Protein folding kinetics exhibit an Arrhenius temperature dependence when corrected for the temperature dependence of protein stability

The anomalous temperature dependence of protein folding has received considerable attention. Here we show that the temperature dependence of the folding of protein L becomes extremely simple when the effects of temperature on protein stability are corrected for; the logarithm of the folding rate is a linear function of 1/T on constant stability contours in the temperature-denaturant plane. This convincingly demonstrates that the anomalous temperature dependence of folding derives from the temperature dependence of the interactions that stabilize proteins, rather than from the super Arrhenius temperature dependence predicted for the configurational diffusion constant on a rough energy landscape. However, because of the limited temperature range accessible to experiment, the results do not rule out models with higher order temperature dependences. The significance of the slope of the stability-corrected Arrhenius plots is discussed.     

pH dependence of corneal oxygen consumption

PURPOSE: To determine whether corneal acidosis, which occurs during contact lens wear, alters corneal O2 consumption (QO2) and if so, whether increased ion transport activity could contribute to altered QO2 during acidosis. METHODS: PO2 was measured, using the phosphorescence quenching of Pd-meso-tetra-(4-carboxyphenyl) porphine, in an airtight chamber that held a trephined rabbit cornea. The rate of change in chamber PO2 was used as a measure of QO2. QO2 was measured at pH 7.5 and then at either pH 6.7, 7.1, or 7.3. Measurements of QO2 at pHs 7.5 and 6.7 were repeated in the presence of 0.5 mM amiloride and 0.5 mM ouabain. RESULTS: When pH was changed from 7.5 to 6.7, 7.1, or 7.3, O2 consumption increased by a factor of 1.80+/-0.11 (+/-SE), 1.65+/-0.12, and 1.44+/-0.06, respectively. Amiloride (0.5 mM) and ouabain (0.5 mM) inhibited 50% and 65%, respectively, of the increase in QO2 at pH 6.7. CONCLUSIONS: Corneal acidosis leads to increased QO2 in a dose-dependent manner. The increased QO2 is in part secondary to the activation of pH regulatory mechanisms, including Na+/H+ exchange, which then stimulates Na+/ K+-ATPase activity. These findings indicate that contact lens-induced acidosis can exacerbate corneal hypoxia and related complications.     

pH dependence of the reaction catalyzed by yeast Mg-enolase

The pH dependence of the chemical shifts of the 31P resonances of enzyme-bound substrates 2-phosphoglycerate (PGA) and phosphoenolpyruvate (PEP) were measured to obtain further insight into the catalytic mechanism of yeast enolase. The 31P resonances of PGA and PEP bound to the enolase-Mg complex are individually observed by NMR. The Keq,internal = 1.5 favoring PEP was measured. A pH dependence of the 31P chemical shifts gives pKa values of 5.82 and 6.16 for bound PGA and PEP, respectively, indicating that both ligands bind predominantly with their phosphate groups as the dianionic species and their ionization has been altered. The phosphoryl group of PGA has been suggested as playing a role in catalysis [Nowak, T., Mildvan, A. S., and Kenyon, G. L. (1973) Biochemistry 12, 1690-1701]. The pH dependence of the kinetic parameters for Mg-enolase shows a single break in the plot of pKm, PGA vs pH at pH 6.27 with a pH independence above pH 7. This is consistent with the trianion of PGA preferably binding to the enzyme. The kcat profile gives pKA values of 5.94 and 8.35, and kcat/Km profiles give pKA values of 5.85, 6.25, and 8.39. Activation studies with Mg2+ show a pH independence for the activator constant (Ka), but a pH-dependent inhibition at higher concentrations of Mg2+. The log kcat and kcat/Ka profiles from Mg2+ activation give pKA values of about 5.9 and 8.4. These results confirm the importance of residues with pKA values of about 5.9 and 8.4 (His and Lys residues?) but do not support a function for the phosphoryl group of the substrate. The pH dependence of the Ki,Mg2+ gives pKA fits of 5. 95, 7.13, and 8.35. Data from cation inhibition suggest that the phosphate of the substrate and a His residue on enolase may bind the inhibitory Mg2+.     

The pH dependence of silicon-iron interaction in rats

A 2 x 2 x 2 factorial experiment was conducted using two dietary levels each (mg/kg of diet) of silicon, 0 and 500; iron, 35 and 187; and ascorbic acid, 0 and 900, to identify biochemical interactions occurring among these nutrients. Supplemental silicon, in conjunction with the higher dietary-iron level, prevented the plasma-iron decreasing effect observed for the higher level of iron in the absence of silicon. In the absence of ascorbic acid, silicon also increased iron concentration in the liver. Lower growth of the silicon and iron-supplemented rats is believed to be a response to a subsequent iron-imposed aberration of copper or zinc metabolism. This is supported by decreased intestinal metallothionein, increased weights (g/100 g body weight) of liver, heart, and testes, and decreased packed-cell volume and hemoglobin concentration. The lower plasma-iron level associated with the higher level of dietary iron appeared to be an expression of the iron-imposed reduction of liver copper stores. Ascorbic acid decreased plasma-iron concentration and prevented the silicon-related increase in liver iron.     

