Kerr effect of charged dipolar macromolecules without condensed counterions in conducting solution.

The theoretical treatment of the Kerr constant of rigid, dipolar, conducting ellipsoidal macromolecules of O'Konski and Krause (1970. J. Phys. Chem. 74:3243) has been extended to very low ionic strength solutions for charged macromolecules. The O'Konski and Krause theoretical treatment postulated a surface conductivity directly on the surface of each macromolecule. For charged macromolecules, this surface conductivity was generally assumed to be caused by movement of condensed counterions on the macromolecules. In the present work, it has been assumed that, at very low ionic strength, the average counterion is at the Debye characteristic distance from the surface of each charged macromolecule and contributes to surface conductivity at that distance, with no additional surface conductivity on the true surface of the macromolecule. Essentially, these considerations change the calculated interaction energy of the macromolecule with an externally applied electric field via a change in both the internal field components and in the reaction field of the macromolecular dipole. The new interaction energy is used to calculate the orientation distribution function of the macromolecules in solution and this distribution function can, in principle, be used to calculate the steady state electric linear or circular dichroism, electric light scattering, anisotropy of conductivity, etc., using the appropriate theoretical treatment for each of these quantities.     

Macromolecules mediate prostacyclin release from human umbilical artery

Previous reports regarding the modulation of prostaglandin release from tissues by serum components did not identify these components. We have found that inhibition of prostacyclin release from human umbilical artery by human serum is attributable to serum macromolecules. We demonstrate that such inhibitory activity depends on macromolecular size and may result from macromolecule/cell surface interactions.     

Hyaluronan conformations on surfaces: effect of surface charge and hydrophobicity

Extended, relaxed, condensed, and interacting forms of the polysaccharide hyaluronan have been observed by atomic force microscopy (AFM). The types of images obtained depend on the properties of the surfaces used. We have investigated several different surface conditions for HA imaging, including unmodified mica, mica chemically modified with two different kinds of amino-terminated silanes (3-aminopropyltriethoxysilane and N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride), and highly oriented pyrolytic graphite. We found the degree of HA molecular extension or condensation to be variable, and the number of bound chains per unit area was low, for all of the mica-based surfaces. HA was more easily imaged on graphite, a hydrophobic surface. Chains were frequently observed in high degrees of extension, maintained by favorable interaction with the surface after molecular combing. This observation suggests that the HA macromolecule interacts with graphite through hydrophobic patches along its surface. AFM studies of HA behavior on differing surfaces under well-controlled environmental conditions provides useful insight into the variety of conformations and interactions likely to be found under differing physiological conditions.     

Assessing accumulated solvent near a macromolecular solute by preferential interaction coefficients

Biological macromolecules are often studied in mixed solvents. To understand cosolvent-macromolecule interactions, the preferential interaction coefficient, Gamma(3), may help determine surface solvent compositions. Gamma(3) measures the amounts of water, B(1), and cosolvent, B(3), within the "local domain," the (possibly far-reaching) region surrounding the macromolecule where the solvent is non-bulk-like. The local domain's boundary is, however, vague and it is unclear which molecules are counted in B(i). It is useful to explore a simple model system to make B(i) more concrete and to understand which aspects of the surface solvent distribution, rho(x), are sampled by Gamma(3). We performed computer simulations on a two-dimensional (2D) system consisting of a hard-wall solute (the macromolecule) in a mixed solvent (hard disks of different radii). We simultaneously calculated Gamma(3) and rho(x). We found that 1) in practice, the local domain's boundary is demarked by the outer limit of the first cosolvent (not water) layer; B(i) mainly counts the solvent near the macromolecule; 2) assuming B(1) to count only the waters within the first water layer is a poor approximation; 3) when determining B(1) and B(3), water and cosolvent molecules must be counted from the same region of space. We speculate that these 2D results may serve as a first-order approximation for the dominant contributions to Gamma(3) even in three dimensions, so long as the cosolvent is not strongly excluded from the macromolecular surface and there is no significant long-ranged solvent structure.     

