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Bosisio C, Quercioli V, Collini M, D'Alfonso L, Baldini G, Bettati S, Campanini B, Raboni S, Chirico G. Protonation and conformational dynamics of GFP mutants by two-photon excitation fluorescence correlation spectroscopy. J Phys Chem B 2008; 112:8806-14. [PMID: 18582099 DOI: 10.1021/jp801164n] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
GFP mutants are known to display fluorescence flickering, a process that occurs in a wide time range. Because serine 65, threonine 203, glutamate 222, and histidine 148 have been indicated as key residues in determining the GFP fluorescence photodynamics, we have focused here on the role of histidine 148 and glutamate 222 by studying the fluorescence dynamics of GFPmut2 (S65A, V68L, and S72A GFP) and its H148G (Mut2G) and E222Q (Mut2Q) mutants. Two relaxation components are found in the fluorescence autocorrelation functions of GFPmut2: a 10-100 micros pH-dependent component and a 100-500 micros laser-power-dependent component. The comparison of these three mutants shows that the mutation of histidine 148 to glycine induces a 3-fold increase in the protonation rate, thereby indicating that the protonation-deprotonation of the chromophore occurs via a proton exchange with the solution mediated by the histidine 148 residue. The power-dependent but pH-independent relaxation mode, which is not affected by the E222Q and H148G mutations, is due to an excited-state process that is probably related to conformational rearrangements of the chromophore after the photoexcitation, more than to the chromophore excited-state proton transfer.
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Affiliation(s)
- C Bosisio
- Dipartimento G. Occhialini, Universita di Milano Bicocca
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2
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Croci S, Babalola OJ, Bettati S, Valenti C, Ortalli I, Parak F. Quadrupole splitting temperature dependence of high and low affinity deoxyhemoglobin encapsulated in wet silica gel. ACTA ACUST UNITED AC 2006. [DOI: 10.1007/s10751-006-9286-5] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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3
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Mozzarelli A, Bettati S, Bruno S. Protein function in the crystalline state. Acta Crystallogr A 2004. [DOI: 10.1107/s0108767304099684] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022] Open
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4
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Abbruzzetti S, Viappiani C, Bruno S, Bettati S, Bonaccio M, Mozzarelli A. Functional characterization of heme proteins encapsulated in wet nanoporous silica gels. J Nanosci Nanotechnol 2001; 1:407-415. [PMID: 12914082 DOI: 10.1166/jnn.2001.058] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Myoglobin and hemoglobin were encapsulated in wet, nanoporous silica gels. A rigorous evaluation of the effect of the encapsulation on protein dynamics and function was carried out by measuring the kinetics of carbon monoxide rebinding after nanosecond laser flash-photolysis with transient absorption detection, and the oxygen affinity with absorption microspectrophotometry. The time course of carbon monoxide binding to myoglobin evidenced a strongly enhanced geminate recombination and a faster bimolecular rebinding with respect to solution, whereas T and R quaternary states of hemoglobin exhibited a geminate phase and a bimolecular binding rate very similar to those observed in solution. Oxygen affinity of T-state hemoglobin was found to be close to that observed for the binding of the first oxygen to T-state hemoglobin in solution. Results indicate that some conformational transitions are kinetically restricted, allowing to isolate distinct tertiary and quaternary states. This opens the way to their detailed functional characterization and application to biodevices.
