Correlation proton magnetic resonance studies at 250 MHz of bovine pancreatic ribonuclease. I. Reinvestigation of the histidine peak assignments

1H NMR spectroscopic studies of calcium-binding proteins. 2. Histidine microenvironments in alpha- and beta-parvalbumins as determined by protonation and laser photochemically induced dynamic nuclear polarization effects

The microenvironments of the histidines in three isoforms of Ca(II)-bound parvalbumin (carp, pI = 4.25; pike, pI = 5.00; rat, pI = 5.50) have been examined with 1H NMR techniques to probe their protonation characteristics and photochemically induced dynamic nuclear polarizability (photo-CIDNP). The histidine at position 26 (or 25), present in all three of these proteins, shows absolutely no photo-CIDNP enhancement of its C2H or C5H resonances. Nor does this nonpolarizable histidine possess a normal pKa: values range only from 4.20 for carp to 4.32 for pike to 4.44 for rat. The C2H and C5H resonances of the histidine in this carp isoform split into doublets as the pH is lowered. The magnitude of this splitting depends on the magnetic field strength, temperature, and pH; however, the line intensities within each doublet are temperature-independent. Although the crystal structure of carp parvalbumin indicates that His-26 is exposed to solvent [Kretsinger, R. H., & Nockolds, C. E. (1973) J. Biol. Chem. 248, 3313-3326], we conclude that in solution this residue, in its unprotonated state, is part of the hydrophobic core of the protein. In contrast, His-48 in rat parvalbumin and His-106 in pike III parvalbumin show dramatic photo-CIDNP enhancements of their C2H, C5H, and beta-CH2 1H NMR resonances. Combined with its nearly normal pKa, 6.14, and exchange-broadened C2H resonance, the photo-CIDNP enhancement results for His-48 indicate that its microenvironment differs little from random-coil exposure, consistent with its presumed position on the solvent surface of helix C.(ABSTRACT TRUNCATED AT 250 WORDS)

Local structure involving histidine-12 in reduced S-sulfonated ribonuclease A detected by proton NMR spectroscopy under folding conditions

The C epsilon H proton resonance of His-12 of reduced cysteine S-sulfonated bovine pancreatic ribonuclease A exhibits a nonlinear temperature dependence of the chemical shift in its 1H-NMR spectrum at an apparent pH of 3.0. At temperatures below ca. 35 degrees C, the temperature dependence of the chemical shift of the His-12 C epsilon H resonance is opposite in sign to those of His-48, His-105, and His-119. At temperatures above ca. 35 degrees C, the temperature dependence of the chemical shift of the His-12 C epsilon H resonance is similar to those of the other three His C epsilon H resonances. These data indicate the existence of an equilibrium between locally ordered and locally disordered environments of His-12 in the sulfonated protein at temperatures below ca. 35 degrees C. The ordered and disordered conformations interconvert at a rate that is fast relative to the 1H-NMR chemical shift time scale--i.e., the locally ordered structure has a lifetime of much less than 7 msec. These results demonstrate that short- and medium-range interactions can define short-lived local structures under conditions of temperature and solution composition at which the native protein structure is stable. Furthermore, they demonstrate the utility of reduced derivatives of disulfide-containing proteins as model systems for the identification of local structures that may play a role as early-forming chain-folding initiation structures.

Comparative proton NMR studies of bovine semen and pancreas ribonucleases

The fine structure of bovine semen RNAase was studied with proton NMR spectroscopy making use of the four-protein system constituted by dimeric bovine semen RNAase, its catalytically active monomeric bis-(S-carboxymethyl-31,32) derivative, the naturally monomeric RNAase A from the pancrease of the same species, and dimerized RNAase A. Only four histidine C-2 H resonances were observed in the aromatic spectrum of bovine semen RNAase, which belong to the four histidine residues present in the sequence of bovine semen RNAase subunits at positions identical with those of the histidines of RNAase A. This is indicative of identical environments for the individual histidine residues in both subunits. These resonances were assigned (i) by comparing their titration curves with the corresponding curves obtained with RNAase A and with monomeric bovine semen RNAase and (ii) by evaluating the effects on their titration curves of nucleotide binding. Very similar NMR parameters were measured for His-105 and also for His-119 of seminal and pancreatic RNAase, while His-12 was found to have different environments in the two proteins. The distinctive NMR features of His-48 in bovine semen RNAase confirmed the role of the hinge regions of the subunits in maintaining the dimeric structure of the protein. While monomerization of the seminal enzyme reduced the differences between the histidine C-2 H resonances of RNAase A and bovine semen RNAase, dimerization of RNAase A did not affect the NMR spectrum of this protein, thus indicating as unlikely the possibility that the quaternary structure of bovine semen RNAase resembles that of dimerized RNAase A.