A simple procedure for the derivation of electron density based surfaces of drug-receptor complexes from a combination of X-ray data and theoretical calculations

To contribute to an understanding of biological recognition and interaction, an easy-to-use procedure was developed to generate and display molecular surfaces and selected electron density based surface properties. To overcome the present limitations to derive electron densities of macromolecules, the considered systems were reduced to appropriate substructures around the active centers. The combination of experimental X-ray structural information and aspherical atomic electron density data from theoretical calculations resulted in properties like the electrostatic potential and the Hirshfeld surface which allowed a study of electronic complementarity and the identification of sites and strengths of drug-receptor interactions. Applications were examined for three examples. The anilinoquinazoline gefitinib (Iressa®) belongs to a new class of anticancer drugs that inhibit the tyrosine kinase activity of the epidermal growth factor receptor (EGFR). In the second example, the interaction of epoxide inhibitors with the main protease of the SARS coronavirus was investigated. Furthermore, the progesterone receptor complex was examined. The quantitative analysis of hydrogen bonding in the chosen substructure systems follows a progression elaborated earlier on the basis of accurate small molecule crystal structures. This finding and results from modified substructures suggest that also the surface properties seem robust enough to provide stable information about the recognition of interacting biomolecular species although they are obtained from medium molecular weighted subfragments of macromolecular complexes, which consist of no more than ～40 residues.     

Conformation and assembly of polypeptide scaffolds in templating the synthesis of silica: an example of a polylysine macromolecular "switch"

Although the role of polycationic macromolecules in catalyzing the synthesis of silica structures is well established, detailed understanding of the mechanisms behind the production of silica structures of controlled morphologies remains unclear. In this study, we have used both poly-L-lysine (PLL) and/or poly-D-lysine (PDL) for silica synthesis to investigate mechanisms controlling inorganic morphologies. The formation of both spherical silica particles and hexagonal plates was observed. The formation of hexagonal plates was suggested, via circular dichroic spectroscopy (CD), to result from the assembly of helical polylysine molecules. We confirm that the formation of PLL helices is a prerequisite to the hexagonal silica synthesis. In addition, we present for the first time that the handedness of the helicity of the macromolecule does not affect the formation of hexagonal silica. We also show, by using two different silica precursors, that the precursor does not have a direct effect on the formation of hexagonal silica plates. Furthermore, when polylysine helices were converted to beta-sheet structure, only silica particles were obtained, thus suggesting that the adoption of a helical conformation by PLL is required for the formation of hexagonally organized silica. These results demonstrate that the change in polylysine conformation can act as a "switch" in silica structure formation and suggest the potential for controlling morphologies and structures of inorganic materials via control of the conformation of soft macromolecular templates.     

GTP-mediated macromolecular interactions: the common features of different systems

G proteins that serve to transduce external signals in membranes share with protein synthesis factors and tubulin structural and functional features that are common to proteins that participate in reversible GTP-mediated macromolecular interactions. These proteins can bind GTP and GDP with high affinity, adopting different structures depending on whether they are complexed with the nucleotide diphosphate or triphosphate. The GTP.protein complex has high affinity for an acceptor macromolecule (or complex of macromolecules) and interacts with it, affecting its activity. These GTP-binding proteins also possess an intrinsic GTPase activity that is generally stimulated by its interaction with the acceptor. The GTPase activity converts the bound GTP to GDP, switching the configuration of the complexed protein to one of low affinity for the acceptor and causing its dissociation. The protein.GDP complex must exchange its GDP for GTP to allow the protein to acquire the high-affinity structure that can cycle back to the acceptor macromolecule. This exchange of guanine nucleotides requires in several instances exchange factors that can regulate the whole process. A detailed comparison of the features of the different systems is made with respect to structural similarities, regulation by protein phosphorylation, ADP ribosylation by bacterial toxins, and requirements for exchange factors. It is also proposed that there is a similar mechanism that involves ATP/ADP-binding proteins.     