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Affiliation(s)
- S Abbruzzetti
- National Institute for the Physics of Matter (INFM), Department of Physics, University of Parma, Via Parco delle Scienze, 43100 Parma, Italy
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5
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Bruno S, Bonaccio M, Bettati S, Rivetti C, Viappiani C, Abbruzzetti S, Mozzarelli A. High and low oxygen affinity conformations of T state hemoglobin. Protein Sci 2001; 10:2401-7. [PMID: 11604545 PMCID: PMC2374069 DOI: 10.1110/ps.20501] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2001] [Revised: 08/07/2001] [Accepted: 08/20/2001] [Indexed: 10/16/2022]
Abstract
To understand the interplay between tertiary and quaternary transitions associated with hemoglobin function and regulation, oxygen binding curves were obtained for hemoglobin A fixed in the T quaternary state by encapsulation in wet porous silica gels. At pH 7.0 and 15 degrees C, the oxygen pressure at half saturation (p50) was measured to be 12.4 +/- 0.2 and 139 +/- 4 torr for hemoglobin gels prepared in the absence and presence of the strong allosteric effectors inositol hexaphosphate and bezafibrate, respectively. Both values are in excellent agreement with those found for the binding of the first oxygen to hemoglobin in solution under similar experimental conditions. The corresponding Hill coefficients of hemoglobin gels were 0.94 +/- 0.02 and 0.93 +/- 0.03, indicating, in the frame of the Monod, Wyman, and Changeux model, that high and low oxygen-affinity tertiary T-state conformations have been isolated in a pure form. The values, slightly lower than unity, reflect the different oxygen affinity of alpha- and beta-hemes. Significantly, hemoglobin encapsulated in the presence of the weak effector phosphate led to gels that show intermediate oxygen affinity and Hill coefficients of 0.7 to 0.8. The heterogeneous oxygen binding results from the presence of a mixture of the high and low oxygen-affinity T states. The Bohr effect was measured for hemoglobin gels containing the pure conformations and found to be more pronounced for the high-affinity T state and almost absent for the low-affinity T state. These findings indicate that the functional properties of the T quaternary state result from the contribution of two distinct, interconverting conformations, characterized by a 10-fold difference in oxygen affinity and a different extent of tertiary Bohr effect. The very small degree of T-state cooperativity observed in solution and in the crystalline state might arise from a ligand-induced perturbation of the distribution between the high- and low-affinity T-state conformations.
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Affiliation(s)
- S Bruno
- Department of Biochemistry and Molecular Biology, University of Parma, 43100 Parma, Italy
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6
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Marabotti A, De Biase D, Tramonti A, Bettati S, Mozzarelli A. Allosteric communication of tryptophan synthase. Functional and regulatory properties of the beta S178P mutant. J Biol Chem 2001; 276:17747-53. [PMID: 11278986 DOI: 10.1074/jbc.m011781200] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The alpha(2)beta(2) tryptophan synthase complex is a model enzyme for understanding allosteric regulation. We report the functional and regulatory properties of the betaS178P mutant. Ser-178 is located at the end of helix 6 of the beta subunit, belonging to the domain involved in intersubunit signaling. The carbonyl group of betaSer-178 is hydrogen bonded to Gly-181 of loop 6 of the alpha subunit only when alpha subunit ligands are bound. An analysis by molecular modeling of the structural effects caused by the betaS178P mutation suggests that the hydrogen bond involving alphaGly-181 is disrupted as a result of localized structural perturbations. The ratio of alpha to beta subunit concentrations was calculated to be 0.7, as for the wild type, indicating the maintenance of a tight alpha-beta complex. Both the activity of the alpha subunit and the inhibitory effect of the alpha subunit ligands indole-3-acetylglycine and d,l-alpha-glycerol-3-phosphate were found to be the same for the mutant and wild type enzyme, whereas the beta subunit activity of the mutant exhibited a 2-fold decrease. In striking contrast to that observed for the wild type, the allosteric effectors indole-3-acetylglycine and d,l-alpha-glycerol-3-phosphate do not affect the beta activity. Accordingly, the distribution of l-serine intermediates at the beta-site, dominated by the alpha-aminoacrylate, is only slightly influenced by alpha subunit ligands. Binding of sodium ions is weaker in the mutant than in the wild type and leads to a limited increase of the amount of the external aldimine intermediate, even at high pH, whereas binding of cesium ions exhibits the same affinity and effects as in the wild type, leading to an increase of the alpha-aminoacrylate tautomer absorbing at 450 nm. Crystals of the betaS178P mutant were grown, and their functional and regulatory properties were investigated by polarized absorption microspectrophotometry. These findings indicate that (i) the reciprocal activation of the alpha and beta activity in the alpha2beta2 complex with respect to the isolated subunits results from interactions that involve residues different from betaSer-178 and (ii) betaSer-178 is a critical residue in ligand-triggered signals between alpha and beta active sites.
