NMR study of the cold, heat, and pressure unfolding of ribonuclease A

The reversible cold, heat, and pressure unfolding of RNase A and RNase A--inhibitor complex were studied by 1D and 2D 1H NMR spectroscopy. The reversible pressure denaturation experiments in the pressure range from 1 bar to 5 kbar were carried out at pH 2.0 and 10 degrees C. The cold denaturation was carried out at 3 kbar, where the protein solution can be cooled down to -25 degrees C without freezing. Including heat denaturation experiments, the experimental data obtained allowed us to construct the pressure--temperature phase diagram of RNase A. The experimental results suggest the possibility that all three denaturation processes (cold, heat, and pressure) lead to non-cooperative unfolding. The appearance of a new histidine resonance in the cold-denatured and pressure-denatured RNase A spectra, compared to the absence of this resonance in the heat-denatured state, indicates that the pressure-denatured and cold-denatured states may contain partially folded structures that are similar to that of the early folding intermediate found in the temperature-jump experiment reported by Blum et al. [Blum, A. D., et al. (1978) J. Mol. Biol. 118, 305]. A hydrogen-exchange experiment was performed to confirm the presence of partially folded structures in the pressure-denatured state. Stable hydrogen-bonded structures protecting the backbone amide hydrogens from solvent exchange were observed in the pressure-denatured state. These experimental results suggest that the pressure-denatured RNase A displays the characteristics of a the inhibitor 3'-UMP show that the RNase A-inhibitor complex is more stable than RNase without the inhibitor.

Effects of amino acid substitutions on the pressure denaturation of staphylococcal nuclease as monitored by fluorescence and nuclear magnetic resonance spectroscopy

In the present study we have used high hydrostatic pressure coupled with either time-resolved and steady-state fluorescence or NMR spectroscopy in order to investigate the effects of amino acid substitutions on the high-pressure denaturation properties of staphylococcal nuclease. This protein has been shown previously to be structurally heterogeneous in its native state. On the NMR time scale, four distinct interconverting conformational forms arise from the population of both cis and trans Xaa-Pro peptide bonds (His46-Pro47 and Lys116-Pro117) [Evans et al. (1989) Biochemistry 28, 362; Loh et al. (1991) in Techniques in Protein Chemistry II, pp 275-282, Academic Press, New York]. Mutations in the protein sequence have been shown to change the distribution among the various forms [Alexandrescu et al. (1989) Biochemistry 28, 204; Alexandrescu et al. (1990) Biochemistry 29, 4516]. Time-resolved fluorescence on a series of mutants with altered equilibria for cis/trans isomerism about the 116-117 peptide bond did not reveal any simple relationship between the position of the cis/trans equilibrium in the folded state and the heterogeneity of the fluorescence decay. However, the specific dynamic properties of each mutant, as revealed by time-resolved fluorescence, do appear to be correlated with their partial molar volume changes of denaturation. A striking finding is that mutation of either (or both) of the prolines that exhibits structural heterogeneity to glycine greatly alters the stability of the protein to pressure. These mutations also result in decreased chain mobility as assessed by time-resolved fluorescence. It appears that packing defects, which allow for peptide bond cis/trans heterogeneity in the wild-type protein, are removed by the Pro-->Gly substitutions.

High-resolution NMR study of the pressure-induced unfolding of lysozyme

The pressure-induced reversible unfolding of lysozyme was investigated by high-resolution proton magnetic resonance spectroscopy by following the proton spectra of the following residues: His-15 epsilon 1, Trp-28 epsilon 3, Leu-17 delta 2, Cys-64 alpha, and Trp-108 epsilon 3. The experiments were performed at pH 3.9 and 68.5 degrees C in the pressure range from 1 bar to 5 kbar both in the absence and presence of tri-N-acetylglucosamine (tri-NAG). From the pressure-induced changes of the equilibrium between the native and denatured forms of lysozyme, the reaction volumes (delta V) were calculated for each residue. Small but statistically significant differences in delta V were found for residues located in different regions of the protein. For example, delta V for the disulfide bonded Cys-64 alpha is smaller than the delta V's found for the other residues. In particular, the effect of tri-NAG binding to lysozyme was a change of delta V from -10.3 +/- 0.6 cm3/mol to -18.1 +/- 1.7 cm3/mol for the Trp-108 epsilon 3 residue which is located close to the active site. It is important to note that the Cys-64 alpha residue also senses the binding of the substrate analog. The ability to detect statistically significant differences for delta V of individual residues located in different regions of lysozyme represents the main result of these experiments.

