Tyrosine B10 and heme-ligand interactions of Lucina pectinata hemoglobin II: control of heme reactivity

Bioinformatic Characterization and Molecular Evolution of the Lucina pectinata Hemoglobins

(1) Introduction: Lucina pectinata is a clam found in sulfide-rich mud environments that has three hemoglobins believed to be responsible for the transport of hydrogen sulfide (HbILp) and oxygen (HbIILp and HbIIILp) to chemoautotrophic endosymbionts. The physiological roles and evolution of these globins in sulfide-rich environments are not well understood. (2) Methods: We performed bioinformatic and phylogenetic analyses with 32 homologous mollusk globin sequences. Phylogenetics suggests a first gene duplication resulting in sulfide binding and oxygen binding genes. A more recent gene duplication gave rise to the two oxygen-binding hemoglobins. Multidimensional scaling analysis of the sequence space shows evolutionary drift of HbIILp and HbIIILp, while HbILp was closer to the Calyptogena hemoglobins. Further corroboration is seen by conservation in the coding region of hemoglobins from L. pectinata compared to those from Calyptogena. (3) Conclusions: Presence of glutamine in position E7 in organisms living in sulfide-rich environments can be considered an adaptation to prevent loss of protein function. In HbILp a substitution of phenylalanine in position B10 is accountable for its unique reactivity towards H2S. It appears that HbILp has been changing over time, apparently not subject to functional constraints of binding oxygen, and acquired a unique function for a specialized environment.

Fluoride binding to characteristic heme-pocket centers: Insights into ligand stability

The studies on the L. pectinata hemoglobins (HbI, HbII, and HbIII) are essential because of their biological roles in hydrogen sulfide transport and metabolism. Variation in the pH could also play a role in the transport of hydrogen sulfide by HbI and oxygen by HbII and HbIII, respectively. Here, fluoride binding was used to further understand the structural properties essential for the molecular mechanism of ligand stabilization as a function of pH. The data allowed us to gain insights into how the physiological roles of HbI, HbII, HbIII, adult hemoglobin (A-Hb), and horse heart myoglobin (Mb) have an impact on the heme-bound fluoride stabilization. In addition, analysis of the vibrational assignments of the met-cyano heme complexes shows varied strength interactions of the heme-bound ligand. The heme pocket composition properties differ between HbI (GlnE7 and PheB10) and HbII/HbIII (GlnE7 and TyrB10). Also, the structural GlnE7 stereo orientation changes between HbI and HbII/HbIII. In HbI, its carbonyl group orients towards the heme iron, while in HbII/HbIII, the amino group occupies this position. Therefore, in HbI, the interactions to the heme-bound fluoride ion, cyanide, and oxygen with GlnE7 via H-bonding are not probable. Still, the aromatic cage PheB10, PheCD1, and PheE11 may contribute to the observed stabilization. However, a robust H-bonding networking stabilizes HbII and HbIII, heme-bound fluoride, cyanide, and oxygen ligand with the OH and NH2 groups of TyrB10 and GlnE7, respectively. At the same time, A-Hb and Mb have moderate but similar ligand interactions controlled by their respective distal E7 histidine.

SAXS structure of homodimeric oxyHemoglobin III from bivalve Lucina pectinata

Hemoglobin III (HbIII) is one of the two oxygen reactive hemoproteins present in the bivalve, Lucina pectinata. The clam inhabits a sulfur-rich environment and HbIII is the only hemoprotein present in the system which does not yet have a structure described elsewhere. It is known that HbIII exists as a heterodimer with hemoglobin II (HbII) to generate the stable Oxy(HbII-HbIII) complex but it remains unknown if HbIII can form a homodimeric species. Here, a new chromatographic methodology to separate OxyHbIII from the HbII-HbIII dimer has been developed, employing a fast performance liquid chromatography and ionic exchange chromatography column. The nature of OxyHbIII in solution at concentrations from 1.6 mg/mL to 20.4 mg/mL was studied using small angle X-ray scattering (SAXS). The results show that at all concentrations, the Oxy(HbIII-HbIII) dimer dominates in solution. However, as the concentration increases to nonphysiological values, 20.4 mg/mL, HbIII forms a 30% tetrameric fraction. Thus, there is a direct relationship between the Oxy(HbIII-HbIII) oligomeric form and hemoglobin concentration. We suggest it is likely that the OxyHbIII dimer contributes to active oxygen transport in tissues of L pectinata, where the Oxy(HbII-HbIII) complex is not present.