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Affiliation(s)
- A Marabotti
- Institute of Biochemical Sciences and National Institute for the Physics of Matter, University of Parma, 43100 Parma, Italy
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7
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Chirico G, Bettati S, Mozzarelli A, Chen Y, Müller JD, Gratton E. Molecular heterogeneity of O-acetylserine sulfhydrylase by two-photon excited fluorescence fluctuation spectroscopy. Biophys J 2001; 80:1973-85. [PMID: 11259310 PMCID: PMC1301386 DOI: 10.1016/s0006-3495(01)76167-x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
O-acetylserine sulfhydrylase, a homo-dimeric enzyme from Salmonella typhimurium, covalently binds one pyridoxal 5'-phosphate molecule per subunit as a fluorescent coenzyme. Different tautomers of the Schiff base between the coenzyme and lysine 41 generate structured absorption and fluorescence spectra upon one-photon excitation. We investigated the protein population heterogeneity by fluorescence correlation spectroscopy and lifetime techniques upon two-photon excitation. We sampled the fluorescence intensity from a small number of molecules (approximately 10) and analyzed the distribution of photon counts to separately determine the number and the fluorescence brightness of excited protein molecules. The changes in the average number of molecules and in the fluorescence brightness with the excitation wavelength indicate the presence of at least two fluorescent species, with two-photon excitation maxima at 660 and 800 nm. These species have been identified as the enolimine and ketoenamine tautomers of the protein-coenzyme internal aldimine. Their relative abundance is estimated to be 4:1, whereas the ratio of their two-photon cross sections is reversed with respect to the single-photon excitation case. Consistent results are obtained from the measurement of the lifetime decays, which are sensitive to the excited-state heterogeneity. At least two components were detected, with lifetimes of approximately 2.5 and 0.5 ns. The lifetimes are very close to the values measured in bulk solutions upon one-photon excitation and attributed to the ketoenamine tautomer and to a dipolar species formed upon proton dissociation in the excited state.
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Affiliation(s)
- G Chirico
- Istituto Nazionale per la Fisica della Materia, Università di Milano-Bicocca, Milano 20133, Italy.
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8
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Bettati S, Benci S, Campanini B, Raboni S, Chirico G, Beretta S, Schnackerz KD, Hazlett TL, Gratton E, Mozzarelli A. Role of pyridoxal 5'-phosphate in the structural stabilization of O-acetylserine sulfhydrylase. J Biol Chem 2000; 275:40244-51. [PMID: 10995767 DOI: 10.1074/jbc.m007015200] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Proteins belonging to the superfamily of pyridoxal 5'-phosphate-dependent enzymes are currently classified into three functional groups and five distinct structural fold types. The variation within this enzyme group creates an ideal system to investigate the relationships among amino acid sequences, folding pathways, and enzymatic functions. The number of known three-dimensional structures of pyridoxal 5'-phosphate-dependent enzymes is rapidly increasing, but only for relatively few have the folding mechanisms been characterized in detail. The dimeric O-acetylserine sulfhydrylase from Salmonella typhimurium belongs to the beta-family and fold type II group. Here we report the guanidine hydrochloride-induced unfolding of the apo- and holoprotein, investigated using a variety of spectroscopic techniques. Data from absorption, fluorescence, circular dichroism, (31)P nuclear magnetic resonance, time-resolved fluorescence anisotropy, and photon correlation spectroscopy indicate that the O-acetylserine sulfhydrylase undergoes extensive disruption of native secondary and tertiary structure before monomerization. Also, we have observed that the holo-O-acetylserine sulfhydrylase exhibits a greater conformational stability than the apoenzyme form. The data are discussed in light of the fact that the role of the coenzyme in structural stabilization varies among the pyridoxal 5'-phosphate-dependent enzymes and does not seem to be linked to the particular enzyme fold type.
