characteristic of high spin Fe(3+) in a strong ligand field of low (orthorhombic) symmetry. (31)P NMR experiments demonstrated the presence of bound phosphate in the holo form of nFbp and showed that phosphate can be dialyzed away in the absence of Fe(3+) in apo-nFbp. Finally, an uncorrected Fe(3+/2+) redox potential for Fe-nFbp was determined to be -290 mV (NHE) at pH 6.5, 20 degrees C. Whereas our findings show that nFbp and mammalian transferrin have similar Fe(3+) binding constants and EPR spectra, they differ greatly in their redox potentials. This has implications for the mechanism of Fe transport across the periplasmic space of Gram-negative bacteria.

Fe(III) coordination properties of two new saccharide-based enterobactin analogues: methyl 2,3,4-tris-O-[N-[2,3-di(hydroxy)benzoyl-glycyl]-aminopropyl]-alpha-D-glucopyranoside and methyl 2,3,4-tris-O-[N-[2,3-di-(hydroxy)-benzoyl]-aminopropyl]-alpha-D-glucopyranoside.

The synthesis of two saccharide-based enterobactin analogues, methyl 2,3,4-tris-O[-N[2,3-di(hydroxy)benzoyl-glycyl]-aminopropyl]-alpha-D-glucopyranoside (H(6)L(A)) and methyl 2,3,4-tris-O-[N-[2,3-di(hydroxy)benzoyl]-aminopropyl]-alpha-D-glucopyranoside (H(6)L(B)), are reported along with their pK(a) values, Fe(III) binding constants, and aqueous solution speciation as determined by spectrophotometric and potentiometric titration techniques. Use of a saccharide platform to synthesize a hexadentate triscatechol chelator provides some advantages over other approaches to enterobactin models, including significant water solubility, resistance to hydrolysis, and backbone chirality which may provide favorable recognition and availability to cells. The protonation constants for the catechol ligand hydroxyl moieties were determined for both ligands and found to be significantly different, which is attributed to the differences in the spacer chain of the two triscatechols. Proton dependent Fe(III)-ligand equilibrium constants were determined using a model involving the sequential protonation of the Fe(III)-ligand complex. These results were used to calculate the formation constants, log beta(110) = 41.38 for Fe(III)-H(6)L(A) and log beta(110) = 46.38 for Fe(III)-H(6)L(B). The calculated pM values of 28.6 for H(6)L(A) and 28.3 for H(6)L(B) indicate that these ligands possess Fe(III) affinities comparable to or greater than other enterobactin models and are thermodynamically capable of removing Fe(III) from transferrin.

Treatment of iron deficiency anemia: are monomeric iron compounds suitable for parenteral administration?

Iron deficiency is the most common nutritional problem worldwide, especially in the developing countries. Oral iron supplementation programs have failed because of noncompliance and gastrointestinal toxicity, thereby necessitating parenteral administration of iron. For parenteral administration, only iron-carbohydrate complexes are currently used, because monomeric iron salts release free iron, thereby causing oxidant injury. However, iron-carbohydrate complexes also have significant toxicity, and they are expensive. We have proposed the hypothesis that monomeric iron salts can be safely administered by the parenteral route if iron is tightly complexed to the ligand, thereby causing clinically insignificant release of free iron, and the kinetic properties of the compound allow rapid transfer of iron to plasma transferrin. A detailed analysis of the physicochemical and kinetic properties reveals that ferric iron complexed to pyrophosphate or acetohydroxamate anions may be suitable for parenteral administration. We have demonstrated that infusion of ferric pyrophosphate into the circulation via the dialysate is safe and effective in maintaining iron balance in patients undergoing maintenance hemodialysis. Parenteral administration of monomeric iron compounds is a promising approach to the treatment of iron deficiency in the general population and merits further investigation.

Concentration-dependent effects of anions on the anaerobic oxidation of hemoglobin and myoglobin.

