Rodlike complexes of a polyelectrolyte (hyaluronan) and a protein (lysozyme) observed by SANS

We study by small-angle neutron scattering (SANS) the structure of hyaluronan -lysozyme complexes. Hyaluronan (HA) is a polysaccharide of 9 nm intrinsic persistence length that bears one negative charge per disaccharide monomer (M(mol) = 401.3 g/mol); two molecular weights, M(w) = 6000 and 500,000 Da were used. The pH was adjusted at 4.7 and 7.4 so that lysozyme has a global charge of +10 and +8, respectively. The lysozyme concentration was varied from 3 to 40 g/L at constant HA concentration (10 g/L). At low protein concentration, samples are monophasic, and SANS experiments reveal only fluctuations of concentration, although, at high protein concentration, clusters are observed by SANS in the dense phase of the diphasic samples. In between, close to the onset of the phase separation, a distinct original scattering is observed. It is characteristic of a rod-like shape, which could characterize "single" complexes involving one or a few polymer chains. For the large molecular weight (500,000), the rodlike rigid domains extend to much larger length scale than the persistence length of the HA chain alone in solution and the range of the SANS investigation. They can be described as a necklace of proteins attached along a backbone of diameter of one or a few HA chains. For the short chains (M(w) ≈ 6000), the rod length of the complexes is close to the chain contour length (～ 15 nm).     

Interaction of surfactant protein A with peroxiredoxin 6 regulates phospholipase A2 activity

Peroxiredoxin 6 (Prdx6) is a "moonlighting" protein with both GSH peroxidase and phospholipase A(2) (PLA(2)) activities. This protein is responsible for degradation of internalized dipalmitoylphosphatidylcholine, the major phospholipid component of lung surfactant. The PLA(2) activity is inhibited by surfactant protein A (SP-A). We postulate that SP-A regulates the PLA(2) activity of Prdx6 through direct protein-protein interaction. Recombinant human Prdx6 and SP-A isolated from human alveolar proteinosis fluid were studied. Measurement of kinetic constants at pH 4.0 (maximal PLA(2) activity) showed K(m)0.35 mm and V(max) 138 nmol/min/mg of protein. SP-A inhibited PLA(2) activity non-competitively with K(i) 10 mug/ml and was Ca(2+) -independent. Activity at pH 7.4 was approximately 50% less, and inhibition by SP-A was partially dependent on Ca(2+). Interaction of SP-A and Prdx6 at pH 7.4 was shown by Prdx6-mediated inhibition of SP-A binding to agarose beads, a pull-down assay using His-tagged Prdx6 and Ni(2) -chelating beads, co-immunoprecipitation from lung epithelial cells and from a binary mixture of the two proteins, binding after treatment with a trifunctional cross-linker, and size-exclusion chromatography. Analysis by static light scattering and surface plasmon resonance showed calcium-independent SP-A binding to Prdx6 at pH 4.0 and partial Ca(2+) dependence of binding at pH 7.4. These results indicate a direct interaction between SP-A and Prdx6, which provides a mechanism for regulation of the PLA(2) activity of Prdx6 by SP-A.     

Spectroscopic and kinetic studies on the oxygen-centered radical formed during the four-electron reduction process of dioxygen by Rhus vernicifera laccase.

The oxygen-centered radical bound to the trinuclear copper center was detected as an intermediate during the reoxidation process of the reduced Rhus vernicifera laccase with dioxygen and characterized by using absorption, stopped-flow, and electron paramagnetic resonance (EPR) spectroscopies and by super conducting quantum interface devices measurement. The intermediate bands appeared at 370 nm (epsilon approximately 1000), 420 nm (sh), and 670 nm (weak) within 15 ms, and were observable for approximately 2 min at pH 7.4 but for less than 5 s at pH 4.2. The first-order rate constant for the decay of the intermediate has been determined by stopped-flow spectroscopy, showing the isotope effect, k(H)/k(D) of 1.4 in D(2)O. The intermediate was found to decay mainly from the protonated form by analyzing pH dependences. The enthalpy and entropy of activation suggested that a considerable structure change takes place around the active site during the decay of the intermediate. The EPR spectra at cryogenic temperatures (<27 K) showed two broad signals with g approximately 1.8 and 1.6 depending on pH. We propose an oxygen-centered radical in magnetic interaction with the oxidized type III copper ions as the structure of the three-electron reduced form of dioxygen.     