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Affiliation(s)
- S Bettati
- Institute of Physical Sciences, Institute of Biochemical Sciences, and National Institute for the Physics of Matter, University of Parma, Parma 43100, Italy
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9
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Bruno S, Bettati S, Manfredini M, Mozzarelli A, Bolognesi M, Deriu D, Rosano C, Tsuneshige A, Yonetani T, Henry ER. Oxygen binding by alpha(Fe2+)2beta(Ni2+)2 hemoglobin crystals. Protein Sci 2000; 9:683-92. [PMID: 10794410 PMCID: PMC2144622 DOI: 10.1110/ps.9.4.683] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
Abstract
Oxygen binding by hemoglobin fixed in the T state either by crystallization or by encapsulation in silica gels is apparently noncooperative. However, cooperativity might be masked by different oxygen affinities of alpha and beta subunits. Metal hybrid hemoglobins, where the noniron metal does not bind oxygen, provide the opportunity to determine the oxygen affinities of alpha and beta hemes separately. Previous studies have characterized the oxygen binding by alpha(Ni2+)2beta(Fe2+)2 crystals. Here, we have determined the three-dimensional (3D) structure and oxygen binding of alpha(Fe2+)2beta(Ni2+)2 crystals grown from polyethylene glycol solutions. Polarized absorption spectra were recorded at different oxygen pressures with light polarized parallel either to the b or c crystal axis by single crystal microspectrophotometry. The oxygen pressures at 50% saturation (p50s) are 95 +/- 3 and 87 +/- 4 Torr along the b and c crystal axes, respectively, and the corresponding Hill coefficients are 0.96 +/- 0.06 and 0.90 +/- 0.03. Analysis of the binding curves, taking into account the different projections of the alpha hemes along the optical directions, indicates that the oxygen affinity of alpha1 hemes is 1.3-fold lower than alpha2 hemes. Inspection of the 3D structure suggests that this inequivalence may arise from packing interactions of the Hb tetramer within the monoclinic crystal lattice. A similar inequivalence was found for the beta subunits of alpha(Ni2+)2beta(Fe2+)2 crystals. The average oxygen affinity of the alpha subunits (p50 = 91 Torr) is about 1.2-fold higher than the beta subunits (p50 = 110 Torr). In the absence of cooperativity, this heterogeneity yields an oxygen binding curve of Hb A with a Hill coefficient of 0.999. Since the binding curves of Hb A crystals exhibit a Hill coefficient very close to unity, these findings indicate that oxygen binding by T-state hemoglobin is noncooperative, in keeping with the Monod, Wyman, and Changeux model.
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Affiliation(s)
- S Bruno
- Institute of Biochemical Sciences, University of Parma, Italy
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10
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Mozzarelli A, Bettati S, Pucci AM, Burkhard P, Cook PF. Catalytic competence of O-acetylserine sulfhydrylase in the crystal probed by polarized absorption microspectrophotometry. J Mol Biol 1998; 283:135-46. [PMID: 9761679 DOI: 10.1006/jmbi.1998.2038] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The reactions of the pyridoxal 5'-phosphate-dependent enzyme O-acetylserine sulfhydrylase with the substrate O-acetyl-L-serine and substrate analogs have been investigated in the crystalline state by single-crystal polarized absorption microspectrophotometry. This approach has allowed us to examine the catalytic competence of the enzyme in different crystalline states, one of which was used to determine the three-dimensional structure; experimental conditions were defined for the accumulation of catalytic intermediates in the crystal suitable for crystallographic analyses.O-Acetyl-L-serine reacts with the enzyme in one of the crystal forms leading via a beta-elimination reaction to the accumulation of the alpha-aminoacrylate Schiff base, absorbing maximally at 320 and 470 nm, as in solution. The dissociation constant for the alpha-aminoacrylate Schiff base is in the millimolar range, 500-fold higher than in solution, suggesting that crystal lattice interactions may oppose functionally relevant conformational changes. The dissociation constant exhibits a bell-shaped dependence on pH centered at pH 7. At this pH the alpha-aminoacrylate species slowly decays with time (30% decrease in 24 hours). The alpha-aminoacrylate intermediate readily reacts with sodium azide, an analog of sulfide, the natural nucleophilic agent, to give a new amino acid and the native enzyme, indicating that the crystalline enzyme catalyzes the overall beta-replacement reaction as in solution. In other crystal forms, including that used for the X-ray investigation, O-acetyl-L-serine either has an even higher dissociation constant or causes crystal damage upon binding. When the crystalline enzyme reacts with either L-cysteine or L-serine, the external aldimine intermediate is formed. The dissociation constants for both substrate analogs are closer to those observed in solution and are modulated by pH as in solution. Findings demonstrate that O-acetylserine sulfhydrylase is catalytically competent in the crystal although some regions of the molecule, likely involved in an open-closed transition induced by O-acetyl-L-serine binding, may have a limited flexibility. The accumulation in the crystal of both the external aldimine and the alpha-aminoacrylate intermediate makes feasible their structural determination and, therefore, the elucidation of the catalytic pathway at the molecular level.