Effects of introduced aspartic and glutamic acid residues on the P'1 substrate specificity, pH dependence and stability of carboxypeptidase Y

Carboxypeptidase Y is a serine carboxypeptidase isolated from Saccharomyces cerevisiae with a preference for C-terminal hydrophobic amino acid residues. In order to alter the inherent substrate specificity of CPD-Y into one for basic amino acid residues in P'1, we have introduced Asp and/or Glu residues at a number of selected positions within the S'1 binding site. The effects of these substitutions on the substrate specificity, pH dependence and protein stability have been evaluated. The results presented here demonstrate that it is possible to obtain significant changes in the substrate preference by introducing charged amino acids into the framework provided by an enzyme with a quite different specificity. The introduced acidic amino acid residues provide a marked pH dependence of the (kcat/Km)FA-A-R-OH/(kcat/Km)FA-A-L-OH ratio. The change in stability upon introduction of Asp/Glu residues can be correlated to the difference in the mean buried surface area between the substituted and the substituting amino acid. Thus, the effects of acidic amino acid residues on the protein stability depend upon whether the introduced amino acid protrudes from the solvent accessible surface as defined by the surrounding residues in the wild type enzyme or is submerged below.     

pH dependence of ligand binding to D2 dopamine receptors

The binding of a range of ligands to D2 dopamine receptors in bovine caudate nucleus and recombinant CHO cells expressing the receptor has been determined at different pH values between 4.5 and 8.5. The maximum number of D2 dopamine receptor binding sites in each tissue was not affected by the change in pH, but the affinity of ligands for binding to the receptors was decreased as the pH was decreased. For classical dopamine antagonists, e.g. spiperone and haloperidol, the data on pH dependence of the dissociation constant for receptor binding indicated that the protonation of a single ionizing group on the receptor (pKa approximately 6) influenced the binding process. For antagonists of the substituted benzamide class, the data indicated that the protonation of two ionizing groups (pKa between 6 and 7) influenced the ligand binding process. These ionizing residues may correspond to Asp 114 for the classical antagonists and Asp 114 and Asp 80 for the substituted benzamide antagonists. Further evidence for the participation of carboxyl residues in the ligand binding process was obtained from the inhibition by N,N'-dicyclohexylcarbodiimide of the binding of [3H]spiperone and [3H]YM 09151-2 to D2 receptors in the recombinant CHO cells.     

Temporal stability of field dependence among hospitalized alcoholics.

Administered the Rod and Frame Test (RFT) to 37 male alcoholics after 10 days (RFT1) of hospitalization and again 33 days (RFT2) later. Contrary to the data reported by G. Goldstein and J. W. Chotlos (see 41:2), there were no significant differences between performance on RFT1 (mean sum of errors = 66.45– of deviation from vertical) and RFT2 (M = 64.53–), suggesting that sobriety and abstinence are not significant variables in the reduction of perceptual field dependence among alcoholics. Research involving alteration of the internal and/or external sensory environment to permit S greater awareness of proprioceptive, kinesthetic, labyrinthine, and other interoceptive cues to the vertical was emphasized as a means of achieving reductions in field dependence. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

pH dependence of the formation of an M-type intermediate in the photocycle of 13-cis-bacteriorhodopsin

Phosphatidic acid (PA) dose-dependently induced superoxide (O2-) production of electropermeabilized human neutrophils but not of intact neutrophils, indicating that PA induces the activation of NADPH oxidase by acting on an intracellular target. The O2- production by PA was not inhibited by protein kinase C (PKC) inhibitors, such as staurosporine and calphostin C, and an inhibitor of PA phosphohydrolase, propranolol. These observations suggest that the activation of the oxidase by PA is independent of the activity of PKC and may dominate the activation by diacylglycerol which is formed from PA via the action of PA phosphohydrolase. Furthermore, the production by PA, as well as that by phorbol myristate acetate, was inhibited by cyclic AMP and GDP beta S. Therefore, PA seems to act at a site downstream of PKC.     

不同pH下好氧颗粒污泥的稳定性

为考察好氧颗粒污泥（AGS）对不同pH环境下的耐受力,研究了不同pH废水（pH分别为3、4、5、10、11、12）下AGS的稳定性变化规律.在酸性环境中,AGS的理化特性随pH下降而变差（胞外聚合物（EPS）含量由102.5 mg/g下降到76.43 mg/g MLSS,比耗氧速率（SOUR）由17.94 mg O₂/（g MLSS·h））下降到16.19 mg O₂/（g MLSS·h））,pH为3和4时均出现不同程度的污泥上浮,对污染物的去除效果也呈下降趋势,但COD和TP的去除率维持在80 %以上.在碱性环境中,AGS出现了明显的上浮及解体.随着pH的增大,AGS的理化特性不断恶化（EPS由93.27 mg/g MLSS上升到178.45 mg/g MLSS,SOUR由6.81 mg O₂/（g MLSS·h））下降到4.30 mg O₂/（g MLSS·h））.同时,AGS对污染物的去除能力急剧下降,对COD、TN、TP的去除率均下降到50 %以下.研究结果表明AGS在酸性环境中有较强的耐冲击负荷能力,而碱性环境对AGS有较大的抑制作用.      