Assessing the flexibility of intermediate filaments by atomic force microscopy

Eukaryotic cells contain three cytoskeletal filament systems that exhibit very distinct assembly properties, supramolecular architectures, dynamic behaviour and mechanical properties. Microtubules and microfilaments are relatively stiff polar structures whose assembly is modulated by the state of hydrolysis of the bound nucleotide. In contrast, intermediate filaments (IFs) are more flexible apolar structures assembled from a approximately 45 nm long coiled-coil dimer as the elementary building block. The differences in flexibility that exist among the three filament systems have been described qualitatively by comparing electron micrographs of negatively stained dehydrated filaments and by directly measuring the persistence length of F-actin filaments (approximately 3-10 microm) and microtubules (approximately 1-8 mm) by various physical methods. However, quantitative data on the persistence length of IFs are still missing. Toward this goal, we have carried out atomic force microscopy (AFM) in physiological buffer to characterise the morphology of individual vimentin IFs adsorbed to different solid supports. In addition, we compared these images with those obtained by transmission electron microscopy (TEM) of negatively stained dehydrated filaments. For each support, we could accurately measure the apparent persistence length of the filaments, yielding values ranging between 0.3 microm and 1 microm. Making simple assumptions concerning the adsorption mechanism, we could estimate the persistence length of an IF in a dilute solution to be approximately 1 microm, indicating that the lower measured values reflect constraints induced by the adsorption process of the filaments on the corresponding support. Based on our knowledge of the structural organisation and mechanical properties of IFs, we reason that the lower persistence length of IFs compared to that of F-actin filaments is caused by the presence of flexible linker regions within the coiled-coil dimer and by postulating the occurrence of axial slipping between dimers within IFs.     

Flexibility of collagen determined from dilute solution viscoelastic measurements

The frequency dependences of the storage and loss shear moduli, G′ and G″, of pronase-treated collagen dissolved in acetate buffer at pH 4.0 were measured at 17.0°C by use of the Birnboim-Schrag multiple lumped resonator apparatus. Some of the solutions contained 70% glycerol. The infinite-dilution moduli were determined and compared with theoretical models for a rigid cylinder and a semiflexible rod. Only the latter could fit the data. A rotational time of 144 μs and a slowest flexural relaxation time of 21 μs, both reduced to water at 20°C, were determined from the fit. The intrinsic viscosity and rotational relaxation time correspond to a semiflexible rod with persistence length of about 170 nm and a Young's modulus of 4 × 10¹⁰ dyn/cm².     

Characterization of human ASIC2a homomeric channels stably expressed in murine Ltk- cells

ASIC2a (BNaC1 or MDEG) is distributed throughout the nervous system and potentially involved in mechanosensation, hearing, vision, and taste functions. However, pharmacological properties of ASIC2 homomers including the mechanism of inhibition by amiloride remain unclear. In this study, we describe the properties of hASIC2a stably expressed in Ltk(-) cells, the first reported stable cell line expressing any ASICs subunit, by standard whole cell voltage clamp method. In response to pH 4.0, at -80 mV, hASIC2a cells exhibited rapidly activating fast transient inward current ( approximately 100 pA/pF) that was followed by a sustained current ( approximately 13 pA/pF). In contrast, untransfected Ltk(-) cells showed only a very small rapidly activating non-inactivating inward current ( approximately 4 pA/pF). The magnitude of hASIC2a transient current was pH dependent with pH(50) values for activation and inactivation of approximately 4.2 and approximately 5.5, respectively. Ion substitution experiments revealed the following rank order of permeability: Na(+)>K(+)>Ca(2+) for the transient current. Amiloride reversibly inhibited the pH 4.0 evoked transient current with IC(50) values of approximately 20 microM at both -30 and -80 mV holding potentials, indicating that the interactions are voltage independent when nearly all amiloride is protonated. Amiloride (100 microM) did not inhibit ASIC2a transient current when pre-applied in pH 7.4 and pH 4.0 currents obtained in absence of amiloride, but it did inhibit currents when co-applied at pH 4.0 suggesting open channel blockade. In summary, ASIC2a stable cell line serves as a useful model system to study the pharmacological properties of ASIC2a currents, potentially contributing to pH-evoked responses in cells of the dorsal root ganglion and the central nervous system.     