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Affiliation(s)
- A Mozzarelli
- Institute of Biochemical Sciences, Istituto Nazionale per la Fisica della Materia, University of Parma 43100 Parma, Italy
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11
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Abstract
The T-structure of human haemoglobin is linked by salt-bridges between its four subunits, formed by the C-terminal arginine residues of the alpha-subunits and the C-terminal histidine residues of the beta-subunits. In the R-structure, these salt-bridges are absent. The oxygen affinity of the T-structure is lower than that of the R-structure by the equivalent of 3.5 kcal/mol haem. This difference has been attributed to the constraints imposed upon the T-structure by the salt-bridges, which were thought to hinder the changes in tertiary structure needed for firm oxygen binding. We have subjected this postulate to a rigorous test by measuring the oxygen equilibria of T-state crystals of an abnormal human haemoglobin in which the C-terminal histidine residues of the beta-chains are replaced by leucine residues. This replacement removes the salt-bridges from the histidine imidazole groups to the neighbouring aspartate residues. The crystals have an oxygen affinity about three times greater than that of crystals of normal haemoglobin. Hill's coefficient is close to unity. The oxygen affinity is unaffected by pH, chloride or the allosteric effector bezafibrate. Equilibrium curves determined by single crystal microspectrophometry using light polarised parallel and normal to the crystallographic a-axis show no significant difference between the oxygen affinities of alpha and beta-haems. Our results show that rupture of salt-bridges raises the oxygen affinity of the T-structure even when this is clamped firmly by the crystal lattice.
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Affiliation(s)
- S Bettati
- Institute of Biochemical Sciences, Istituto Nazionale per la Fisica della Materia, University of Parma, Parma, 43100, Italy
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12
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Bettati S, Kwiatkowski LD, Kavanaugh JS, Mozzarelli A, Arnone A, Rossi GL, Noble RW. Structure and oxygen affinity of crystalline des-his-146beta human hemoglobin in the T state. J Biol Chem 1997; 272:33077-84. [PMID: 9407091 DOI: 10.1074/jbc.272.52.33077] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
To correlate directly structure with function, the oxygen affinity and the three-dimensional structure of crystals of the T quaternary state of des-His-146beta human hemoglobin have been determined by polarized absorption microspectrophotometry and x-ray diffraction crystallography. In des-His-146beta, the COOH-terminal histidine residues of the beta chains of hemoglobin A have been removed. Oxygen binding to crystalline des-His hemoglobin is non-cooperative and independent of pH. The oxygen affinity is 1.7-fold greater than that of the crystalline state of hemoglobin A. Removal of His-146beta results in a small movement of the truncated COOH-terminal peptide and in a very small change in quaternary structure. Previously, similar studies on T state crystals of des-Arg-141alpha hemoglobin showed that removal of the COOH termini of the alpha chains results in much larger effects on oxygen affinity and on quaternary structure. Kinetic studies in solution reveal that at pH 7.0, the rates of CO combination with deoxygenated des-His-146beta in the absence and presence of inositol hexaphosphate are 2.5- and 1.3-fold, respectively, more rapid than for hemoglobin A. The values for des-Arg are 7.6- and 3.9-fold. The properties of the T state of hemoglobin both in the crystal and in solution are influenced to a greater degree by the interactions associated with Arg-141alpha than those associated with His-146beta.
