Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:3.1.31.1 (micrococcal nuclease)
2,818 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The pK(a) values of internal ionizable groups are usually very different from the normal pK(a) values of ionizable groups in water. To examine the molecular determinants of pK(a) values of internal groups, we compared the properties of Lys, Asp, and Glu at internal position 38 in staphylococcal nuclease. Lys38 titrates with a normal or elevated pK(a), whereas Asp38 and Glu38 titrate with elevated pK(a) values of 7.0 and 7.2, respectively. In the structure of the L38K variant, the buried amino group of the Lys38 side chain makes an ion pair with Glu122, whereas in the structure of the L38E variant, the buried carboxyl group of Glu38 interacts with two backbone amides and has several nearby carboxyl oxygen atoms. Previously, we showed that the pK(a) of Lys38 is normal owing to structural reorganization and water penetration concomitant with ionization of the Lys side chain. In contrast, the pK(a) values of Asp38 and Glu38 are perturbed significantly owing to an imbalance between favorable polar interactions and unfavorable contributions from dehydration and from Coulomb interactions with surface carboxylic groups. Their ionization is also coupled to subtle structural reorganization. These results illustrate the complex interplay between local polarity, Coulomb interactions, and structural reorganization as determinants of pK(a) values of internal groups in proteins. This study suggests that improvements to computational methods for pK(a) calculations will require explicit treatment of the conformational reorganization that can occur when internal groups ionize.
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PMID:The pK(a) values of acidic and basic residues buried at the same internal location in a protein are governed by different factors. 1932 49

Prior computational studies of the acid-unfolding behavior of staphylococcal nuclease (SNase) suggest that the pK(a) values of its carboxylic groups are difficult to reproduce with electrostatics calculations with continuum methods. To examine the molecular determinants of the pK(a) values of carboxylic groups in SNase, the pK(a) values of all 20 Asp and Glu residues were measured with multidimensional and multinuclear NMR spectroscopy in an acid insensitive variant of SNase. The crystal structure of the protein was obtained to describe the microenvironments of the carboxylic groups. Fourteen Asp and Glu residues titrate with relatively normal pK(a) values that are depressed by less than 1.1 units relative to the normal pK(a) of Asp and Glu in water. Only six residues have pK(a) values shifted by more than 1.5 units. Asp-21 has an unusually high pK(a) of 6.5, which is probably the result of interactions with other carboxylic groups at the active site. The most perturbed pK(a) values appear to be governed by hydrogen bonding and not by Coulomb interactions. The pK(a) values calculated with standard continuum electrostatics methods applied to static structures are more depressed than the measured values because Coulomb effects are exaggerated in the calculations. The problems persist even when the protein is treated with the dielectric constant of water. This can be interpreted to imply that structural relaxation is an important determinant of the pK(a) values; however, no major pH-sensitive conformational reorganization of the backbone was detected using NMR spectroscopy.
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PMID:Molecular determinants of the pKa values of Asp and Glu residues in staphylococcal nuclease. 1953 44

In order to examine the properties specific to the folded protein, the effect of the conformational states on protein dynamical transition was studied by incoherent elastic neutron scattering for both wild type and a deletion mutant of staphylococcal nuclease. The deletion mutant of SNase which lacks C-terminal 13 residues takes a compact denatured structure, and can be regarded as a model of intrinsic unstructured protein. Incoherent elastic neutron scattering experiments were carried out at various temperature between 10 K and 300 K on IN10 and IN13 installed at ILL. Temperature dependence of mean-square displacements was obtained by the q-dependence of elastic scattering intensity. The measurements were performed on dried and hydrated powder samples. No significant differences were observed between wild type and the mutant for the hydrated samples, while significant differences were observed for the dried samples. A dynamical transition at approximately 140 K observed for both dried and hydrated samples. The slopes of the temperature dependence of MSD before transition and after transition are different between wild type and the mutant, indicating the folding induces hardening. The hydration water activates a further transition at approximately 240 K. The behavior of the temperature dependence of MSD is indistinguishable for wild type and the mutant, indicating that hydration water dynamics dominate the dynamical properties.
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PMID:Effect of conformational states on protein dynamical transition. 1959 99

Understanding protein stabilization by small organic compounds is a topic of great practical importance. The effect of mannosylglycerate, a charged compatible solute typical of thermophilic microorganisms, on a variant of staphylococcal nuclease was investigated using several NMR spectroscopy methods. No structural changes were apparent from the chemical shifts of amide protons. Measurements of (15)N relaxation and model-free analysis, water-amide saturation transfer (phase-modulated CLEAN chemical exchange), and hydrogen/deuterium exchange rates provided a detailed picture of the effects of mannosylglycerate on the backbone dynamics and time-averaged structure of this protein. The widest movements of the protein backbone were significantly constrained in the presence of mannosylglycerate, as indicated by the average 5-fold decrease of the hydrogen/deuterium exchange rates, but the effect on the millisecond timescale was small. At high frequencies, internal motions of staphylococcal nuclease were progressively restricted with increasing concentrations of mannosylglycerate or reduced temperature, while the opposite effect was observed with urea (a destabilizing solute). The order parameters showed a strong correlation with the changes in the T(m) values induced by different solutes, determined by differential scanning calorimetry. These data show that mannosylglycerate caused a generalised reduction of backbone motions and demonstrate a correlation between protein stabilization and protein rigidification.
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PMID:Relationship between protein stabilization and protein rigidification induced by mannosylglycerate. 1974 13