1H-NMR study on the tautomerism of the imidazole ring of histidine residues. II. Microenvironments of histidine-12 and histidine-119 of bovine pancreatic ribonuclease A

The NMR titration curves of proton chemical shifts were observed for the C2 protons of histidine residues in intact bovine pancreatic RNAase A (EC 3.1.27.5) and carboxyalkylated RNAase A. By comparing the methyl region of NMR spectra, the 250-340 nm region of circular dichoic spectra, and the NMR titration curves of tyrosine ring protons among intact and modified RNAase A, it was ascertained that the carboxyalkylation of histidine residues at position 12 or 119 did not make any appreciable conformational changes to RNAase A. With the pK values determined for intact and modified RNAase A, the microscopic pK values and molar ratios of tautomers were estimated for His-12 and His-119 by means of the procedure described in the preceding paper. The estimated microscopic pK values of tautomers were 6.2 for the N1-H tautomer of His-12, more than 8 for the N3-H tautomer of His-12, 7.0 for the N1-H tautomer of His-119, and 6.4 for the N3-H tautomer of His-119, respectively. These values were interpreted in terms of the microscopic environments surrounding the histidine residues. The microscopic structure estimated in the present study was discussed, comparing it with those from X-ray crystallography and hydrogen-tritium (or hydrogen-deuterium) exchange technique.

Anion binding and pH-dependent electrostatic effects in ribonuclease

The solvent-accessibility-modified, Tanford-Kirkwood, discrete charge model for electrostatic effects is applied to both ribonuclease A and ribonuclease S. The behavior of individual titratable sites and the pH-dependent free energy of denaturation are correctly predicted. The use of the solvent-accessibility factor in reducing charge-site interactions introduces a higher Coulombic shielding for solvent-exposed sites. This shielding is interpreted as a higher local strength or alternatively a higher effective dielectric constant. Specific anion binding sites are determined by locating areas of high positive electrostatic potential at the protein solvent interface. The potential and thus the anion affinity of a given site are calculated and shown to vary with the pH-dependent charge array. pH-dependent anion binding constants are calculated for the ribonuclease S active site. These binding constants and the predicted response of the active-site histidine pK1/2 values to anion binding are shown to agree with experimental determinations.

Local conformational transition of toxin B from Naja naja as studied by nuclear magnetic resonance and circular dichroism

The 270-MHz proton NMR spectra of toxin B, a long neurotoxin from Naja naja, in 2H2O solution were analyzed. From the analysis of the discontinuous pH dependence of His-21 C-2 proton chemical shift a 23 degrees C, toxin B is found to exhibit a slow-exchange conformational transition between the states A and B with the transition midpoint at pH 5.75. Separate peaks due to the states A and B were also observed for His-21 C-4 proton, Tyr-25 ring protons, phenylalanine ring protons and two C-alpha protons. This local conformational transition as induced by the protonation of His-21 residue is further confirmed by the measurement of the pH dependence of circular dichroism. In state A, the pKa value of His-21 is 6.6-6.9, indicating that His-21 residue is not significantly perturbed. The His-21 residue in state B is accessible to solvent, as found by the analysis of hydrogen leads to deuterium exchange rate. The low pKa value (4.6-4.9) of His-21 in stat B is probably due to the formation of a hydrogen bond of the imidazole ring. The conformational transition rate at 23 degrees C is obtained as 2.5 s-1 at the transition midpoint (pH 5.75), by the saturation transfer method. The conformational transition at 60 degrees C corresponds to a fast exchange case for the Tyr-25 ring proton resonances and to an intermediate exchange case for the His-21 C-2 and C-4 proton resonances. The transition rate at the midpoint is about 300 s-1 at 60 degrees C.