Roles of surface electrochemistry and macromolecular adsorption in heparin-induced red blood cell aggregation

Red blood cell (RBC) aggregation in heparin-saline solution was quantified by microscopic observation. The adsorption isotherms of heparin onto normal and neuraminidase-treated RBC surfaces were determined by radioactive heparin labeled with 125I-Bolton-Hunter Reagent. RBC aggregation by heparin requires the presence of sialic acids at cell surface and was enhanced by reduction of ionic strength of the suspending medium. Adsorption of heparin onto RBC surface was increased by removal of sialic acids. These findings not only serve to elucidate the basic mechanism of cell-cell interaction mediated by negatively charged macromolecules, but also provide experimental evidence for the possible conformational change of macromolecules at the charged surface.     

Specificity of marine microbial surface interactions

The macromolecular surface components involved in intraspecific cell surface interactions of the green microalga Chlorella vulgaris and closely associated bacteria were investigated. The specific surface attachment between this alga and its associated bacteria is mediated by lectin-like macromolecules associated with the surfaces of these cells. The binding activity of these surface polymers was inhibited by specific simple sugars; this suggests the involvement of specific receptor-ligand binding sites on the interactive surfaces. Epifluorescent microscopic evaluation of bacteria-alga interactions in the presence and absence of the macromolecules that mediate these interactions showed that the glycoproteins active in these processes were specific to the microbial sources from which they were obtained. The demonstration and definition of the specificity of these interactions in mixed microbial populations may play an important role in our understanding of the dynamics of marine microbial populations in the sea.     

Specificity of marine microbial surface interactions.

The macromolecular surface components involved in intraspecific cell surface interactions of the green microalga Chlorella vulgaris and closely associated bacteria were investigated. The specific surface attachment between this alga and its associated bacteria is mediated by lectin-like macromolecules associated with the surfaces of these cells. The binding activity of these surface polymers was inhibited by specific simple sugars; this suggests the involvement of specific receptor-ligand binding sites on the interactive surfaces. Epifluorescent microscopic evaluation of bacteria-alga interactions in the presence and absence of the macromolecules that mediate these interactions showed that the glycoproteins active in these processes were specific to the microbial sources from which they were obtained. The demonstration and definition of the specificity of these interactions in mixed microbial populations may play an important role in our understanding of the dynamics of marine microbial populations in the sea.     

Differential modulation of the active site environment of human carbonic anhydrase XII by cationic quantum dots and polylysine

Due to prevalence of negative charges on the protein surface, opposite to the active site pocket of human carbonic anhydrase XII (hCA XII), both positively charged CdTe quantum dots (Qds⁺) and polylysine electrostatically interact with the enzyme, and such interaction does not influence the catalytic activity of the enzyme. However, both these cationic macromolecules differently modulate the active site environment of the enzyme. The steady-state kinetic data revealed that whereas polylysine exhibited no influence on dansylamide (DNSA) dependent inhibition of the enzyme, Qds⁺ overcame such an inhibitory effect, leading to almost 70% restoration of the catalytic activity of the enzyme. We provide evidence that DNSA remains bound to the enzyme upon interaction with both polylysine and Qds⁺. Arguments are presented that the above differential feature of polylysine and Qds⁺ on hCA XII is encoded in the “rigidity” versus “flexibility” of these cationic macromolecules.     