The redox potentials of hemoglobin and myoglobin and the shapes of their anaerobic oxidation curves are sensitive indicators of globin alterations surrounding the active site. This report documents concentration-dependent effects of anions on the ease of anaerobic oxidation of representative hemoglobins and myoglobins. Hemoglobin (Hb) oxidation curves reflect the cooperative transition from the T state of deoxyHb to the more readily oxidized R-like conformation of metHb. Shifts in the oxidation curves for Hb A(0) as Cl(-) concentrations are increased to 0.2 m at pH 7.1 indicate preferential anion binding to the T state and destabilization of the R-like state of metHb, leading to reduced cooperativity in the oxidation process. A dramatic reversal of trend occurs above 0.2 m Cl(-) as anions bind to lower affinity sites and shift the conformational equilibrium toward the R state. This pattern has been observed for various hemoglobins with a variety of small anions. Steric rather than electronic effects are invoked to explain the fact that no comparable reversal of oxygen affinity is observed under identical conditions. Evidence is presented to show that increases in hydrophilicity in the distal heme pocket can decrease oxygen affinity via steric hindrance effects while increasing the ease of anaerobic oxidation.

Kinetics and mechanism of iron(III) dissociation from the dihydroxamate siderophores alcaligin and rhodotorulic acid.

The kinetics and mechanism of siderophore ligand dissociation from their fully chelated Fe(III) complexes is described for the highly preorganized cyclic tetradentate alcaligin and random linear tetradentate rhodotorulic acid in aqueous solution at 25 degrees C (Fe2L3 + 6H+ reversible 2 Fe3+ aq + 3 H2L). At siderophore:Fe(III) ratios where Fe(III) is hexacoordinated, kinetic data for the H(+)-driven ligand dissociation from the Fe2L3 species is consistent with a singly ligand bridged structure for both the alcaligin and rhodotorulic acid complexes. Proton-driven ligand dissociation is found to proceed via parallel reaction paths for rhodotorulic acid, in contrast with the single path previously observed for the linear trihydroxamate siderophore ferrioxamine B. Parallel paths are also available for ligand dissociation from Fe2(alcaligin)3, although the efficiency of one path is greatly diminished and dissociation of the bis coordinated complex Fe(alcaligin)(OH2)2+ is extremely slow (k = 10(-5) M-1 s-1) due to the high degree of preorganization in the alcaligin siderophore. Mechanistic interpretations were further confirmed by investigating the kinetics of ligand dissociation from the ternary complexes Fe(alcaligin)(L) in aqueous acid where L = N-methylacetohydroxamic acid and glycine hydroxamic acid. The existence of multiple ligand dissociation paths is discussed in the context of siderophore mediated microbial iron transport.

Multiple-path dissociation mechanism for mono- and dinuclear tris(hydroxamato)iron(III) complexes with dihydroxamic acid ligands in aqueous solution.

Linear synthetic dihydroxamic acids ([CH3N(OH)C=O)]2(CH2)n; H2Ln) with short (n = 2) and long (n = 8) hydrocarbon-connecting chains form mono- and dinuclear complexes with Fe(III) in aqueous solution. At conditions where the formation of Fe2(Ln)3 is favored, complexes with each of the two ligand systems undergo [H+]-induced ligand dissociation processes via multiple sequential and parallel paths, some of which are common and some of which are different for the two ligands. The pH jump induced ligand dissociation proceeds in two major stages (I and II) where each stage is shown to be comprised of multiple components (Ix, where x = 1-3 for L2 and L8, and IIy, where y = 1-3 for L2 and y = 1-4 for L8). A reaction scheme consistent with kinetic and independent ESI-MS data is proposed that includes the tris-chelated complexes (coordinated H2O omitted for clarity) (Fe2(Ln)3, Fe2(L2)2(L2H)2, Fe(LnH)3, Fe(L8)(L8H)), bis-chelated complexes (Fe2(Ln)2(2+), Fe(LnH)2+, Fe(L8)+), and monochelated complexes (Fe(LnH)2+). Analysis of kinetic data for ligand dissociation from Fe2(Ln)(LnH)3+ (n = 2, 4, 6, 8) allows us to estimate the dielectric constant at the reactive dinuclear Fe(III) site. The existence of multiple ligand dissociation paths for the dihydroxamic acid complexes of Fe(III) is a feature that distinguishes these systems from their bidentate monohydroxamic acid and hexadentate trihydroxamic acid counterparts and may be a reason for the biosynthesis of dihydroxamic acid siderophores, despite higher environmental molar concentrations necessary to completely chelate Fe(III).