lac Repressor: a proton magnetic resonance look at the deoxyribonucleic acid binding fragment

The DNA binding fragment from Escherichia coli lac repressor, the N-terminal 56 amino acid residue "headpiece", has been examined by high-resolution 1H NMR spectroscopy at 360 MHz. The aromatic region has been examined in detail along with the four headpieces of altered repressors that are each missing one of the tyrosines, respectively. The spectra here show more resolved resonances and correct errors in the resonance assignments that have been published by Ribeiro et al. (1981b) Ribeiro, A. A., Wemmer, D., Bray, R. P., Wade-Jardetzky, N. G., & Jardetzky, O. (1981) Biochemistry 20, 818-823]. These corrections allow an interpretation of the spectroscopic observations that is now consistent with the extensive genetic analysis that has been carried out with the lac repressor gene. In addition, nuclear Overhauser enhancement measurements give a guide to the interresidue distances among the aromatic residues in this protein fragment.

The 1H nuclear-magnetic-resonance spectra and spatial structure of the Naja mossambica mossambica neurotoxin III

The proton NMR spectra at 300 MHz of neurotoxin III from venom of Naja mossambica mossambica are reported. By the use of double resonance techniques, pH dependence chemical shifts, isotope labeling technique, and comparison with homologous neurotoxins all proton signals in the aromatic and methyl regions as well as epsilon-CH2 proton signals of some lysine residues have been assigned to individual amino acid residues and their spatial microenvironment has been determined. The results deduced on the solution structure of neurotoxin III are in complete agreement with the crystal structure of sea snake erabutoxins as well as with the previously established backbone folding and inter-residue interactions for the Naja naja oxiana short-chain neurotoxin in solution. In addition evidence has been obtained (a) that the conformation of the beta turn in the 31-34 segment depends on the ionization state of Asp-31 and His-32 side chain groups and (b) that an intricate electrostatic interaction exists in a system of ionogenic groups of the invariant Lys-27, Lys-47, Asp-31, Arg-33, Glu-38 and His-32 residues. These aspects of dynamic conformation are related to an interaction mechanism of a neurotoxin molecule and a nicotinic acetylcholine receptor.

1H-NMR studies on the binding subsites of bovine pancreatic ribonuclease A

The titration curves of the C-2 histidine protons of an RNAase derivative (a covalent derivative obtained by reaction of bovine pancreatic RNAase A (EC 3.1.27.5) with 6-chloropurine 9-beta-D-ribofuranosyl 5'-monophosphate) were studied by means of 1H-NMR spectroscopy at 270 MHz. The interaction of natural (5'AMP, 5'GMP, 5'IMP) and halogenated purine mononucleotides (cl6RMP, br8AMP) with RNAase A was also monitored by using the same technique. The slight change observed in the pK values of the active centre histidine residues of the RNAase derivative, with respect to those in the native enzyme, can be considered as evidence that the phosphate of the label does not interact directly either with His-12 or 119 in the p1 site, but the p2 site as proposed previously (Parés, X., Llorens, R., Arús, C. and Cuchillo, C.M. (1980) Eur. J. Biochem. 105, 571--579). Lys-7 and/or Arg-10 are proposed as part of the p2 phosphate-binding subsite. The pK values of His-12 and 119 and the shift of an aromatic resonance of the native enzyme found on interaction with some purine nucleotides, can be interpreted by postulating that the interaction of 5'AMP, 5'GMP and 5'IMP takes place not only in the so-called purine-binding site B2R2p1 but also in the primary pyrimidine-binding site B1R1 and p0 of RNAase A.

Genetic assignment of resonances in the NMR spectrum of a protein: lac repressor

By using a systematic genetic approach, the resonances in the 19F NMR spectrum of 3-fluorotyrosine-substituted lac repressor protein have been assigned. The NMR data indicate that each monomer of the repressor consists of two distinct and independent domains. One domain, the NH2-terminal sixth of the primary sequence, which has been shown to be very important for DNA binding, is very mobile. The remaining COOH-terminal sequence is more rigid. Ligands of the repressor, which affect its DNA binding capability, lead to conformational changes in the COOH-terminal domain. The approach to the assignment of spectral features taken here can be extended to other systems.