Apolipoprotein E4 forms a molten globule. A potential basis for its association with disease

The amino-terminal domain of apolipoprotein (apo) E4 is less susceptible to chemical and thermal denaturation than the apoE3 and apoE2 domains. We compared the urea denaturation curves of the 22-kDa amino-terminal domains of the apoE isoforms at pH 7.4 and 4.0. At pH 7.4, apoE3 and apoE4 reflected an apparent two-state denaturation. The midpoints of denaturation were 5.2 and 4.3 m urea, respectively. At pH 4.0, a pH value known to stabilize folding intermediates, apoE4 and apoE3 displayed the same order of denaturation but with distinct plateaus, suggesting the presence of a stable folding intermediate. In contrast, apoE2 proved the most stable and lacked the distinct plateau observed with the other two isoforms and could be fitted to a two-state unfolding model. Analysis of the curves with a three-state unfolding model (native, intermediate, and unfolded) showed that the apoE4 folding intermediate reached its maximal concentration ( approximately 90% of the mixture) at 3.75 m, whereas the apoE3 intermediate was maximal at 4.75 m ( approximately 80%). These results are consistent with apoE4 being more susceptible to unfolding than apoE3 and apoE2 and more prone to form a stable folding intermediate. The structure of the apoE4 folding intermediate at pH 4.0 in 3.75 m urea was characterized using pepsin proteolysis, Fourier transform infrared spectroscopy, and dynamic light scattering. From these studies, we conclude that the apoE4 folding intermediate is a single molecule with the characteristics of a molten globule. We propose a model of the apoE4 molten globule in which the four-helix bundle of the amino-terminal domain is partially opened, generating a slightly elongated structure and exposing the hydrophobic core. Since molten globules have been implicated in both normal and abnormal physiological function, the differential abilities of the apoE isoforms to form a molten globule may contribute to the isoform-specific effects of apoE in disease.     

Adsorption of DNA onto positively charged amidine colloidal spheres and the resultant bridging interaction

The complexation behaviour of duplex linear DNA (negatively charged) with amidine functionalised sub-micron latex spheres (positively charged) was studied using dynamic light scattering (DLS) and a PALS interferrometric zeta potential sizer. Four types of DNA-sphere complex were investigated as a function of component concentration by combining amidine functionalised polystyrene microspheres with radii of 10.5 nm and 60 nm, and herring DNA of lengths of 35 nm and 85 nm. At low DNA concentrations (c(DNA)), the undercharged complexes showed a small increase in measured hydrodynamic radius (R(h)) and a decrease in zeta potential with increasing c(DNA). Within a critical DNA concentration range R(h) was seen to peak sharply, and the zeta potentials were approximately 0 mV, corresponding to the formation of unstable neutral complexes. Immediately above this concentration region the measured R(h) values became comparable with those at low c(DNA), and the zeta potential became negative, indicating the formation of stable overcharged complexes. The small and large spheres formed multi-sphere and single sphere overcharged aggregates respectively, which is thought to be determined by the relative magnitude of the chain persistence length (approximately 50 nm) and the sphere radius, switching on or off the DNA bridging interaction.     

Fluorescence and chemical modification of tryptophan as probes of structure of GTP:AMP phosphotransferase from beef heart mitochondria