The pK(a) values measured previously for the internal Lys-66, Asp-66, and Glu-66 in variants of a highly stable form of staphylococcal nuclease are shifted by as many as 5 pK(a) units relative to normal pK(a) values in water. These shifts cannot be reproduced with continuum electrostatics calculations with static structures unless the protein is treated with high dielectric constants near 10. These high apparent dielectric constants are inconsistent with the highly hydrophobic microenvironments of the ionizable moieties in crystal structures. To examine the origins of these high apparent dielectric constants, we showed that the pK(a) values of these internal residues are sensitive to the global stability of the protein; the shifts tend to be smaller in less stable forms of nuclease. This implies that the apparent dielectric constants reported by these internal ionizable groups are high because they reflect conformational reorganization coupled to their ionization. To detect this directly, acid-base titrations monitored with Trp fluorescence and near-UV and far-UV CD spectroscopy were performed on variants with Lys-66, Glu-66, or Asp-66 in background proteins with different stabilities. Conformational reorganization coupled to the ionization of the internal groups was spectroscopically detectable, especially in the less stable background proteins. The data show that to improve the accuracy of structure-based pK(a) calculations of internal groups the calculations will have to treat explicitly all structural reorganization coupled to ionization. The data also suggest a novel approach to mapping the folding free energy landscape of proteins by using internal ionizable groups to stabilize partially unfolded states.
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PMID:Conformational consequences of ionization of Lys, Asp, and Glu buried at position 66 in staphylococcal nuclease. 2032 80

Haloacetonitriles, which are water chlorination by-products, are mutagens, carcinogens and teratogens. In vitro, the reaction of the haloacetonitriles bromoacetonitrile (BAN), chloroacetonitrile (CAN), dichloroacetonitrile (DCAN) and trichloroacetonitrile (TCAN) with calf thymus DNA produced fluorescent DNA derivatives. The reactivity of haloacetonitriles towards DNA was in the order of BAN > CAN > DCAN > TCAN. The emission fluorescence spectra of the reaction product(s) of various haloacetonitriles with DNA has a peak at 404 nm at fixed excitation wavelength (300 nm). The fluorescence intensity of the reaction product(s) was pH dependent with an optimum intensity at pH 7.4. The DNA interaction was dependent on haloacetonitrile concentration. Higher affinity of haloacetonitriles towards single-stranded DNA (SS-CT-DNA) than towards double-stranded DNA (DS-CT-DNA) was observed. To characterize the type of fluorescent product(s) formed, samples of the reaction product of SS-CT-DNA with BAN were hydrolysed (a) enzymatically by micrococcal nuclease and spleen phosphodiesterase to nucleotides and nucleotide derivative(s) or (b) chemically by formic acid to nucleobases and nucleobase adducts. The hydrolysates were analysed by reversed phase HPLC. A major fluorescent peak was detected in the enzymatic hydrolysate that was not present in unreacted DNA. In the acid hydrolysate, one fluorescent peak was detected that was not present in unreacted DNA. Authentic 7-(cyanomethyl)guanine was synthesized by the reaction of CAN with 2'-deoxyguanosine and the product was purified and characterized spectroscopically. The product, 7-(cyanomethyl)guanine, was found to be chromatographically and spectroscopically identical to the fluorescence product that was obtained following haloacetonitrile-DNA interaction. This study shows that haloacetonitriles, in vitro, are capable of alkylating DNA at the guanine moiety to form a 7-(cyanomethyl)guanine adduct.
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PMID:Studies on the mechanisms of haloacentronitrile-induced genotoxicity IV: In vitro interaction of haloacetonitriles with DNA. 2065 Jan 78

Charges are inherently incompatible with hydrophobic environments. Presumably for this reason, ionizable residues are usually excluded from the hydrophobic interior of proteins and are found instead at the surface, where they can interact with bulk water. Paradoxically, ionizable groups buried in the hydrophobic interior of proteins play essential roles, especially in biological energy transduction. To examine the unusual properties of internal ionizable groups we measured the pK(a) of glutamic acid residues at 25 internal positions in a stable form of staphylococcal nuclease. Two of 25 Glu residues titrated with normal pK(a) near 4.5; the other 23 titrated with elevated pK(a) values ranging from 5.2-9.4, with an average value of 7.7. Trp fluorescence and far-UV circular dichroism were used to monitor the effects of internal charges on conformation. These data demonstrate that although charges buried in proteins are indeed destabilizing, charged side chains can be buried readily in the hydrophobic core of stable proteins without the need for specialized structural adaptations to stabilize them, and without inducing any major conformational reorganization. The apparent dielectric effect experienced by the internal charges is considerably higher than the low dielectric constants of hydrophobic matter used to represent the protein interior in electrostatic continuum models of proteins. The high thermodynamic stability required for proteins to withstand the presence of buried charges suggests a pathway for the evolution of enzymes, and it underscores the need to mind thermodynamic stability in any strategy for engineering novel or altered enzymatic active sites in proteins.
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PMID:Charges in the hydrophobic interior of proteins. 2079 41