The pentaammineruthenium(III)-histidine complex in ribonuclease A as an optical probe of conformational change

The formation of stable transition metal complexes between pentaammineruthenium(III) and the imidazole moiety of histidine residues in ribonuclease A is described. Studies of the kinetics of incorporation suggest that three of the four histidines are reactive and ion-exchange chromatography affords the isolation of three derivatives containing a single ruthenium-histidine complex. Equilibrium studies of the thermal- and urea-induced unfolding of one of these derivatives show that the ligand-to-metal change-transfer absorptions above 300 nm can be used to follow the conformational changes in the vicinity of the complex. For the urea-induced unfolding of this derivative, the coincidence of the transition curves obtained from the change-transfer absorptions and from difference ultraviolet spectroscopy at 286 nm, which measures the exposure of buried tyrosine residues to solvent, suggest that only two conformations, native and unfolded, are present at equilibrium in significant concentrations.

Guanyl-specific ribonuclease from the fungus Penicillium chrysogenum strain 152 and its complex with guanosine 3'-phosphate studied by nuclear magnetic resonance

The method of proton magnetic resonance was used to obtain information on the active site of the guanyl-specific ribonuclease from Penicillium chrysogenum, strain 152A. Four pH-dependent signals in the aromatic region of the proton NMR spectrum of the enzyme were assigned to the C-2 and C-4 protons of the two histidine residues. To determine the pK values and the environment of the histidine residues the pH dependence of their chemical shifts was studied and experimental curves thus obtained were analyzed taking into account the effect of other dissociating groups of the enzyme. The pK values of the histidine residues were found to be equal to 7.92 +/- 0.04 and 7.86 +/- 0.09. The results of the calculations indicate that each histidine residue should interact with an acidic group (carboxylic) of the protein (pK 4.33 and 3.48) and the distance between two histidine residues does not exceed 0.85 nm. The rate constants for the quasi-first order reaction of deuterium exchange of the histidine residues (11.2 s-1 and 3.7 x-1) suggest that both residues are accessible, though to a different degree to solvent. Formation of a complex between the enzyme and guanosine 3'-phosphate (Guo3'P) is accompanied by the shift of the histidine pK toward the alkaline region by 0.5. The existence of the complex is controlled by dissociation of a histidine residue with pK 8.7 in alkaline medium and by protonation of the N-7 of Guo3'P (pK 2.4) in acid medium. Nuclear Overhauser effect measurements were used to determine the glycosidic torsion angle for the Guo3'P in the complex and to estimate the distances between the histidine residues of the enzyme and ribose ring of Guo-3'P. The results obtained suggest that the nucleotide in the complex has an anti conformation and the least exposed histidine is spaced not more than 0.5 nm from the C-1' proton of the nucleotide ribose ring. A model for the enzyme-nucleotide complex is presented.

The 1H nuclear-magnetic-resonance spectroscopy of cobalt(II)-beta-lactamase II

The 1H n.m.r. spectra of beta-lactamase II in the presence of Co(II) were studied. Analysis of the spectra suggests that Co(II) binds at the same two metal-binding sites as does Zn(II). The binding of Co(II) at the first site is much weaker than the binding of Zn(II) at this site, whereas the binding of Co(II) at the second site is tighter than the binding of Zn(II). The binding of Co(II) to the mono-zinc(II)-enzyme caused only one marked change in the spectrum, namely a decrease in the intensity of the resonances assigned to the C-2 and C-4 protons of one histidine residue (residue E). However, when the spectra of the apoenzyme and the Co(II)-enzyme were compared, there were many differences. A significant fraction of the protons in the whole molecule are affected by the binding of Co(II) at the first metal-ion-binding site (where the ligands are the enzyme's sole thiol group and three histidine residues). This may be because the first site is internal, or because of a difference in conformation between the apoenzyme and the mono-Co(II)-enzyme. The second site may be located on the surface of the molecule.