Focal electrical intracerebral stimulation of a face-sensitive area causes transient prosopagnosia

Face perception is subtended by a large set of areas in the human ventral occipito-temporal cortex. However, the role of these areas and their importance for face recognition remain largely unclear. Here we report a case of transient selective impairment in face recognition (prosopagnosia) induced by focal electrical intracerebral stimulation of the right inferior occipital gyrus. This area presents with typical face-sensitivity as evidenced by functional neuroimaging right occipital face area (OFA). A face-sensitive intracerebral N170 was also recorded in this area, supporting its contribution as a source of the well-known N170 component typically recorded on the scalp. Altogether, these observations indicate that face recognition can be selectively impaired by local disruption of a single face-sensitive area of the network subtending this function, the right OFA.

Impaired mechanical strength of bone in experimental copper deficiency

Copper, through its role as cofactor for lysyl oxidase, is essential for intra- and inter-molecular cross-links in collagen. Copper deficiency, in man and in animals, is associated with bone fragility ascribed to defective cross-links. To assess bone strength in copper-deficient animals, we designed a sensitive torsion-testing apparatus according to biomechanical considerations. Femora from 7 copper-deficient rats and from their pair-fed controls were tested in torsional loading until fracture. Significant decreases in the maximal sustained torque (t = 2.93, p < 0.05), in the ultimate angular deformation (t = 2.52, p < 0.05) and in the toughness (t = 2.88, p < 0.05) were demonstrated. In a complementary study, it was shown that the ash weight and the calcium content of the femora from the copper-deficient animals did not differ from those of the controls. It was likely, therefore, that the impaired mechanical strength was related to defects in the collagen component of bone.

Structure of pressure-assisted cold denatured lysozyme and comparison with lysozyme folding intermediates

At high (> 3.5 kbar) pressures and low (< -10 degrees C) temperatures, hen egg-white lysozyme denatures readily and reversibly. Amide hydrogen exchange methods were used to investigate the structure of the pressure-assisted cold-denatured state of lysozyme. Protection factors were obtained for 52 backbone amide protons. The extent of protection of many of these protons is markedly different from that in lysozyme denatured by high temperature, high urea concentration, or chemical modification; specifically, the protection factors are higher and are strongly correlated with elements of secondary structure present in the native state. Furthermore, the pattern of protection factors is similar to that observed in lysozyme during refolding from highly denatured states, particularly during the early stages (< 3.5 ms) of refolding [Gladwin, S. T., & Evans, P. A. (1996) Folding Des. 1, 407]. Previous data on cold-denatured ribonuclease A were reevaluated and compared to known folding intermediates [Houry, W. A. & Scheraga, H. A. (1996) Biochemistry 35, 11734; Udgaonkar, J. B., & Baldwin, R. L. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 8197] to further test the supposition that the pressure-assisted cold-denatured states of proteins resemble the early folding stages.

Molten-globule conformation of Arc repressor monomers determined by high-pressure 1H NMR spectroscopy

The conformation of the pressure-dissociated monomer of Arc repressor was characterized by 1H NMR spectroscopy. The NMR spectra of the monomer under pressure (up to 5.0 kbar; 1 bar = 100 kPa) are typical of a molten globule and they are considerably different from those of the native dimer and thermally denatured monomer. The two-dimensional nuclear Overhauser effect spectra suggest that the pressure-induced molten globule retains some secondary structure. The presence of nuclear Overhauser effects in the beta-sheet region in the dissociated state suggests that the intermonomer beta-sheet (residues 8-14) in the native dimer is replaced by an intramonomer beta-sheet. Changes in one-dimensional and two-dimensional NMR spectra prior to pressure dissociation were found and suggest the existence of a "predissociated" state.

High pressure 2H nuclear magnetic resonance study of the gel phases of dipalmitoylphosphatidylcholine

The 2H-NMR lineshapes of dipalmitoylphosphatidylcholine perdeuterated in the acyl chains were studied in a 15% dispersion in water as a function of pressure from 1 bar to 5 kbar over the temperature range from 7 degrees C to 75 degrees C. Increasing pressure in the gel state had the same effect as lowering the temperature: the lineshape gradually changed from a motionally averaged to a rigid lattice type spectrum with much of the intensity in the shoulders at +/- 63 kHz. At very high pressures and low temperatures (7 degrees C, 2.5 kbar; 25 degrees C, 5 kbar) even the methyl portion of the spectrum became a rigid lattice type spectrum at +/- 21 kHz. In addition to the liquid crystalline phase, five gel phases were detected. Using different techniques to determine the phase transitions, a general pressure-temperature phase diagram was constructed.