The side chains of Lys66, Asp66, and Glu66 in staphylococcal nuclease are fully buried and surrounded mainly by hydrophobic matter, except for internal water molecules associated with carboxylic oxygen atoms. These ionizable side chains titrate with pK(a) values of 5.7, 8.8, and 8.9, respectively. To reproduce these pK(a) values with continuum electrostatics calculations, we treated the protein with high dielectric constants. We have examined the structural origins of these high apparent dielectric constants by using NMR spectroscopy to characterize the structural response to the ionization of these internal side chains. Substitution of Val66 with Lys66 and Asp66 led to increased conformational fluctuations of the microenvironments surrounding these groups, even under pH conditions where Lys66 and Asp66 are neutral. When Lys66, Asp66, and Glu66 are charged, the proteins remain almost fully folded, but resonances for a few backbone amides adjacent to the internal ionizable residues are broadened. This suggests that the ionization of the internal groups promotes a local increase in dynamics on the intermediate timescale, consistent with either partial unfolding or increased backbone fluctuations of helix 1 near residue 66, or, less likely, with increased fluctuations of the charged side chains at position 66. These experiments confirm that the high apparent dielectric constants reported by internal Lys66, Asp66, and Glu66 reflect localized changes in conformational fluctuations without incurring detectable global structural reorganization. To improve structure-based pK(a) calculations in proteins, we will need to learn how to treat this coupling between ionization of internal groups and local changes in conformational fluctuations explicitly.
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PMID:Structural origins of high apparent dielectric constants experienced by ionizable groups in the hydrophobic core of a protein. 2105 59

Molecular dynamics simulations were used to examine the effects of ionization of internal groups on the structures of eighteen variants of staphylococcal nuclease (SNase) with internal Lys, Asp, or Glu. In most cases the RMSD values of internal ionizable side chains were larger when the ionizable moieties were charged than when they were neutral. Calculations of solvent-accessible surface area showed that the internal ionizable side chains were buried in the protein interior when they were neutral and moved toward crevices and toward the protein-water interface when they were charged. The only exceptions are Lys-36, Lys-62, and Lys-103, which remained buried even after charging. With the exception of Lys-38, the number of internal water molecules surrounding the ionizable group increased upon charging: the average number of water oxygen atoms within the first hydration shell increased by 1.7 for Lys residues, by 5.2 for Asp residues, and by 3.2 for Glu residues. The polarity of the microenvironment of the ionizable group also increased when the groups were charged: the average number of polar atoms of any kind within the first hydration shell increased by 2.7 for Lys residues, by 4.8 for Asp residues, and by 4.0 for Glu residues. An unexpected correlation was observed between the absolute value of the shifts in pK(a) values measured experimentally, and several parameters of structural relaxation: the net difference in the polarity of the microenvironment of the charged and neutral forms of the ionizable groups, the net difference in hydration of the charged and neutral forms of the ionizable groups, and the difference in RMSD values of the charged and neutral forms of the ionizable groups. The effects of ionization of internal groups on the conformation of the backbone were noticeable but mostly small and localized to the area immediately next to the internal ionizable moiety. Some variants did exhibit local unfolding.
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PMID:Conformational relaxation and water penetration coupled to ionization of internal groups in proteins. 2142 36

The pK(a) Cooperative (http://www.pkacoop.org) was organized to advance development of accurate and useful computational methods for structure-based calculation of pK(a) values and electrostatic energies in proteins. The Cooperative brings together laboratories with expertise and interest in theoretical, computational, and experimental studies of protein electrostatics. To improve structure-based energy calculations, it is necessary to better understand the physical character and molecular determinants of electrostatic effects. Thus, the Cooperative intends to foment experimental research into fundamental aspects of proteins that depend on electrostatic interactions. It will maintain a depository for experimental data useful for critical assessment of methods for structure-based electrostatics calculations. To help guide the development of computational methods, the Cooperative will organize blind prediction exercises. As a first step, computational laboratories were invited to reproduce an unpublished set of experimental pK(a) values of acidic and basic residues introduced in the interior of staphylococcal nuclease by site-directed mutagenesis. The pK(a) values of these groups are unique and challenging to simulate owing to the large magnitude of their shifts relative to normal pK(a) values in water. Many computational methods were tested in this first Blind Prediction Challenge and critical assessment exercise. A workshop was organized in the Telluride Science Research Center to objectively assess the performance of many computational methods tested on this one extensive data set. This volume of Proteins: Structure, Function, and Bioinformatics introduces the pK(a) Cooperative, presents reports submitted by participants in the Blind Prediction Challenge, and highlights some of the problems in structure-based calculations identified during this exercise.
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PMID:The pKa Cooperative: a collaborative effort to advance structure-based calculations of pKa values and electrostatic effects in proteins. 2200 77


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