Selective modification of the two Trp residues of GTP:AMP phosphotransferase from beef heart mitochondria (Mr 26 000; MgGTP + AMP in equilibrium MgGDP + ADP) has been attained by treatment of the enzyme with N-bromosuccinimide at pH 4.0. Almost complete loss of activity is observed when one Trp is oxidized. Fluorescence emission spectra (lambda exc 295 nm) were recorded over the pH range 1.9-12.2. Quenching constants, K, with acrylamide were 4.9, 3.4, 3.1, 2.4, 9.2 and 9.4 M-1 at respective pH values of 11.1, 7.5, 5.5, 4.0, 1.9 and 7.5 with 6 M guanidine/HCl. Over the pH range 8.0-5.5 the fluorescence peak has a constant height with maximum at 333-334 nm, which can be segregated by acrylamide quenching into a peak with maximum at 338 nm and another with maximum at 330 nm. Dropping the pH from 5.5 to 4.0 results in the fluorescence at 338 nm decreasing to 335 nm (indicative of less exposure of the Trp) while that at 330 nm remains constant. Thus the limitation of reactivity to N-bromosuccinimide to pH 4.0 or lower cannot be accounted for by increased exposure of the Trp residues but rather must be explained by a change in the microenvironment of each Trp. As shown by K values above, at pH 2.0 Trp residues are exposed to the solvent, as in the case of treatment with 6 M guanidine hydrochloride. In raising the pH from 8.0 to 12.0 a number of changes occur: (a) the lambda max of emission shifts from 333-334 nm to 343 nm; (b) residue(s) become(s) more available to acrylamide quenching; (c) fluorescence decreases and enzymatic activity increases, both with a midpoint at about 10.6; (d) absorption difference spectra show a maximum at 295 nm typical of Tyr ionization. These data are consistent with conformational change as the pH becomes more alkaline making the Trp residue(s) more exposed to the solvent and/or to non-radiative energy transfer to tyrosinate.     

Characterization of beta-connectin (titin 2) from striated muscle by dynamic light scattering.

Connectin (titin) is a large filamentous protein (single peptide) with a molecular mass of approximately 3 MDa, contour length approximately 900 nm, and diameter approximately 4 nm, and resides in striated muscle. Connectin links the thick filaments to the Z-lines in a sarcomere and produces a passive elastic force when muscle fiber is stretched. The aim of this study is to elucidate some aspects of physical properties of isolated beta-connectin (titin 2), a proteolytic fragment of connectin, by means of dynamic light-scattering (DLS) spectroscopy. The analysis of DLS spectra for beta-connectin gave the translational diffusion coefficient of 3.60 x 10(-8) cm2/s at 10 degrees C (or the hydrodynamic radius of 44.1 nm), molecular mass little smaller than 3.0 MDa (for a literature value of sedimentation coefficient), the root-mean-square end-to-end distance of 163 nm (or the radius of gyration of 66.6 nm), and the Kuhn segment number of 30 and segment length of 30 nm (or the persistence length of 15 nm). These results permitted to estimate the flexural rigidity of 6.0 x 10(-20) dyn x cm2 for filament bending, and the elastic constant of 7 dyn/cm for extension of one persistence length. Based on a simple model, implications of the present results in muscle physiology are discussed.     

Impaired receptor binding and activation associated with a human prostacyclin receptor polymorphism

The human prostacyclin receptor (hIP) is a seven transmembrane-spanning G-protein-coupled receptor that plays an important role in vascular homeostasis. Recent genetic analyses (SNP database, NCBI) have revealed the first two polymorphisms within the coding sequence, V25M and R212H. Here we present structure-function characterizations of these polymorphisms at physiological pH (7.4) and at an acidic pH (6.8) that would be encountered during stress such as renal, respiratory, or heart failure. Through a series of competition binding and G-protein activation assays (measured by cAMP production), we determined that the V25M polymorph exhibited agonist binding and G-protein activation similar to wild-type receptor at normal pH (7.4). However, the R212H variant demonstrated a significant decrease in binding affinity at lower pH (R212H at pH 7.4, K(i) = 2.2 +/- 1.2 nm; pH 6.8 K(i) = 45.6 +/- 12.0 nm). The R212H polymorph also exhibited abnormal activation at both pH 7.4 and pH 6.8 (pH 7.4, R212H EC(50) = 2.8 +/- 0.5 nm versus wild-type hIP EC(50) = 0.5 +/- 0.1 nm; pH 6.8, R212H EC(50) = 3.2 +/- 1.6 nm versus wild-type hIP EC(50) = 0.5 +/- 0.2 nm). Polymorphisms of the human prostacyclin receptor potentially may be important predictors of disease progress during biological stressors such as acidosis in which urgent correction of bodily pH may be required to restore normal hemostasis and vasodilation. This study provides the mechanistic basis for further research into genetic risk factors and pharmacogenetics of cardiovascular disease associated with hIP.     