Lucina pectinata oxyhemoglobin (II-III) heterodimer pH susceptibility

Lucina pectinata live in high concentrations of hydrogen sulfide (H₂S) and contains one hemoglobin, Hemoglobin I (HbI), transporting H₂S and two hemoglobins, Hemoglobin II (HbII) and Hemoglobin (HbIII), transferring dioxygen to symbionts. HbII and HbIII contain B10 tyrosine (Tyr) and E7 glutamine (Gln) in the heme pocket generating an efficient hydrogen bonding network with the (HbII-HbIII)-O₂ species, leading to very low ligand dissociation rates. The results indicate that the oxy-hemeprotein is susceptible to pH from 4 to 9, at acidic conditions, and as a function of the potassium ferricyanide concentration, 100% of the met-aquo derivative is produced. Without a strong oxidant, pH 5 generates a small concentration of the met-aquo complex. The process is accelerated by the presence of salts, as indicated by the crystallization structures and UV-Vis spectra. The results suggest that acidic pH generates conformational changes associated with B10 and E7 heme pocket amino acids, weakening the (HbII-HbIII)-O₂ hydrogen bond network. The observation is supported by X-ray crystallography, since at pH 4 and 5, the heme-Fe tends to oxidize, while at pH 7, the oxy-heterodimer is present. Conformational changes also are observed at higher pH by the presence of a 605 nm transition associated with the iron heme-Tyr interaction. Therefore, pH is one crucial factor regulating the (HbII-HbIII)-O₂ complex hydrogen-bonding network. Thus, it can be proposed that the hydrogen bonding adjustments between the heme bound O₂ and the Tyr and Gln amino acids contribute to oxygen dissociation from the (HbII-HbIII)-O₂ system.

Chemical Biology of H2S Signaling through Persulfidation

Signaling by H2S is proposed to occur via persulfidation, a posttranslational modification of cysteine residues (RSH) to persulfides (RSSH). Persulfidation provides a framework for understanding the physiological and pharmacological effects of H2S. Due to the inherent instability of persulfides, their chemistry is understudied. In this review, we discuss the biologically relevant chemistry of H2S and the enzymatic routes for its production and oxidation. We cover the chemical biology of persulfides and the chemical probes for detecting them. We conclude by discussing the roles ascribed to protein persulfidation in cell signaling pathways.

Characterization and Expression of the Lucina pectinata Oxygen and Sulfide Binding Hemoglobin Genes

The clam Lucina pectinata lives in sulfide-rich muds and houses intracellular symbiotic bacteria that need to be supplied with hydrogen sulfide and oxygen. This clam possesses three hemoglobins: hemoglobin I (HbI), a sulfide-reactive protein, and hemoglobin II (HbII) and III (HbIII), which are oxygen-reactive. We characterized the complete gene sequence and promoter regions for the oxygen reactive hemoglobins and the partial structure and promoters of the HbI gene from Lucina pectinata. We show that HbI has two mRNA variants, where the 5'end had either a sequence of 96 bp (long variant) or 37 bp (short variant). The gene structure of the oxygen reactive Hbs is defined by having 4-exons/3-introns with conservation of intron location at B12.2 and G7.0 and the presence of pre-coding introns, while the partial gene structure of HbI has the same intron conservation but appears to have a 5-exon/ 4-intron structure. A search for putative transcription factor binding sites (TFBSs) was done with the promoters for HbII, HbIII, HbI short and HbI long. The HbII, HbIII and HbI long promoters showed similar predicted TFBSs. We also characterized MITE-like elements in the HbI and HbII gene promoters and intronic regions that are similar to sequences found in other mollusk genomes. The gene expression levels of the clam Hbs, from sulfide-rich and sulfide-poor environments showed a significant decrease of expression in the symbiont-containing tissue for those clams in a sulfide-poor environment, suggesting that the sulfide concentration may be involved in the regulation of these proteins. Gene expression evaluation of the two HbI mRNA variants indicated that the longer variant is expressed at higher levels than the shorter variant in both environments.