Conformation of cobrotoxin in aqueous solution as studied by nuclear magnetic resonance

The 270-MHz proton NMR spectra of cobrotoxin from Naja naja atra were observed in 2H2O solution. The pKa value (5.93) of His-32 is slightly lower than the pKa value (6.65) of the reference model of N-acetylhistidine methylamide, because of the electrostatic interaction with Arg-33 and Asp-31. The pKa value (5.3--5.4) of His-4 is appreciably low, because of the interaction with the positively charged guanidino group possibly of Arg-59. The hydrogen-deuterium exchange rates in 2H2O solution were measured of cobrotoxin and imidazole-bearing models. The second-order rate constants of N-acetylhistidine methylamide, N-acetylhistidine and imidazole acetic acid satisfy the Brønsted relation. With reference to this Brønsted relation, the imidazole ring of His-32 is confirmed to be exposed. The imidazole ring of His-4 is also exposed and the exchange rate is excessively promoted by the presence possibly of Arg-59 in the proximity. All the methyl proton resonances are assigned to amino-acid types, by conventional double-resonance method and more effectively by the spin-echo double-resonance method. Eight methyl proton resonances are identified as due to the gamma and/or delta-methyl groups of Val-46, Leu-1, Ile-50 and Ile-52 residues. The proximity of aromatic ring protons and methyl protons is elucidated by the analyses of nulcear Overhauser effect enhancements. The aromatic proton resonances of Trp-29 are affected by the ionizable groups of Asp-31, His-32 and Tyr-35. The methyl groups of Ile-50 are in the proximity to the aromatic ring of Trp-29 and the methyl groups of Ile-52 are in the proximity to Tyr-25. The highest-field methyl proton resonance is due to a threonine residue in the proximity to His-4. The appreciable temperature-dependent chemical shift of this methyl proton resonance suggests a temperature-dependent local conformational equilibrium around the His-4 residue of the first loop of the cobrotoxin molecule.

The aromatic residues of bovine pancreatic ribonuclease studied by 1H nuclear magnetic resonance

1. The aromatic proton resonances in the 360-MHz 1H nuclear magnetic resonance (NMR) spectrum of bovine pancreatic ribonuclease were divided into histidine, tyrosine and phenylalanine resonances by means of pH titrations and double resonance experiments. 2. Photochemically induced dynamic nuclear polarization spectra showed that one histidine (His-119) and two tyrosines are accessibly to photo-excited flavin. This permitted the identification of the C-4 proton resonance of His-119. 3. The resonances of the ring protons of Tyr-25, Tyr-76 and Tyr-115 and the C-4 proton of His-12 were identified by comparison with subtilisin-modified and nitrated ribonucleases. Other resonances were assigned tentatively to Tyr-73, Tyr-92 and Phe-46. 4. On addition of active-site inhibitors, all phenylalanine resonances broadened or disappeared. The resonance that was most affected was assigned tentatively to Phe-120. 5. Four of the six tyrosines of bovine RNase, identified as Tyr-76, Tyr-115 and, tentatively, Tyr-73 and Tyr-92, are titratable above pH 9. The rings of Tyr-73 and Tyr-115 are rapidly rotating or flipping by 180 degrees about their C beta--C gamma bond and are accessible to flavin in photochemically induced dynamic nuclear polarization experiments. Tyr-25 is involved in a pH-dependent conformational transition, together with Asp-14 and His-48. A scheme for this transition is proposed. 6. Binding of active-site inhibitors to bovine RNase only influences the active site and its immediate surroundings. These conformational changes are probably not connected with the pH-dependent transition in the region of Asp-14, Tyr-25 and His-48. 7. In NMR spectra of RNase A at elevated temperatures, no local unfolding below the temperature of the thermal denaturation was observed. NMR spectra of thermally unfolded RNase A indicated that the deviations from a random coil are small and might be caused by interactions between neighbouring residues.

Carbon-13 NMR investigations on ribonuclease A

The proposed interaction between the amino acid residues Asp 14 and His 48 of ribonuclease A has been confirmed by 13C-NMR spectroscopy. The titration behaviour of the resonance of the side-chain carboxyl group of Asp 14 suggests a pKa of 6.5--7.0 for His 48. An equilibrium between different conformation process of His 48. Upon this deprotonation a hydrogen bond between the side-chains of Asp 14 or His 48 and Tyr 25 seems to be formed as is suggested by the behaviour of a tyrosine C zeta resonance assigned to Tyr 25. One phenylalanine resonance broadens and moves upfield on the addition of the inhibitor Cyd-2'-P, being therefore assigned to Phe 120. The behaviour of this resonance suggests that the upfield shift results from the anisotropy of the cytidine ring. Three signals are assigned to the three Phe residues.