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Affiliation(s)
- S Bettati
- Institute of Biochemical Sciences, University of Parma, 43100 Parma, Italy
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13
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Bettati S, Mozzarelli A. T state hemoglobin binds oxygen noncooperatively with allosteric effects of protons, inositol hexaphosphate, and chloride. J Biol Chem 1997; 272:32050-5. [PMID: 9405399 DOI: 10.1074/jbc.272.51.32050] [Citation(s) in RCA: 105] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Hemoglobin is the paradigm of allosteric proteins. Over the years, cooperative oxygen binding has been explained by different models predicting that the T state of hemoglobin binds oxygen either noncooperatively or with some degree of cooperativity or with strong cooperativity. Therefore, a critical test that discriminates among models is to determine the oxygen binding by the T state of hemoglobin. Fixation of hemoglobin in the T state has been achieved either by crystallization from polyethylene glycol solutions or by encapsulation in wet porous silica gels. Hemoglobin crystals bind oxygen noncooperatively with reduced affinity compared with solution, with no Bohr effect and with no influence of other allosteric effectors. In this study, we have determined accurate oxygen-binding curves to the T state of hemoglobin in silica gels with the same microspectrophotometric apparatus and multiwavelengths analysis used in crystal experiments. The T state of hemoglobin in silica gels binds oxygen noncooperatively with an affinity and a Bohr effect similar to those observed in solution for the binding of the first oxygen molecule. Other allosteric effectors such as inositol hexaphosphate, bezafibrate, and chloride significantly affect oxygen affinity. Therefore, T state hemoglobins that are characterized by strikingly different functional properties share the absence of cooperativity in the binding of oxygen. These findings are fully consistent with the Monod, Wyman, and Changeux model and with most features of Perutz's stereochemical model, but they are not consistent with models of both Koshland and Ackers.
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Affiliation(s)
- S Bettati
- Institute of Biochemical Sciences, University of Parma, 43100 Parma, Italy
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14
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Bettati S, Mozzarelli A, Rossi GL, Tsuneshige A, Yonetani T, Eaton WA, Henry ER. Oxygen binding by single crystals of hemoglobin: the problem of cooperativity and inequivalence of alpha and beta subunits. Proteins 1996; 25:425-37. [PMID: 8865338 DOI: 10.1002/prot.3] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Oxygen binding by the human hemoglobin tetramer in the T quaternary structure is apparently noncooperative in the crystalline state (Hill n = 1.0), as predicted by the two-state allosteric model of Monod, Wyman, and Changeux (MWC) (Mozzarelli et al., Nature 351:416-419, 1991; Rivetti et al., Biochemistry 32:2888-2906, 1993). However, cooperativity within the tetramer can be masked by a difference in affinity between the alpha and beta subunits. Indeed, analysis of the binding curves derived from absorption of light polarized along two different crystal directions, for which the projections of the alpha and beta hemes are slightly different, revealed an inequivalence in the intrinsic oxygen affinity of the alpha and beta subunits (p50(alpha) approximately 80 torr, p50(beta) approximately 370 torr at 15 degrees C) that compensates a small amount of cooperativity (Rivetti et al., Biochemistry 32:2888-2906, 1993). To further investigate this problem, we have measured oxygen binding curves of single crystals of hemoglobin (in a different lattice) in which the iron in the alpha subunits has been replaced by the non-oxygen-binding nickel(II). The Hill n is 0.90 +/- 0.06, and the p50 is slightly different for light polarized parallel to different crystal directions, indicating a very small difference in affinity between the two crystallographically inequivalent beta subunits. The average crystal p50 is 110 +/- 20 torr at 15 degrees C, close to the p50 of 80 torr observed in solution, but about threefold less than the p50 calculated by Rivetti et al. (Biochemistry 32:2888-2906, 1993) for the beta subunits of the unsubstituted tetramer. These results suggest that Rivetti et al., if anything, overestimated the alpha/beta inequivalence. They therefore did not underestimate the cooperativity within the T quaternary structure, when they concluded that it represents a small deviation from the perfectly noncooperative binding of an MWC allosteric model. Our conclusion of nearly perfect MWC behavior for binding to the T state of unmodified hemoglobin raises the question of the relevance of the large T-state cooperativity inferred for cyanide binding to partially oxidized hemoglobin (Ackers et al., Science 255:54-63, 1992).