pH-induced quaternary assembly of Vitreoscilla hemoglobin: the monomer exhibits better peroxidase activity

pH-dependent (pH6.0-8.0) quaternary structural changes of ferric Vitreoscilla hemoglobin (VHb) have been investigated using dynamic light scattering. The VHb exhibits a monomeric state under neutral conditions at pH7.0, while the protein forms distinct homodimeric species at pH6.0 and 8.0, respectively. The dissociation constant obtained using the Bio-Layer Interferometry technology indicates that, at pH7.0, the monomer-monomer dissociation of VHb is about 6-fold or 5-fold higher (KD=6.34μM) compared with that at slightly acidic pH (KD=1.05μM) or slightly alkaline pH (KD=1.22μM). The pH-dependent absorption spectra demonstrate that the heme microenvironment of VHb is sensitive to the changes of pH value. The maximum absorption band of heme group of VHb shifts from 402nm to 407nm when pH changes from 6.0 to 8.0. In addition, the fluorescence emission spectra of VHb, taken at excitation wavelength of 295nm, suggest that the single Trp122 fluorescence quantum yields in VHb are decreased due to the formation of the homodimeric species. However, the circular dichroism spectra data display that the secondary structures of VHb are little affected by pH transitions. The pH-dependent peroxidase activity of VHb was also investigated in this study. The optimum pH for VHb using 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) as substrate is 7.0, which implies that the monomer state of VHb would exhibit better peroxidase activity than the homodimeric species of VHb at pH6.0 and 8.0.

Tyrosine B10 triggers a heme propionate hydrogen bonding network loop with glutamine E7 moiety

Propionates, as peripheral groups of the heme active center in hemeproteins have been described to contribute in the modulation of heme reactivity and ligand selection. These electronic characteristics prompted the question of whether the presence of hydrogen bonding networks between propionates and distal amino acids present in the heme ligand moiety can modulate physiological relevant events, like ligand binding association and dissociation activities. Here, the role of these networks was evaluated by NMR spectroscopy using the hemoglobin I PheB10Tyr mutant from Lucina pectinata as model for TyrB10 and GlnE7 hemeproteins. (1)H-NMR results for the rHbICN PheB10Tyr derivative showed chemical shifts of TyrB10 OHη at 31.00ppm, GlnE7N(ε1)H/N(ε2)H at 10.66ppm/-3.27ppm, and PheE11 C(δ)H at 11.75ppm, indicating the presence of a crowded, collapsed, and constrained distal pocket. Strong dipolar contacts and inter-residues crosspeaks between GlnE7/6-propionate group, GlnE7/TyrB10 and TyrB10/CN suggest that this hydrogen bonding network loop between GlnE7, TyrB10, 6-propionate group, and the heme ligand contribute significantly to the modulation of the heme iron electron density as well as the ligand stabilization mechanism. Therefore, the network loop presented here support the fact that the electron withdrawing character of the hydrogen bonding is controlled by the interaction of the propionates and the nearby electronic environments contributing to the modulation of the heme electron density state. Thus, we hypothesize that in hemeproteins with similar electrostatic environment the flexibility of the heme-6-propionate promotes a hydrogen bonding network loop between the 6-propionate, the heme ligand and nearby amino acids, tailoring in this way the electron density in the heme-ligand moiety.