Nuclear magnetic resonance at high pressure

Nuclear magnetic resonance relaxation measurements at high pressure provide unique information about the microscopic behavior of liquids. This article presents the principles of this technique; illustrates its usefulness by several specific examples of studies of molecular liquids, water, and supercritical dense fluids; and indicates the promising future of high-resolution nuclear magnetic resonance spectroscopy at high pressure with examples of studies of chemical exchange and homogeneous catalytic processes.

Neuromonitoring of the external branch of the superior laryngeal nerve during thyroid surgery

Changes of the voice caused by injury of the external branch of the superior laryngeal nerve (EBSLN) during thyroid surgery may have essential consequences for a patient's life. Therefore, intraoperative effort has to be made to prevent EBSLN damage. Neuromonitoring has already been described as helpful to improve the identification rate of the recurrent laryngeal nerve. We describe our surgical procedure of upper thyroid pole preparation using neuromonitoring to avoid intraoperative damage of the superior laryngeal nerve. Neuromonitoring allowed safe preparation of the upper thyroid pole without injury of the ESBLN in 108 consecutive patients.

High-resolution, high-pressure NMR studies of proteins

Advanced high-resolution NMR spectroscopy, including two-dimensional NMR techniques, combined with high pressure capability, represents a powerful new tool in the study of proteins. This contribution is organized in the following way. First, the specialized instrumentation needed for high-pressure NMR experiments is discussed, with specific emphasis on the design features and performance characteristics of a high-sensitivity, high-resolution, variable-temperature NMR probe operating at 500 MHz and at pressures of up to 500 MPa. An overview of several recent studies using 1D and 2D high-resolution, high-pressure NMR spectroscopy to investigate the pressure-induced reversible unfolding and pressure-assisted cold denaturation of lysozyme, ribonuclease A, and ubiquitin is presented. Specifically, the relationship between the residual secondary structure of pressure-assisted, cold-denatured states and the structure of early folding intermediates is discussed.

Central retinal artery occlusion in Sneddon's disease associated with antiphospholipid antibodies

A 33-year-old woman with retinal artery occlusion also had Sneddon's disease (livedo reticularis, neurologic abnormalities, and labile hypertension). While retinal artery occlusive disease obviously must be added to the symptomatic spectrum of this rare disorder, the detection of antiphospholipid antibodies in this patient has pathogenetic implications.

High-pressure NMR study of the dissociation of Arc repressor

Different denatured states of Arc repressor were characterized by one-dimensional and two-dimensional NMR and by fluorescence spectroscopy. Increasing pressure promoted sequential changes in the structure of Arc repressor: from the native dimer through a predissociated state to a denatured molten globule monomer. A compact state (molten globule) of Arc repressor was obtained in the dissociation of Arc repressor by pressure whereas high temperature and urea induced dissociation and unfolding to less structured conformations. The NMR spectra of the monomer under pressure (up to 5.0 kbar) are typical of a molten globule, and they are considerably different from those of the native dimer and the thermally or chemically denatured monomer. The substantial line broadening and overlap of many resonances in the NMR spectra at high pressures indicate that there is interconversion between a number of different conformations of the molten globule at an intermediate exchange rate. The two-dimensional NOE spectra show that the pressure-denatured monomer retains substantial secondary structure. The presence of NOEs in the beta-sheet region in the dissociated state suggests that the intersubunit beta-sheet (residues 6-14) in the native-dimer is replaced by an intramonomer beta-sheet. Changes in 2D NMR spectra prior to dissociation indicate the existence of a predissociated state that may represent an intermediate in the folding and subunit association pathway of Arc repressor.

Pressure effects on dipalmitoylphosphatidylcholine bilayers measured by 2H nuclear magnetic resonance

The effects of pressure, up to 5 kbar, on multilamellar vesicles of 1,2-dipalmitoyl-sn-phosphatidylcholine perdeuterated in the acyl chains (DPPC-d62) were examined by using high-pressure NMR techniques. A deuterium probe was built, and the quadrupole splitting was measured against pressure at various temperatures. The experiments were performed on pure lipid bilayers in the liquid-crystalline state and on bilayers in the liquid-crystalline state containing the local anesthetic tetracaine. The results show that the order parameter of all segments of the acyl chains increases with pressure in the liquid-crystalline state. The more highly ordered regions of the chains are affected slightly more than the regions near the methyl ends. The addition of tetracaine increases the disorder of the chains, and pressure reverses the effect of anesthetic on the lipid as seen by the reversal of the changes in line shape and the measured order parameter.