Microcalorimetric determination of thermodynamic parameters for ionophore-siderophore host-guest complex formation.

Thermodynamic parameters (delta H, delta S, and delta G) were determined by microcalorimetry in wet chloroform for host-guest assembly formation involving second-sphere complexation of the siderophore ferrioxamine B by crown ether (18-crown-6, cis-dicyclohexano-18-crown-6, benzo-18-crown-6) and cryptand (2.2.2 cryptand) hosts. Similar data were also collected for the same hosts with the pentylammonium ion guest, which corresponds to the pendant pentylamine side chain of ferroxamine B. Host-guest assembly formation constants (Ka) obtained from microcalorimetry agree with values obtained indirectly from chloroform/water extraction studies in those cases where comparable data are available. On the basis of a trend established by the pentylammonium guest, an enhanced stability relative to the crown ethers is observed for the assembly composed of ferrioxamine B and 2.2.2 cryptand that is due to entropic effects. Trends in delta H and delta S with changes in host and guest structure are discussed and attributed directly to host-guest complex formation, as solvation effects were determined to be insignificant (delta Cp = 0).

Spectroelectrochemistry of heme proteins: effects of active-site heterogeneity on Nernst plots.

In order to detect and model the effect of functional chain heterogeneity on Nernst plots for heme proteins, we examined the redox properties of various myoglobins (Mbs) and their mixtures using an improved spectroelectrochemical method. Specific redox responses and formal half potentials (E1/2) were obtained for Aplysia, horse, and sperm whale Mbs, as well as 1:1 mixtures of Mbs consisting of Aplysia/sperm whale, sperm whale/horse, and horse/Aplysia. Linear Nernst plots with slopes near unity were observed for horse, sperm whale, and Aplysia Mbs, with E1/2 values of 14, 19, and 96 mV (vs. NHE) respectively, consistent with previous reports using indirect methods. The Nernst plot responses for mixtures of some of these Mbs allowed us to evaluate and model the non-Nernstian behavior that results from intrinsically different values of E1/2 and from incomplete spectral overlap. The data demonstrate that increasing the E1/2 differences between the components of a Mb mixture increases the changes in shape of the resulting Nernst plots, the dominant effect being a decrease in the observed Nernst coefficient (nNernst). Comparison of Nernst plots for redox data with Hill plots for O2 binding data shows that the redox process is more affected by the structural differences in the distal heme pockets of the Mbs studied than is O2 binding. Similar effects of chain heterogeneity may give rise to disproportionate reductions in the slopes of Nernst and Hill plots for hemoglobins (Hbs). This possibility is discussed in relation to Hbs investigated for redox and O2 binding activity in our laboratories where we find nNernst to be commonly less than nHill over a range of experimental conditions.

Internal electron transfer between hemes and Cu(II) bound at cysteine beta93 promotes methemoglobin reduction by carbon monoxide.