Hydrogen sulfide and hemeproteins: knowledge and mysteries

Historically, hydrogen sulfide (H(2)S) has been regarded as a poisonous gas, with a wide spectrum of toxic effects. However, like ·NO and CO, H(2)S is now referred to as a signaling gas involved in numerous physiological processes. The list of reports highlighting the physiological effects of H(2)S is rapidly expanding and several drug candidates are now being developed. As with ·NO and CO, not a single H(2)S target responsible for all the biological effects has been found till now. Nevertheless, it has been suggested that H(2)S can bind to hemeproteins, inducing different responses that can mediate its effects. For instance, the interaction of H(2)S with cytochrome c oxidase has been associated with the activation of the ATP-sensitive potassium channels, regulating muscle relaxation. Inhibition of cytochrome c oxidase by H(2)S has also been related to inducing a hibernation-like state. Although H(2)S might induce these effects by interacting with hemeproteins, the mechanisms underlying these interactions are obscure. Therefore, in this review we discuss the current state of knowledge about the interaction of H(2)S with vertebrate and invertebrate hemeproteins and postulate a generalized mechanism. Our goal is to stimulate further research aimed at evaluating plausible mechanisms that explain H(2)S reactivity with hemeproteins.

Extreme differences between hemoglobins I and II of the clam Lucina pectinalis in their reactions with nitrite

The clam Lucina pectinalis supports its symbiotic bacteria by H₂S transport in the open and accessible heme pocket of Lucina Hb I and by O₂ transport in the narrow and crowded heme pocket of Lucina Hb II. Remarkably, air-equilibrated samples of Lucina Hb I were found to be more rapidly oxidized by nitrite than any previously studied Hb, while those of Lucina Hb II showed an unprecedented resistance to oxidation induced by nitrite. Nitrite-induced oxidation of Lucina Hb II was enabled only when O₂ was removed from its active site. Structural analysis revealed that O₂ "clams up" the active site by hydrogen bond formation to B10Tyr and other distal-side residues. Quaternary effects further restrict nitrite entry into the active site and stabilize the hydrogen-bonding network in oxygenated Lucina Hb II dimers. The dramatic differences in nitrite reactivities of the Lucina Hbs are not related to their O₂ affinities or anaerobic redox potentials, which were found to be similar, but are instead a result of differences in accessibility of nitrite to their active sites; i.e. these differences are due to a kinetic rather than thermodynamic effect. Comparative studies revealed heme accessibility to be a factor in human Hb oxidation by nitrite as well, as evidenced by variations of rates of nitrite-induced oxidation that do not correlate with R and T state differences and inhibition of oxidation rate in the presence of O₂. These results provide a dramatic illustration of how evolution of active sites with varied heme accessibility can moderate the rates of inner-sphere oxidative reactions of Hb and other heme proteins.

Factors controlling the reactivity of hydrogen sulfide with hemeproteins

Hemoglobin I (HbI) from the clam Lucina pectinata is an intriguing hemeprotein that binds and transports H(2)S to sulfide-oxidizing chemoautotrophic bacteria to maintain a symbiotic relationship and to protect the mollusk from H(2)S toxicity. Single point mutations at E7, B10, and E11 were introduced into the HbI heme pocket to define the reactivity of sulfide with hemeproteins. The functional and structural properties of mutant and wild-type recombinant proteins were first evaluated using the well-known ferrous CO and O(2) derivatives. The effects of these mutations on the ferric environment were then studied in the metaquo and hydrogen sulfide derivatives. The results obtained with the ferrous HbI mutants show that all the E7 substitutions and the PheB10Tyr mutation influence directly CO and O(2) binding and stability while the B10 and E11 substitutions induce distal structural rearrangements that affect ligand entry and escape indirectly. For the metaquo-GlnE7His, -PheB10Val, -PheB10Leu, and -E11 variants, two individual distal structures are suggested, one of which is associated with H-bonding interactions between the E7 residues and the bound water. Similar H-bonding interactions are invoked for these HbI-H(2)S mutant derivatives and the rHbI, altering in turn sulfide reactivity within these protein samples. This is evident in the resonance Raman spectra of these HbI-H(2)S complexes, which show reduction of heme iron as judged by the appearance of the nu(4) oxidation state marker at 1356 cm(-1), indicative of heme-Fe(II) species. This reduction process depends strongly on distal mutations showing faster reduction for those HbI mutants exhibiting the strongest H-bonding interactions. Overall, the results presented here show that (a) H(2)S association is regulated by external kinetic barriers, (b) H(2)S release is controlled by two competing reactions involving simple sulfide dissociation and heme reduction, (c) at high H(2)S concentrations, reduction of the ferric center dominates, and (d) reduction of the heme is also enhanced in those HbI mutants having polar distal environments.