High-pressure 31P NMR study of dipalmitoylphosphatidylcholine bilayers

High-pressure 31P NMR was used for the first time to investigate the effects of pressure on the structure and dynamics of the phosphocholine headgroup in pure 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multilamellar aqueous dispersions and in DPPC bilayers containing the positively charged form of the local anesthetic tetracaine (TTC). The 31P chemical shift anisotropies, delta sigma, and the 31P spin-lattice relaxation times, T1, were measured as a function of pressure from 1 bar to 5 kbar at 50 degrees C for both pure DPPC and DPPC/TTC bilayers. This pressure range permitted us to explore the rich phase behavior of DPPC from the liquid-crystalline (LC) phase through various gel phases such as gel I (P beta'), gel II (L beta'), gel III, gel IV, gel X, and the interdigitated, Gi, gel phase. For pure DPPC bilayers, pressure had an ordering effect on the phospholipid headgroup within the same phase and induced an interdigitated Gi gel phase which was formed between the gel I (P beta') and gel II (L beta') phases. The 31P spin-lattice relaxation time measurements showed that the main phase transition (LC to gel I) was accompanied by the transition between the fast and slow correlation time regimes. Axially symmetric 31P NMR lineshapes were observed at pressures up to approximately 3 kbar but changed to characteristic axially asymmetric rigid lattice lineshapes at higher pressures (3.1-5.1 kbar).(ABSTRACT TRUNCATED AT 250 WORDS)

High-resolution 13C NMR study of pressure effects on the main phase transition in L-alpha-dipalmitoyl phosphatidylcholine vesicles

The effects of pressure on the liquid-crystalline to gel transition in vesicles of L-alpha-dipalmitoyl phosphatidylcholine were investigated by high-resolution proton-decoupled natural-abundance 13C NMR spectroscopy. The linewidths of several 13C resonances, including the choline methyl groups, carbonyl carbons, and choline methylene groups and the palmitoyl methyl groups are reported as a function of pressure at 52.7 degrees C. These preliminary NMR experiments clearly demonstrate that high-pressure, high-resolution proton-decoupled natural-abundance 13C NMR spectra are a promising tool to study the phase-transition behavior and the dynamics of model membrane systems.

The pattern-evoked electroretinogram (PERG): age-related alterations and changes in glaucoma

Pattern-onset electroretinograms (PERGs) were studied in 147 normal subjects of different ages (14-79 years) and in 110 eyes of 65 patients with glaucoma or ocular hypertension. The responses showed an increase (P less than 0.001) in peak latency with increasing age and a decrease (P less than 0.001) in amplitude which approximately parallels the loss of ganglion cells estimated by other authors. Many glaucoma eyes showed a loss of the normally present spatial tuning. In the age group above 50 years 50% of the onset responses were significantly diminished and the peak latencies were not significantly different. A negative correlation (P less than 0.001) was found between the size of the PERG and the cup/disc ratio and a positive correlation (P less than 0.001) with the area of the neuroretinal rim of the optic disc. The PERG decreased (P less than 0.01) with increasing visual field losses.

Structure of the pressure-assisted cold denatured state of ubiquitin

The pressure-assisted cold denatured state of ubiquitin in aqueous solution was investigated by high resolution NMR. Hydrogen exchange kinetics were measured for backbone amide protons in the cold denatured protein to determine its structure. In contrast to cold denatured ribonuclease A and lysozyme, cold denatured ubiquitin shows little persistent secondary structure. The behavior of ubiquitin supports the idea of a relationship between the residual structure of pressure-assisted cold-denatured states and the structure of early folding intermediates provided they exist.

High-Resolution NMR Probe for Experiments at High Pressures

A 300 MHz high-resolution, high-pressure NMR probe which operates in the pressure range of 1 bar to 9 kbar at temperatures of -30 to 100&deg;C is described. Specialized novel design features of the probe are discussed and test spectra showing resolution better than 1 Hz (<3.0 x 10(-9)) for 8 mm samples are presented. Potential biochemical applications of this probe are illustrated by experiments dealing with the pressure-induced unfolding of hen egg white lysozyme.

Hydrogen-exchange kinetics in the cold denatured state of ribonuclease A

Ribonuclease A (RNase A) exhibits a well-defined cold denaturation transition when examined at high pressure (3 kbar) and low temperatures (below -10 degrees C). Our main interest in this study was to investigate the pressure-assisted cold denatured state of RNase A by hydrogen exchange techniques. The protection factors obtained from the kinetic data are similar to those obtained previously for RNase A denatured by exposure to high pressure near its temperature of maximum stability, but differ markedly from those in thermally denatured RNase A. A qualitative analysis of the hydrogen-exchange rates suggests that cold denatured RNase A behaves markedly differently from a random coil, probably due to patches of residual secondary structure.