The spin trapping of superoxide with M4PO (3,3,5,5-tetramethylpyrroline-N-oxide)

The spin trap 3,3,5,5-tetramethylpyrroline-N-oxide (M4PO) reacts with superoxide to produce a short-lived spin adduct (M4PO/.OOH, AN = 14.0 G, AH = 6.5 G) that decays by a first-order process (t1/2 approximately 35 s at pH 7.4). This short lifetime may limit its usefulness in studies of superoxide.     

Hierarchical assembly and the onset of banding in fibrous long spacing collagen revealed by atomic force microscopy.

The mechanism of formation of fibrillar collagen with a banding periodicity much greater than the 67 nm of native collagen, i.e. the so-called fibrous long spacing (FLS) collagen, has been speculated upon, but has not been previously studied experimentally from a detailed structural perspective. In vitro, such fibrils, with banding periodicity of approximately 270 nm, may be produced by dialysis of an acidic solution of type I collagen and alpha(1)-acid glycoprotein against deionized water. FLS collagen assembly was investigated by visualization of assembly intermediates that were formed during the course of dialysis using atomic force microscopy. Below pH 4, thin, curly nonbanded fibrils were formed. When the dialysis solution reached approximately pH 4, thin, filamentous structures that showed protrusions spaced at approximately 270 nm were seen. As the pH increased, these protofibrils appeared to associate loosely into larger fibrils with clear approximately 270 nm banding which increased in diameter and compactness, such that by approximately pH 4.6, mature FLS collagen fibrils begin to be observed with increasing frequency. These results suggest that there are aspects of a stepwise process in the formation of FLS collagen, and that the banding pattern arises quite early and very specifically in this process. It is proposed that typical 4D-period staggered microfibril subunits assemble laterally with minimal stagger between adjacent fibrils. alpha(1)-Acid glycoprotein presumably promotes this otherwise abnormal lateral assembly over native-type self-assembly. Cocoon-like fibrils, which are hundreds of nanometers in diameter and 10-20 microm in length, were found to coexist with mature FLS fibrils.     

DNA length and concentration dependencies of anisotropic phase transitions of DNA solutions

Critical concentrations for the isotropic to cholesteric phase transitions of double-stranded DNA fragments in simple buffered saline (0.1 M NaCl) solutions were determined as a function of DNA contour length ranging from approximately 50 nm to 2700 nm, by solid-state 31P NMR spectroscopy and polarized light microscopy. As expected for semirigid chains, the critical concentrations decrease sharply with increasing DNA length near the persistence length in the range from 50 to 110 nm, and approach a plateau when the contour length exceeds 190 nm. The biphasic region is substantially wider than observed for xanthan, another semirigid polyelectrolyte approximately twice as stiff as DNA, primarily because of low critical concentrations for first appearance of the anisotropic phase, C(i)*, in DNA samples > or =110 nm (320 base pairs) long. The limiting C(i)* for DNA > or =490 nm long is exceptionally low (only 13 mg/ml) and is substantially lower than the C(i)* of approximately 40 mg/ml reported for the stiffer xanthan polyelectrolyte. The much higher values of the critical concentrations, C(a)*, for the disappearance of the isotropic DNA phase (> or =67 mg/ml) are modestly higher than those observed for xanthan and are predicted reasonably well by a theory that has been applied to other semirigid polymers, if a DNA persistence length in the consensus range of 50-100 nm is assumed. By contrast, the broad biphasic region and low C(i)* values of DNA fragments > or =190 nm long could only be reconciled with theory by assuming persistence lengths of 200-400 nm. The latter discrepancies are presumed to reflect some combination of deficiencies in current theory as applied to chiral, strong polyelectrolytes such as DNA, and sequence-dependent variations in DNA properties such as flexibility, curvature, or interaction potential. The propensity of DNA to spontaneously self-order at low concentrations well in the physiological range may have biological significance.     