Structure and ligand selection of hemoglobin II from Lucina pectinata

Lucina pectinata ctenidia harbor three heme proteins: sulfide-reactive hemoglobin I (HbI(Lp)) and the oxygen transporting hemoglobins II and III (HbII(Lp) and HbIII(Lp)) that remain unaffected by the presence of H(2)S. The mechanisms used by these three proteins for their function, including ligand control, remain unknown. The crystal structure of oxygen-bound HbII(Lp) shows a dimeric oxyHbII(Lp) where oxygen is tightly anchored to the heme through hydrogen bonds with Tyr(30)(B10) and Gln(65)(E7). The heme group is buried farther within HbII(Lp) than in HbI(Lp). The proximal His(97)(F8) is hydrogen bonded to a water molecule, which interacts electrostatically with a propionate group, resulting in a Fe-His vibration at 211 cm(-1). The combined effects of the HbII(Lp) small heme pocket, the hydrogen bonding network, the His(97) trans-effect, and the orientation of the oxygen molecule confer stability to the oxy-HbII(Lp) complex. Oxidation of HbI(Lp) Phe(B10) --> Tyr and HbII(Lp) only occurs when the pH is decreased from pH 7.5 to 5.0. Structural and resonance Raman spectroscopy studies suggest that HbII(Lp) oxygen binding and transport to the host bacteria may be regulated by the dynamic displacements of the Gln(65)(E7) and Tyr(30)(B10) pair toward the heme to protect it from changes in the heme oxidation state from Fe(II) to Fe(III).

Interaction of giant extracellular Glossoscolex paulistus hemoglobin (HbGp) with zwitterionic surfactant N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS): Effects of oligomeric dissociation

The present work focuses on the interaction between the zwitterionic surfactant N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and the giant extracellular hemoglobin of Glossoscolex paulistus (HbGp). Electronic optical absorption, fluorescence emission and circular dichroism spectroscopy techniques, together with Gel-filtration chromatography, were used in order to evaluate the oligomeric dissociation as well as the autoxidation of HbGp as a function of the interaction with HPS. A peculiar behavior was observed for the HPS-HbGp interaction: a complex ferric species formation equilibrium was promoted, as a consequence of the autoxidation and oligomeric dissociation processes. At pH 7.0, HPS is more effective up to 1mM while at pH 9.0 the surfactant effect is more intense above 1mM. Furthermore, the interaction of HPS with HbGp was clearly less intense than the interaction of this hemoglobin with cationic (CTAC) and anionic (SDS) surfactants. Probably, this lower interaction with HPS is due to two factors: (i) the lower electrostatic attraction between the HPS surfactant and the protein surface ionic sites when compared to the electrostatic interaction between HbGp and cationic and anionic surfactants, and (ii) the low cmc of HPS, which probably reduces the interaction of the surfactant in the monomeric form with the protein. The present work emphasizes the importance of the electrostatic contribution in the interaction between ionic surfactants and HbGp. Furthermore, in the whole HPS concentration range used in this study, no folding and autoxidation decrease induced by this surfactant were observed. This is quite different from the literature data on the interaction between surfactants and tetrameric hemoglobins, that supports the occurrence of this behavior for the intracellular hemoglobins at low surfactant concentration range. Spectroscopic data are discussed and compared with the literature in order to improve the understanding of hemoglobin-surfactant interaction as well as the acid isoelectric point (pI) influence of the giant extracellular hemoglobins on their structure-activity relationship.