13C NMR investigations on Npi-[13C1]carboxymethyl-histidine-119 ribonuclease

The ribonuclease A derivative Npi-[13C1]carboxymethyl-histine-119 ribonuclease prepared by using [13C1]bromoacetate as alkylating reagent has been investigated with high resolution 13C NMR spectroscopy. In the 13C NMR spectra two carbon resonances of relatively high intensity appear which can be assigned to carboxyl groups attached to His-119 and Met-30, their intensity ratio being 10 : 1. The pH dependence of the carbon resonance of the carboxy-methyl group bound to the Npi of His-119 differs in the absence and presence of Cyd-2'-P, thus indicating that the catalytically inactive derivative does bind nucleotides. A mechanism of the alkylation reaction at pH 5.6 is proposed in which the epsilon-amino group of Lys-41 acts as the binding site for the carboxyl group of bromoacetate pushing the bromomethylene group towards the Npi of His-119 or the Ntau of His-12.

Application of pulse radiolysis to the study of proteins: chymotrypsin and trypsin

The one-electron reduction of chymotrypsin, trypsin, and their zymogens have been studied by pulse radiolysis. The optical spectra of the transient products from the two active enzymes display a pH-dependent band at 360 nm, associated with the histidine-electron adduct. The yield of the histidyl radical as a function of pH is consistent with a pK(a) less than 4.5, which suggests that the radical is located at the enzyme active site. The histidines of the proenzymes chymotrypsinogen and trypsinogen are unreactive towards the hydrated electron. We conclude that formation of the histidine-electron adduct at the serine protease active site is sensitive to the physical alterations which accompany protease activation.

Conformation of erabutoxins a and b in aqueous solution as studied by nuclear magnetic resonance and circular dichroism

The 270-MHZ nuclear magnetic resonance (NMR) spectra of erabutoxins a and b have been observed in 2H2O solution. By the use of convolution difference and double resonance techniques, proton signals in the aromatic and methyl regions have been assigned. From the pH dependence of NMR chemical shifts, the pKa value of His-26 of erabutoxin b is found to be 5.8, whereas His-7 of erabutoxins a and b is not protonated at pH above 3. The imidazole ring of His-7 is protonated upon the denaturation at pH 2.85. The acid denaturation process has been followed by the His-26 and methyl proton signals and is found to be reversible but is slow as compared with NMR chemical shift time scale. The circular dichroism (CD) of erabutoxin b has also been observed. The denaturation is found to involve a major change from the beta-rich conformation to a disordered one. The NMR and CD changes upon acid denaturation are satisfactorily explained by the two-state process. The deuterium exchange rates of the C-2 protons of His-26 and His-7 of erabutoxin b indicate that His-26 is exposed to the solvent whereas His-7 is tightly buried in the interior of the protein globule. The pKa value of Tyr-25 is as high as about 12.0, possibly due to the hydrogen bond formation between the hydroxyl group of Tyr-25 and a carboxylate group. The hydroxyl group of Tyr-25 is reversibly titrated so that this group is not buried tightly in the interior of the protein globule. The line width of the aromatic proton signals of Tyr-25 is significantly broad at room temperature, suggesting a restricted rotation of the aromatic ring. The aromatic proton signals of Trp-29 are fairly sharp; this aromatic ring is exposed and mobile. Except for His-7, the micro-environments of Tyr-25, His-26, and Trp-29 residues and methyl proton signals of valine and isoleucine are consistent with the locations of alpha carbon atoms as elucidated by X-ray crystal analyses.