Influence of Acidity on Growth and Biochemistry of Pennisetum clandestinum

Hydroponics were used to study the impact of acidity on growth, nutritive properties and metabolic changes in kikuyu grass (Pennisetum clandestinum Hochst). Four treatments (pH 6.0, 5.0, 4.0, and 3.0) were compared for effects on biomass, leaf and root length, crude protein, amino acid content and key enzymes of sugar metabolism. Reduction in biomass, root and leaf length, amino acid contents, glucose-6-phosphate dehydrogenase (G6PDH) and pyruvate kinase (PK) content was observed only at pH 3.0, in association with increased leaf proline content. Kikuyu grass is able to grow normally under mild acidity (down to at least pH 4.0).     

The binding of copper ions to glycine-rich proteins (GRPs) from Cicer arietinum

Cicer arietinum GRP1 and GRP2 are rich in glycine interposed with histidine and tyrosine. In order to study whether or not these proteins bind Cu(2+), circular dichroism (CD) and nuclear magnetic resonance (NMR) were measured for three synthetic peptides corresponding to sections of the protein's sequences including 1, N(1)Y(2)G(3)H(4)G(5)G(6)G(7)N(8)Y(9)G(10)N(11), where all peptides were chemically blocked with an acetyl group at the N-terminus and an -NH(2) group at the C-terminus. The visible CD spectra for 1 showed a positive peak near 590 nm not at pH 6.0 but pH 7.4 in the presence of copper ions. The Cu(2+) binding induced a drastic change in the far-UV CD spectra, showing the occurrence of large conformation changes. In the 2D TOCSY NMR spectra at pH 7.4, the addition of small amounts of CuSO(4) caused a significant broadening of proton resonances of not only His4 but also Gly5, Asn8 and Asn11. CD titration experiment suggested that NYGHGGGNYGN including one repeat unit comprises the fundamental Cu(2+) binding unit.     

Dissociation of double-stranded DNA by small metal nanoparticles

The dissociation of double-stranded DNA (dsDNA) by 5 nm Au nanoparticles was observed through a series of DNA melting transition measurements. Experimental evidence implicates a strong non-specific interaction between the dsDNA and small Au nanoparticles as the cause. Subsequently the relative affinities of mononucleotides and polynucleotides for the 5 nm Au nanoparticle were determined by measuring the stability of mono- or polynucleotide-stabilized 5 nm Au nanoparticles in salt solutions of different concentrations as a function of time and temperature. The experimental data shows that for the mononucleotides, the affinity decreases in the following order: dA>dG>dC>dT. The order from the polynucleotides was however different, with the affinity decreasing as poly A approximately poly C approximately poly T>poly G. The lack of correlation between the two ranked orders indicates that the trend obtained from one cannot be used to infer the trend in the other, or vice versa. The evidence provided suggests that the persistence length of the oligonucleotides plays an important role, and must be considered alongside with the individual nucleotide binding strength to determine the overall interaction between the oligonucleotides and Au nanoparticles.     

Fibrillar beta-lactoglobulin gels: Part 1. Fibril formation and structure

As a prelude to experimental and theoretical work on the mechanical properties of fibrillar beta-lactoglobulin gels, this paper reports the structural characterization of beta-lactoglobulin fibrils by electron and atomic force microscopy (AFM), infrared and Raman spectroscopy, and powder X-ray diffraction. Aggregates formed by incubation of beta-lactoglobulin in various alcohol-water mixtures at pH 2, and in water-trifluoroethanol (TFE) at pH 7, were found to be wormlike (approximately 7 nm in width and <500 nm in length), with a "string-of-beads" appearance. Longer (approximately 7 nm in width, and >1 microm in length), smoother, and seemingly stiffer fibrils formed on heating aqueous beta-lactoglobulin solutions at pH 2 and low ionic strength, although there was little evidence for the higher-order structures common in most amyloid-forming systems. Time-lapse AFM also revealed differences in the formation of these two fibril types: thermally induced aggregation occurring more cooperatively, in keeping with a nucleation and growth process. Only short stiff-rods (<20 nm in length) formed on heating beta-lactoglobulin at pH 7, and only complex three-dimensional "amorphous"aggregates in alcohols other than TFE at this pH. Studies of all of the pH 2 fibrils from beta-lactoglobulin, by Raman and infrared spectroscopy confirmed beta-sheet as mediating the aggregation process. Interestingly, however, some evidence for de novo helix formation for the solvent-induced systems was obtained, although it remains to be seen whether this is actually incorporated into the fibril-structure. In contrast to other amyloid systems, X-ray powder diffraction provided no evidence for extensive repeating "crystalline" structures for any of the pH 2 beta-lactoglobulin fibrils. In relation to amyloid, the lactoglobulin fibrils bear more resemblance to protofilaments than to higher-order fibril structures, these latter appearing more convincingly for thermally induced insulin fibrils (pH 2) also included in the AFM study.     