Dynamic light scattering and optical absorption spectroscopy study of pH and temperature stabilities of the extracellular hemoglobin of Glossoscolex paulistus

The extracellular hemoglobin of Glossoscolex paulistus (HbGp) is constituted of subunits containing heme groups, monomers and trimers, and nonheme structures, called linkers, and the whole protein has a minimum molecular mass near 3.1 x 10(6) Da. This and other proteins of the same family are useful model systems for developing blood substitutes due to their extracellular nature, large size, and resistance to oxidation. HbGp samples were studied by dynamic light scattering (DLS). In the pH range 6.0-8.0, HbGp is stable and has a monodisperse size distribution with a z-average hydrodynamic diameter (D(h)) of 27 +/- 1 nm. A more alkaline pH induced an irreversible dissociation process, resulting in a smaller D(h) of 10 +/- 1 nm. The decrease in D(h) suggests a complete hemoglobin dissociation. Gel filtration chromatography was used to show unequivocally the oligomeric dissociation observed at alkaline pH. At pH 9.0, the dissociation kinetics is slow, taking a minimum of 24 h to be completed. Dissociation rate constants progressively increase at higher pH, becoming, at pH 10.5, not detectable by DLS. Protein temperature stability was also pH-dependent. Melting curves for HbGp showed oligomeric dissociation and protein denaturation as a function of pH. Dissociation temperatures were lower at higher pH. Kinetic studies were also performed using ultraviolet-visible absorption at the Soret band. Optical absorption monitors the hemoglobin autoxidation while DLS gives information regarding particle size changes in the process of protein dissociation. Absorption was analyzed at different pH values in the range 9.0-9.8 and at two temperatures, 25 degrees C and 38 degrees C. At 25 degrees C, for pH 9.0 and 9.3, the kinetics monitored by ultraviolet-visible absorption presents a monoexponential behavior, whereas for pH 9.6 and 9.8, a biexponential behavior was observed, consistent with heme heterogeneity at more alkaline pH. The kinetics at 38 degrees C is faster than that at 25 degrees C and is biexponential in the whole pH range. DLS dissociation rates are faster than the autoxidation dissociation rates at 25 degrees C. Autoxidation and dissociation processes are intimately related, so that oligomeric protein dissociation promotes the increase of autoxidation rate and vice versa. The effect of dissociation is to change the kinetic character of the autoxidation of hemes from monoexponential to biexponential, whereas the reverse change is not as effective. This work shows that DLS can be used to follow, quantitatively and in real time, the kinetics of changes in the oligomerization of biologic complex supramolecular systems. Such information is relevant for the development of mimetic systems to be used as blood substitutes.

Giant extracellular Glossoscolex paulistus Hemoglobin (HbGp) upon interaction with cethyltrimethylammonium chloride (CTAC) and sodium dodecyl sulphate (SDS) surfactants: Dissociation of oligomeric structure and autoxidation