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Affiliation(s)
- S Bettati
- Institute of Biochemical Sciences, University of Parma, Italy
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15
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Mozzarelli A, Bettati S, Rivetti C, Rossi GL, Colotti G, Chiancone E. Cooperative oxygen binding to scapharca inaequivalvis hemoglobin in the crystal. J Biol Chem 1996; 271:3627-32. [PMID: 8631972 DOI: 10.1074/jbc.271.7.3627] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Oxygen binding to homodimeric Scapharca inaequivalvis hemoglobin (HbI) crystals has been investigated by single-crystal polarized absorption microspectrophotometry. The saturation curve, characterized by a Hill coefficient nH = 1.45 and an oxygen pressure at half saturation p50 = 4.8 torr, at 15 degrees C, shows that HbI in the crystalline state retains positive cooperativity in ligand binding. This finding will permit the correlation of the oxygen-linked conformational changes in the crystal with the expression of cooperativity. Polarized absorption spectra of deoxy-HbI, oxy-HbI, and oxidized HbI crystals indicate that oxygenation does not induce heme reorientation, whereas oxidation does. Lattice interactions prevent the dissociation of oxidized dimers that occurs in solution and stabilize an equilibrium distribution of pentacoordinate and hexacoordinate high spin species.
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Affiliation(s)
- A Mozzarelli
- Istituto di Scienze Biochimiche, Università di Parma, 43100 Parma, Italy
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16
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Peracchi A, Bettati S, Mozzarelli A, Rossi GL, Miles EW, Dunn MF. Allosteric regulation of tryptophan synthase: effects of pH, temperature, and alpha-subunit ligands on the equilibrium distribution of pyridoxal 5'-phosphate-L-serine intermediates. Biochemistry 1996; 35:1872-80. [PMID: 8639669 DOI: 10.1021/bi951889c] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
The equilibrium distribution of catalytic intermediates formed in the reaction of L-serine with the tryptophan synthase alpha 2 beta 2-complex from Salmonella typhimurium has been investigated by absorption and fluorescence spectroscopy as a function of pH, temperature, and alpha-subunit ligands. The novel result of this study is that the equilibrium between the two major catalytic species, the external aldimine and the alpha-aminoacrylate, is modulated by the ionization of two groups with apparent pK values of 7.8 +/- 0.3 and 10.3 +/- 0.2. Protonation of these groups stabilizes the alpha-aminoacrylate Schiff base by an estimated 100-fold with respect to the external aldimine. Furthermore, the formation of the alpha-aminoacrylate from the external aldimine is an endothermic process. Temperature slightly affects the apparent pK values but remarkably influences the amplitude of the phase associated with the ionization of each group. At 20 degrees C, each phase accounts for nearly half of the titration. Since the isolated beta 2-dimer does not exhibit a pH-dependent distribution of intermediates, the alpha-beta-subunit interactions seem critical to the onset of this functional property of the beta-subunit. The modulation of intersubunit interactions by the alpha-subunit ligands DL-alpha-glycerol 3-phosphate and phosphate leads to significant changes in the pH-dependent distribution of intermediates. At saturating concentrations of either of these alpha-subunit ligands, the alpha-aminoacrylate Schiff base is the predominant species over a wide pH range while the apparent pK values of the groups that control the equilibrium are not significantly affected. The pH-dependent interconversion of catalytic intermediates here reported has not been previously detected because phosphate buffers have usually been employed in the studies of this enzyme. Our findings are discussed in the light of a model in which specific protein conformations are associated with the external aldimine and the alpha-aminoacrylate Schiff bases, the latter being stabilized by temperature, protons, and alpha-subunit ligands.
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Affiliation(s)
- A Peracchi
- Istituto di Scienze Biochimiche, Università di Parma, Italy
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