Effect of low pH on single skeletal muscle myosin mechanics and kinetics

Acidosis (low pH) is the oldest putative agent of muscular fatigue, but the molecular mechanism underlying its depressive effect on muscular performance remains unresolved. Therefore, the effect of low pH on the molecular mechanics and kinetics of chicken skeletal muscle myosin was studied using in vitro motility (IVM) and single molecule laser trap assays. Decreasing pH from 7.4 to 6.4 at saturating ATP slowed actin filament velocity (V(actin)) in the IVM by 36%. Single molecule experiments, at 1 microM ATP, decreased the average unitary step size of myosin (d) from 10 +/- 2 nm (pH 7.4) to 2 +/- 1 nm (pH 6.4). Individual binding events at low pH were consistent with the presence of a population of both productive (average d = 10 nm) and nonproductive (average d = 0 nm) actomyosin interactions. Raising the ATP concentration from 1 microM to 1 mM at pH 6.4 restored d (9 +/- 3 nm), suggesting that the lifetime of the nonproductive interactions is solely dependent on the [ATP]. V(actin), however, was not restored by raising the [ATP] (1-10 mM) in the IVM assay, suggesting that low pH also prolongs actin strong binding (t(on)). Measurement of t(on) as a function of the [ATP] in the single molecule assay suggested that acidosis prolongs t(on) by slowing the rate of ADP release. Thus, in a detachment limited model of motility (i.e., V(actin) approximately d/t(on)), a slowed rate of ADP release and the presence of nonproductive actomyosin interactions could account for the acidosis-induced decrease in V(actin), suggesting a molecular explanation for this component of muscular fatigue.     

How to develop globular proteins into adhesives

To make globular proteins suitable for application in adhesives, the specific bonds and interactions which shape their structure have to broken. Only then, a layer of relatively large, flexible and interwoven polymer chains, which are firmly attached to the solid surface by adsorption, can be created. Such a network layer is essential to save the adhesive bond under an applied force, because it can distribute the concentration of stresses generated at the interface into the bulk. Unfolding and swelling of a protein can be achieved by changing the solvent quality. For the globular whey protein beta-lactoglobulin, the optimal conditions for unfolding and swelling is found with 98% formic acid as a solvent. In formic acid, beta-lactoglobulin looses its amphoteric character (it is protonated, probably for approximately 20%). In addition, formic acid is less polar than water and thus a better solvent for the apolar parts of the protein. The swelling and unfolding behaviour of beta-lactoglobulin is studied by viscosity and CD-spectroscopy measurements. For the interpretation of the results we apply the Kuhn formalism that the conformation of a protein can be described in terms of a statistical chain which consists of segments of an average persistence length P. The statistical segment length P, which varies with the experimental conditions, is directly related to the adsorption energy required for a strong adhesion between coil and surface. It determines the depletion energy kT P(-2) m(-2) which must be overcome by specific attraction between side groups of the protein chain and the surface. For beta-lactoglobulin in 98% formic acid, we find a P value of approximately 2.2 nm, pointing at a relatively flexible chain. The minimum net adsorption energy kT P(-2) is then approximately 1 mJ m(-2), a relatively small value to be exceeded. Preliminary results of destructive adhesion tests on beech wood lap-shear joints reveal promising tensile strengths of approximately 2.9+/-1.1 N mm(-2), indeed.