The effect of polycations on cell membrane stability and transport processes

Summary The interaction of poly-l-lysines of different molecular weights (PL) with Ehrlich ascites tumor cells was studied experimentally with respect to cell surface binding, cell electrophoresis, cytotoxicity and membrane permeability. Although they decrease the net negative charge of Ehrlich ascites cells similarly at low PL concentrations, low molecular weight PL was less cytotoxic and less damaging to the potassium transport mechanism than was high molecular weight PL. At certain PL concentrations, membrane damage was reversible on reincubation in PL-free media. The amount of bound polylysine as determined with fluorescent labeled polylysine was compared by electrophoresis to the amount of polylysine expressed on the electrokinetic surface. The results indicated that only a small fraction of polylysine bound to Ehrlich ascites tumor cells was electrokinetically detectable. The adsorption of polylysine to Ehrlich ascites tumor cells was not describable by the usual adsorption isotherms. It is suggested that the same number of monomeric lysine units of high and low molecular weight PL are adsorbed at the cell electrokinetic surface, but cytotoxicity is dependent on molecular weight. Although the negative charge of human red blood cells could be reversed at low PL concentrations, no such effect could be observed for ELD (a subline of Ehrlich ascites carcinoma) cells even at high PL concentrations. The relationship of PL binding to the stimulation of macromolecular uptake is discussed.     

Binding sites and endocytosis of heparin and polylysine are changed when the two molecules are given as a complex to Chinese hamster ovary cells

This paper characterizes the complex formed in vitro between polylysine and heparin in the presence of heparin excess, and investigates the interaction of this complex with the surface of Chinese hamster ovary cells. It examines the kinetics of surface binding and cellular uptake of the complex and shows that both processes can be distinguished from those of free heparin and free polylysine. The view that these three ligands bind to different surface sites is further supported by the fact that their interaction with cells is influenced differently by cell detachment with trypsin, detachment with EGTA or exposure to acid pH. Membrane transport of the complex is a saturable process suggestive of receptor-mediated endocytosis. It is, however, less effective than would be expected on the basis of the binding kinetics. Only 40% of the complex bound at 0 degrees C is internalized during a 2 h reincubation period at 37 degrees C, suggesting some degree of uncoupling between binding and endocytosis. These data confirm prior results obtained with methotrexate-polylysine conjugates. We had shown that the addition of heparin to a medium containing a methotrexate-polylysine conjugate leads unexpectedly to a marked cellular uptake of drug conjugate, which is capable of killing cells that are otherwise resistant to free methotrexate (Shen, W.-C. and Ryser, H.J.-P. (1981) Proc. Natl. Acad. Sci. USA 78, 7589-7593). The polylysine X heparin complex is therefore of interest as a potential carrier for intracellular drug delivery through endocytosis.     

A contribution to the theory of preferential interaction coefficients

A simple and complete derivation of the relation between concentration-based preferential interaction coefficients and integrals over the relevant pair correlation functions is presented for the first time. Certain omissions from the original treatment of pair correlation functions in multicomponent thermodynamics are also addressed. Connections between these concentration-based quantities and the more common molality-based preferential interaction coefficients are also derived. The pair correlation functions and preferential interaction coefficients of both solvent (water) and cosolvent (osmolyte) in the neighborhood of a macromolecule contain contributions from short-range repulsions and generic long-range attractions originating from the macromolecule, as well as from osmolyte-solvent exchange reactions beyond the macromolecular surface. These contributions are evaluated via a heuristic analysis that leads to simple insightful expressions for the preferential interaction coefficients in terms of the volumes excluded to the centers of the water and osmolyte molecules and a sum over the contributions of exchanging sites in the surrounding solution. The preferential interaction coefficients are predicted to exhibit the experimentally observed dependence on osmolyte concentration. Molality-based preferential interaction coefficients that were reported for seven different osmolytes interacting with bovine serum albumin are analyzed using the this formulation together with geometrical parameters reckoned from the crystal structure of human serum albumin. In all cases, the excluded volume contribution, which is the volume excluded to osmolyte centers minus that excluded to water centers in units of V1, exceeds in magnitude the contribution of the exchange reactions. Under the assumption that the exchange contribution is dominated by sites in the first surface-contiguous layer, the ratio of the average exchange constant to its neutral random value is determined for each osmolyte. These ratios all lie in the range 1.0 +/- 0.15, which indicates rather slight deviations from random occupation near the macromolecular surface. Finally, a mechanism is proposed whereby the chemical identity of an osmolyte might be concealed from partially ordered multilayers of water in clefts, grooves, and pits, and its consequences are noted.     