Modification of the pH dependence of animal and plant transport ATPases by sulfated polysaccharides

The effects of heparin and dextran sulfate 8,000 on two isoforms of the sarco/endoplasmic reticulum Ca(2+)-ATPase of different animal tissues and on the corn root H(+)-ATPase were examined. In the absence of sulfated polysaccharides the pH profile's of the three transport ATPases were quite different, but after the addition of heparin or dextran sulfate 8,000 the pH profiles of the three enzymes became similar, all showed maximal activity at pH 7.0. Potassium and sodium antagonized the effects of sulfated polysaccharides on the three transport ATPases, but the antagonism was considerably reduced at acidic pH values.     

On the stability of d .

In signal detection theory, the index d -sub(s) is commonly believed to be a more stable measure of discrimination than d ' when the slope of the receiver operating characteristic varies across conditions. The basis for this is examined, and a model is developed that appears to explain both the stability of d -sub(s) and the variation in the slope of the receiver operating characteristic, which may be found when the prior probability of the signal is varied. The model rests on the assumption that sequential influences may affect criterion variance. (20 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

On the stability of arrays of precipitates

The problem of the stability of an array of elastically interacting plate-shaped precipitates is considered. Algebraic conditions for stability against coarsening are developed, and the results of detailed numerical tests of the stability against displacement and of the stability against coarsening, of a particularly simple and symmetric array are reported.     

Native protein fluctuations: the conformational-motion temperature and the inverse correlation of protein flexibility with protein stability

We study the fluctuations of native proteins by exact enumeration using the HP lattice model. The model fluctuations increase with temperature. We observe a low-temperature point, below which large fluctuations are frozen out. This prediction is consistent with the observation by Tilton et al. [R. F. Tilton, Jr., J. C. Dewan, and G. A. Petsko, Biochemistry 31, 2469 (1992)], that the thermal motions of ribonuclease A increase sharply above about 200 K. We also explore protein "flexibility" as defined by Debye-Waller-like factors and solvent accessibilities of core residues to hydrogen exchange. We find that proteins having greater stability tend to have fewer large fluctuations, and hence lower flexibilities. If flexibility is necessary for enzyme catalysis, this could explain why proteins from thermophilic organisms, which are exceptionally stable, may be catalytically inactive at normal temperatures.     

Relations between opacification of lens protein and pH

Some of the acidic reagents or medicines (i.e. HCl, GSH & GSSG, Vit. C, Vit. B6) and basic reagents (Tris) were used to regulate the pH of water soluble protein and urea soluble protein solution of human lenses. At a certain pH, the lens protein solution appears opalescence, the pH of opacity solution which is measured by pH meter is around 5.6-6.3. The opalescence of water soluble and urea soluble lens protein solution of human fetal, adult and cataractous lenses were also determined by gradient acidic and basic solution. The result show that the opacity is more obvious in soluble protein solution of cataractous lenses and clear lenses than in that of fetal lenses, and the lens urea soluble protein has a marked opalescence in comparison with the water soluble protein. So we assume that cataractogenesis might be associated with the change of pH within lens.     

Imidazole binding to horse metmyoglobin: dependence upon pH and ionic strength

A series of substituted chalcone oxides (1,3-diphenyl-2-oxiranyl propanones) and structural analogs was synthesized to investigate the mechanism by which they inhibit soluble epoxide hydrolases (sEH). The inhibitor potency and inhibition kinetics were evaluated using both murine and human recombinant sEH. Inhibition kinetics were well described by the kinetic models of A. R. Main (1982, in Introduction to Biochemical Toxicology, pp. 193-223, Elsevier, New York) supporting the formation of a covalent enzyme-inhibitor intermediate with a half-life inversely proportional to inhibitor potency. Structure-activity relationships describe active-site steric constraints and support a mechanism of inhibition consistent with the electronic stabilization of the covalent enzyme-inhibitor intermediate. The electronic effects induced by altering the ketone functionality and the para-substitution of the phenyl attached to the epoxy C1 (i.e., the alpha-carbon) had the greatest influence on inhibitor potency. The direction of the observed influence was reversed for the inhibitory potency of glycidol (1-phenyl-2-oxiranylpropanol) derivatives. Recent insights into the mechanism of epoxide hydrolase activity are combined with these experimental results to support a proposed mechanism of sEH inhibition by chalcone oxides.     

The dependence of the biological activity of metal complexes on the structural stability of the initial DNA

The authors have formulated the basic principles of the creation of metallo-constructions with DNA which possess biological activity. Thus, the interaction of metal and DNA during formation of metallo-construction must not destabilize the structure of initial DNA. The studies conducted by the authors prove the hypothesis of the working mechanism of these complexes: pronounced activity against viruses which contain DNA and RNA.