Previous studies showed that CO/H2O oxidation provides electrons to drive the reduction of oxidized hemoglobin (metHb). We report here that Cu(II) addition accelerates the rate of metHb beta chain reduction by CO by a factor of about 1000. A mechanism whereby electron transfer occurs via an internal pathway coupling CO/H2O oxidation to Fe(III) and Cu(II) reduction is suggested by the observation that the copper-induced rate enhancement is inhibited by blocking Cys-beta93 with N-ethylmaleimide. Furthermore, this internal electron-transfer pathway is more readily established at low Cu(II) concentrations in Hb Deer Lodge (beta2His --> Arg) and other species lacking His-beta2 than in Hb A0. This difference is consistent with preferential binding of Cu(II) in Hb A0 to a high affinity site involving His-beta2, which is ineffective in promoting electron exchange between Cu(II) and the beta heme iron. Effective electron transfer is thus affected by Hb type but is not governed by the R left arrow over right arrow T conformational equilibrium. The beta hemes in Cu(II)-metHb are reduced under CO at rates close to those observed for cytochrome c oxidase, where heme and copper are present together in the oxygen-binding site and where internal electron transfer also occurs.

The release of iron from different asbestos structures by hydrogen peroxide with concomitant O2 generation.

Treatment of aqueous suspensions of different asbestos fibers (amosite, anthophyllite, chrysotile, and crocidolite) at 0-4 degrees C and pH 7.2 with H2O2 results in the consumption of H2O2 with concomitant release of iron and production of O2. During incubations, [H2O2] decreased in proportion to the mass of the suspended fiber, the duration of incubation, and the initial [H2O2]. The consumption of H2O2, production of O2 and release of iron all vary synergistically with the structure of the asbestos fiber. Release of silicon during the incubation was small relative to the decrement in [H2O2], reflecting a lack of dissolution of the fiber. The data are consistent with a redox process for the release of surface bound iron and it is significant that iron release occurs in the absence of a Fe(II) or Fe(III) chelator. The implications of iron release from the asbestos surface may be important in inflammatory disorders in which both silicate bound iron and H2O2 accumulate.

Conformational fluctuations in deoxy hemoglobin revealed as a major contributor to anionic modulation of function through studies of the oxygenation and oxidation of hemoglobins A0 and Deer Lodge beta2(NA2)His --> Arg.

Organisms rely on regulation at the molecular level, such as the allosteric regulation of hemoglobin (Hb) function by anions, to meet challenges presented by changing environmental and physiological conditions. A comparison of the effects of anions on oxygenation, oxidation, and sulfhydryl reactivity of Hb leads us to suggest that a large and significant part of the shift in oxygen affinity brought about by anion binding occurs as a result of increased conformational rigidity of the T state of deoxy Hb. As conformational rigidity increases, it becomes increasingly difficult for subunits in the deoxygenated T-state tetramer to assume higher oxygen affinity forms (T', T", T"'...) with less steric hindrance. The oxygen affinity reflects the average of the rapidly equilibrating conformations within the T state and is correspondingly decreased when anion levels are increased. The initial stage of the oxidation of Hb is relatively insensitive to steric alterations and thus reflects, primarily, the electronic aspects of the quaternary (T, T', T", T"'...) <--> equilibrium. We show that the reactivity of the sterically obscured sulfhydryl of beta93 Cys in deoxy Hb is much greater in chloride-free buffers than in buffers with added chloride. Anion-induced decreases in the extent and frequency of conformational fluctuations of subunits within the T-quaternary state thus reduce sulfhydryl reactivity as well as oxygen affinity. This parallel in anionic control of function allowed us to test, and disprove, the possibility that uncompensated positive charges in the central cavity of Hb Deer Lodge increase the frequency and extent of conformational fluctuations in its deoxy structure. This Hb variant exhibits increased anion sensitivity, increased oxygen affinity, and increased ease of oxidation, but without increased reactivity of its sulfhydryl groups, indicating that active-site alterations in deoxy Hb Deer Lodge are primarily electronic and not associated with increased conformational fluctuations within its T state. The restoration of normal properties in Hb Deer Lodge by addition of anions reinforces our conclusion that anionic control can be exerted through both steric and electronic alterations. The anionic control of fluctuations within the T state of Hb illustrates an important principle of macromolecular structure-function relationships: that functional regulation can be achieved by alterations in conformational rigidity.