1H NMR studies of the binding of EDTA to bovine pancreatic ribonuclease

EDTA binds at the active site of ribonuclease causing a selective downfield shift of the C2 proton resonance of His 12 at pH 5.5 (pH denotes an uncorrected glass electrode pH meter reading of a 2H2O solution). A dissociation constant for EDTA binding to ribonuclease of 1.70 mM was calculated from this chemical shift change. The pKa of His 12 increased from 5.79 in ribonuclease alone to 6.73 in the RNAase . EDTA complex. Compared to these effects, the other histidine residues were not significantly affected by EDTA. EDTA was shown to act as a competitive inhibitor of cytidine 2',3'-cyclic phosphate hydrolysis by ribonuclease with a Ki of 1.37 mM at pH 5.5, 25 degrees C. Molecular model building suggests that three of the four carboxyl groups of EDTA could simultaneously interact with histidine 12, lysine 41 and lysine 7. A complex of this type would account for the data described herein.

Nuclear-magnetic-resonance study of the histidine residues of S-peptide and S-protein and kinetics of 1H-2H exchange of ribonuclease A

1H NMR spectroscopy at 100 MHz was used to determine the first-order rate constants for the 1H-2H exchange of the H-2 histidine resonances of RNase-A in 2H2O at 35 degrees C and pH meter readings of 7, 9, 10 and 10.5. Prolonged exposure in 2H2O at 35 degrees C and pH meter reading 11 caused irreversible denaturation of RN-ase-A. The rate constants at pH 7 and 9 agreed reasonably well with those obtained in 1H-3H exchange experiments by Ohe, J., Matsuo, H., Sakiyama, F. and Narita, K. [J. Biochem, (Tokyo) 75, 1197-1200 (1974)]. The rate data obtained by various authors is summarised and the reasons for the poor agreement between the data is discussed. The first-order rate constant for the exchange of His-48 increases rapidly from near zero at pH 9 (due to its inaccessibility to solvent) with increase of pH to 10.5 The corresponding values for His-119 show a decrease and those for His-12 a small increase over the same pH range. These changes are attributed to a conformational change in the hinge region of RNase-A (probably due to the titration of Tyr-25) which allows His-48 to become accessible to solvent. 1H NMR spectra of S-protein and S-peptide, and of material partially deuterated at the C-2 positions of the histidine residues confirm the reassignment of the histidine resonances of RNase-A [Bradbury, J. H. & Teh, J. S. (1975) Chem. Commun., 936-937]. The chemical shifts of the C-2 and C-4 protons of histidine-12 of S-peptide are followed as a function of pH and a pK' value of 6.75 is obtained. The reassignment of the three C-2 histidine resonances of S-protein is confirmed by partial deuteration studies. The pK' values obtained from titration of the H-2 resonances of His-48, His-105 and His-119 are 5.3, 6.5 and 6.0, respectively. The S-protein is less stable to acid than RNase-A since the former, but not the latter, shows evidence of reversible denaturation at pH 3 and 26 degrees C. His-48 in S-protein titrates normally and has a lower pK than in RN-ase-A probably because of the absence of Asp-14, which in RN-ase-A forms a a hydrogen bond with His-48 and causes it to be inaccessible to solvent, at pH values below 9.

Carbon-2 proton exchange at histidine-41 in bovine erythrocyte superoxide dismutase

The C-2 proton of one histidine residue in bovine erythrocyte superoxide dismutase is shown to be particularly labile. This residue is identified by tritiation, protein digestion and subsequent peptide 'mapping' as histidine-41. A half-life for the exchange of histidine C-2 1H for 2H in 2H2O as solvent, at pD 8.1 and 40 degrees C, is estimated as approx. 9.2h, by 1H nuclear-magnetic-resonance spectroscopy.

Protein folding

This review describes recent advances in studies on the stabilities of the three-dimensional structures of proteins and on the processes leading to the formation of these structures. The term ‘protein folding’ will be used here to denote the process of the conversion of an open polypeptide chain into the unique three-dimensional conformation of the native protein. Experimental and theoretical aspects of protein folding have been reviewed by anfinsen & Scheraga (1975). In the present article, we emphasize advances made since the writing of that review, together with a brief summary of the background of recent studies.