Isoelectric focusing of interacting systems III. Carrier ampholyte-induced macromolecular association or dissociation into subunits.

A phenomenological theory of isoelectric focusing of interacting systems (Cann, J. R., and Stimpson, D. I. Biophys. Chem., 7, 103, (1977) has been extended to include carrier ampholyte-induced dimerization or dissociation of a subunit macromolecule. Equilibrium as well as transient focusing patterns can show two well-resolved peaks similar to the patterns for a mixture of two noninteracting macromolecules. Characteristically, one of the peaks in the transient pattern grows at the expense of the other, which under certain conditions may disappear completely as equilibrium is approached. The implications of these findings for conventional applications of isoelectric focusing and for the detection and characterization of macromolecular interactions are discussed.     

Surface-Bound Casein Modulates the Adsorption and Activity of Kinesin on SiO2 Surfaces

Conventional kinesin is routinely adsorbed to hydrophilic surfaces such as SiO₂. Pretreatment of surfaces with casein has become the standard protocol for achieving optimal kinesin activity, but the mechanism by which casein enhances kinesin surface adsorption and function is poorly understood. We used quartz crystal microbalance measurements and microtubule gliding assays to uncover the role that casein plays in enhancing the activity of surface-adsorbed kinesin. On SiO₂ surfaces, casein adsorbs as both a tightly bound monolayer and a reversibly bound second layer that has a dissociation constant of 500 nM and can be desorbed by washing with casein-free buffer. Experiments using truncated kinesins demonstrate that in the presence of soluble casein, kinesin tails bind well to the surface, whereas kinesin head binding is blocked. Removing soluble casein reverses these binding profiles. Surprisingly, reversibly bound casein plays only a moderate role during kinesin adsorption, but it significantly enhances kinesin activity when surface-adsorbed motors are interacting with microtubules. These results point to a model in which a dynamic casein bilayer prevents reversible association of the heads with the surface and enhances association of the kinesin tail with the surface. Understanding protein-surface interactions in this model system should provide a framework for engineering surfaces for functional adsorption of other motor proteins and surface-active enzymes.     

Molecular basis for the unique role of the AAA+ chaperone ClpV in type VI protein secretion

Ring-forming AAA(+) ATPases act in a plethora of cellular processes by remodeling macromolecules. The specificity of individual AAA(+) proteins is achieved by direct or adaptor-mediated association with substrates via distinct recognition domains. We investigated the molecular basis of substrate interaction for Vibrio cholerae ClpV, which disassembles tubular VipA/VipB complexes, an essential step of type VI protein secretion and bacterial virulence. We identified the ClpV recognition site within VipB, showed that productive ClpV-VipB interaction requires the oligomeric state of both proteins, solved the crystal structure of a ClpV N-domain-VipB peptide complex, and verified the interaction surface by mutant analysis. Our results show that the substrate is bound to a hydrophobic groove, which is formed by the addition of a single α-helix to the core N-domain. This helix is absent from homologous N-domains, explaining the unique substrate specificity of ClpV. A limited interaction surface between both proteins accounts for the dramatic increase in binding affinity upon ATP-driven ClpV hexamerization and VipA/VipB tubule assembly by coupling multiple weak interactions. This principle ensures ClpV selectivity toward the VipA/VipB macromolecular complex.     

ESR investigation of the binding of acidic biopolymers to synthetic apatite.

The interaction between synthetic crystalline calcium phosphate (apatite) and acidic macromolecules (sodium polyacrylate, sodium poly(L -glutamate), chondroitin sulfate, phosvitin) was investigated by electron spin resonance spectroscopy of mineral-macromolecule complexes doped with vanadyl ion (VO⁺⁺) as a paramagnetic probe. Changes in magnetic parameters were interpreted in terms of bonding between mineral and macromolecule. The VO⁺⁺ probe data indicated that polymer acidic functional groups were bound to mineral surfaces in all cases.