The effects of two ionic surfactants on the oligomeric structure of the giant extracellular hemoglobin of Glossoscolex paulistus (HbGp) in the oxy - form have been studied through the use of several spectroscopic techniques such as electronic optical absorption, fluorescence emission, light scattering, and circular dichroism. The use of anionic sodium dodecyl sulphate (SDS) and cationic cethyltrimethyl ammonium chloride (CTAC) has allowed to differentiate the effects of opposite headgroup charges on the oligomeric structure dissociation and hemoglobin autoxidation. At pH 7.0, both surfactants induce the protein dissociation and a significant oxidation. Spectral changes occur at very low CTAC concentrations suggesting a significant electrostatic contribution to the protein-surfactant interaction. At low protein concentration, 0.08 mg/ml, some light scattering within a narrow CTAC concentration range occurs due to protein-surfactant precipitation. Light scattering experiments showed the dissociation of the oligomeric structure by SDS and CTAC, and the effect of precipitation induced by CTAC. At higher protein concentrations, 3.0 mg/ml, a precipitation was observed due to the intense charge neutralization upon formation of ion pair in the protein-surfactant precipitate. The spectral changes are spread over a much wider SDS concentration range, implying a smaller electrostatic contribution to the protein-surfactant interactions. The observed effects are consistent with the acid isoelectric point (pI) of this class of hemoglobins, which favors the intense interaction of HbGp with the cationic surfactant due to the existence of excess acid anionic residues at the protein surface. Protein secondary structure changes are significant for CTAC at low concentrations while they occur at significantly higher concentrations for SDS. In summary, the cationic surfactant seems to interact more strongly with the protein producing more dramatic spectral changes as compared to the anionic one. This is opposite as observed for several other hemoproteins. The surfactants at low concentrations produce the oligomeric dissociation, which facilitates the iron oxidation, an important factor modulating further oligomeric protein dissociation.

Capillary crystallization and molecular-replacement solution of haemoglobin II from the clam Lucina pectinata

Haemoglobin II is one of three haemoglobins present in the cytoplasm of the Lucina pectinata mollusc that inhabits the Caribbean coast. Using HBII purified from its natural source, crystallization screening was performed using the counter-diffusion method with capillaries of 0.2 mm inner diameter. Crystals of HbII suitable for data collection and structure determination were grown in the presence of agarose at 0.1%(w/v) in order to improve their quality. The crystals belong to the tetragonal space group P4(2)2(1)2, with unit-cell parameters a = b = 73.92, c = 152.35 A, and diffracted X-rays to a resolution of better than 2.0 A. The asymmetric unit is a homodimer with a corresponding Matthews coefficient (VM) of 3.15 A3 Da(-1) and a solvent content of 61% by volume.

Hydrogen-bonding conformations of tyrosine B10 tailor the hemeprotein reactivity of ferryl species

Ferryl compounds [Fe(IV)=O] in living organisms play an essential role in the radical catalytic cycle and degradation processes of hemeproteins. We studied the reactions between H2O2 and hemoglobin II (HbII) (GlnE7, TyrB10, PheCD1, PheE11), recombinant hemoglobin I (HbI) (GlnE7, PheB10, PheCD1, PheE11), and the HbI PheB10Tyr mutant of L. pectinata. We found that the tyrosine residue in the B10 position tailors, in two very distinct ways, the reactivity of the ferryl species, compounds I and II. First, increasing the reaction pH from 4.86 to 7.50, and then to 11.2, caused the the second-order rate constant for HbII to decrease from 141.60 to 77.78 M-1 s-1, and to 2.96 M-1 s-1, respectively. This pH dependence is associated with the disruption of the heme-tyrosine (603 nm) protein moiety, which controls the access of the H2O2 to the hemeprotein active center, thus regulating the formation of the ferryl species. Second, the presence of compound I was evident in the UV-vis spectra (648-nm band) in the reactions of HbI and recombinant HbI with H2O2, This band, however, is completely absent in the analogous reaction with HbII and the HbI PheB10Tyr mutant. Therefore, the existence of a hydrogen-bonding network between the heme pocket amino acids (i.e., TyrB10) and the ferryl compound I created a path much faster than 3.0x10(-2) s-1 for the decay of compound I to compound II. Furthermore, the decay of the heme ferryl compound I to compound II was independent of the proximal HisF8 trans-ligand strength. Thus, the pH dependence of the heme-tyrosine moiety complex determined the overall reaction rate of the oxidative reaction limiting the interaction with H2O2 at neutral pH. The hydrogen-bonding strength between the TyrB10 and the heme ferryl species suggests the presence of a cycle where the ferryl consumption by the ferric heme increases significantly the pseudoperoxidase activity of these hemeproteins.