Investigation of the structure of bovine erythrocyte superoxide dismutase by 1H nuclear magnetic resonance spectroscopy

The 270-MHz 1H nuclear magnetic resonance spectra of the apo, copper(I)-zinc(II), and copper(II)-zinc(II) forms of bovine erythrocyte superoxide dismutase (EC 1.15.1.1) are reported, and assignments of resonances to ten amino acid residues are proposed. The data require that at least four and probably six histidine residues serve as ligands to the metals in each subunit of the enzyme, consistent with x-ray diffraction results. The remaining assigned resonances are associated with His-19, His-41, Tyr-108, and the N-terminal N-acetyl group. The imidazole C(2)H of His-41 exchanges readily at pH greater than 8. The structural implications of the effect of the paramagnetic Cu(II) in the holoenzyme on the proton relaxation times are in reasonable accord with the data from x-ray diffraction studies.

Reactions of gold(III) ions with ribonuclease A and methionine derivatives in aqueous solution

1. Addition of Au(III) (as chloroaurate) to folded ribonuclease A causes the formation of aggregates, as shown by chromatography and extreme broadening of the 270 MHZ 1H NMR spectrum. 2. When RNAase is partially unfolded Au(III) causes oxidation of methionines to the sulphoxide, and leads to almost complete unfolding (at molar equivalent ratios). 3. Reactions between model methionine derivatives and Au(III) show that the oxidation involves the production of Au(I)-methionine species. The stability of these complexes is dependent on the availability of free NH2 groups which catalyse their disproportionation.

Catalytic activity of Ntau-carboxymethylhistidine-12 ribonuclease: pH dependence

The pH-dependence of RNAase A and of Ntau-carboxymethylhistidine-12-RNAase (ribonucleate 3'-pyrimidino-oligonucleotidohydrolase) catalysis was studied. Apparent acid dissociation constants were obtained by least squares analysis of the kinetics data. These dissociation constants were compared with pKa values of model imidazole compounds, and with pKa values of histidine residues 12 and 119 on the protein. The shapes of the kcat versus pH profiles for RNAase A and its carboxymethyl derivative are very similar, from which it is concluded that the mechanism of catalysis is closely similar in the two proteins. Apparent pKa values obtained from the kinetic data are higher for the carboxymethylated protein than for RNAase A, as are the pKa values of residues 12 and 119. The similar shifts are consistent with the conclusions that both these residues are functionally significant in native and modified enzyme, and that an unblocked tau-nitrogen on histidine-12 is not essential for activity. From the enzyme's catalytic dependence on pH, and the NMR determined pKa values we propose that histidine 12 and 119 function catalytically in their basic and acidic forms respectively.

Proton-nuclear-magnetic-resonance study of the conformation of neurotoxin II from Middle-Asian cobra (Naja naja oxiana) venom

A proton nuclear magnetic resonance (NMR) study at 100 and 300 MHz of neurotoxin II from the venom of Middle-Asian cobra Naja naja oxiana has been performed in 2H2O and H2O solutions. By means of chemical modification and double resonance all the aromatic residue resonances have been assigned. From the NMR titration curves, pK values of histidine 4 and histidine 31 residues have been determined. For one of the two neighbouring tryptophan residues pH dependence (in the 2-8-pH range) of the chemical shifts of indole protons has been revealed. According to the different sensitivity of the linewidth of indole NH resonances to pH in H2O solution, the accessibility of each of the tryptophan residues has been estimated. Temperature dependence has been observed for the linewidth of the aromatic resonances of the tyrosine 24 residue. Deuterium exchange rates have been measured for amide protons as well as for C(2)H histidine resonances. The NMR data obtained have allowed the conclusions to be made that the two histidine residues and one of the tryptophan residues should be localized on the surface of the protein globule, that arginine residues should be present in the environment of histidine 4, that histidine 31 and the buried tryptophan are possibly localized in close spatial proximity and that the side chain of tyrosine 24 is buried within the protein globule.

The intrinsic pKa-values of functional groups in enzymes: improper deductions from the pH-dependence of steady-state parameters

The assumptions implicit in the deductions made from the pH-dependence of rate measurements of enzyme-catalyzed reactions are summarized, and the limitations of such determinations are discussed nonalgebraically. The following types of pH-profile are considered (in order of increasing utility): pH-"activity" curves at fixed [S] o;pH-dependences of kcat, Km, kcat/Km, and Ki; pH-dependences of kmodification (by specific reagents) and of competitive labeling (by nonspecific reagents); pH-dependences of elementary steps; and the direct observation of a titrating group.