Surface-attached Protein Research Articles

The effect of charged residue substitutions on the thermodynamics of protein-surface interactions.

The interactions of proteins with surfaces are important in both biological processes and biotechnologies. In contrast to decades of study regarding the biophysics of proteins in bulk solution, however, our mechanistic understanding of the biophysics of proteins interacting with surfaces remains largely qualitative. In response, we have set to explore quantitatively the thermodynamics of protein-surface interactions. In this work, we explore systematically the role of electrostatics in modulating the interaction between proteins and charged surfaces. In particular, we use electrochemistry to explore the extent to which a macroscopic, hydroxyl-coated surface held at a slightly negative potential affects the folding thermodynamics of surface-attached protein variants with different composition of charged amino acids. Doing so, we find that attachment to the surface generally leads to a net stabilization, presumably due to excluded volume effects that reduce the entropy of the unfolded state. The magnitude of this stabilization, however, is strongly correlated with the charged-residue content of the protein. In particular, we find statistically significant correlations with both the net charge of the protein, with greater negative charge leading to less stabilization by the surface, and with the number of arginines, with more arginines leading to greater stabilization. Such findings refine our understanding of protein-surface interactions, providing in turn a guiding rationale to achieve the functional deposition of proteins on artificial surfaces for implementation in, for example, protein-based biotechnologies.

Understanding the Biophysics of Protein-Surface Interactions

Despite the biological and biotechnological importance of protein-surface interactions, our understanding of how and why they occur is still limited: What are the thermodynamic consequences of the interaction of proteins with surfaces? Why do proteins generally remain folded and functional on biological surfaces and cell membranes, but often unfold, adhere to, and inactivate on artificial surfaces? To respond to these questions we need to understand the biophysical origins of protein-surface interactions, however the absence of experimental methods to measure the thermodynamics of such interactions has so far precluded quantitative analysis. In response, we have developed an approach to measure the free energy of protein-surface interactions, which we have employed to explore the extent to which attachment to a specific, macroscopic surface alters the thermodynamic stability of protein L. We have achieved so by modifying protein L with the redox reporter methylene blue and then covalently tethering its N-terminus to a gold electrode passivated with a hydroxyl-terminated alkanethiol monolayer. Denaturant-induced unfolding of the surface-attached protein alters the ease with which the methylene blue reporter transfers electrons to the surface of the electrode, and therefore electrochemical techniques, such as square-wave voltammetry, allow monitoring the unfolding to extract protein stability. Comparing the thus obtained stability of the surface-attached protein to that of the same protein in bulk solution we find that surface-attachment stabilizes the protein due to excluded volume effects that restrict the conformational entropy of the unfolded state. We have also explored the role of macromolecular crowding, solvent composition and electrostatics to find that their surface biophysics are markedly different from those in bulk solution. We believe that our studies refine our understanding of the biophysics underlying protein-surface interactions, which may in turn improve the design of protein-surface pairs for protein-incorporating biotechnologies such as protein-based sensors.

Electrochemical analysis of Os(VI)-modified glycoproteins and label-free glycoprotein detection eluted from lectin capillary column

Earlier we showed that using Os(VI)temed complex, glycans can be modified directly in glycoproteins and detected voltammetrically at μM concentrations at carbon electrodes. Here we used Os(VI)2,2′-bipyridine for modification of ribonucleases and Hg electrodes for voltammetric detection. We show that glycosylated RNase B produced electrocatalytic signal (after separation from the reaction mixture) at pM concentrations while non-glycosylated RNase A yielded negligible signal under the same conditions. Using Os(VI)temed and carbon electrodes voltammetric detection was less sensitive but allowed detection of RNase B in an excess of non-glycosylated protein, directly in the reaction mixture. We also showed that the constant current chronopotentiometric stripping (CPS) peak H (which is due to the ability of some amino acid residues in proteins to catalyze hydrogen evolution at Hg electrodes) could be used for protein structure-sensitive analysis at mercury electrodes. Using this peak, here we show that glycosylation of RNase stabilizes its molecule at the electrode. On the other hand, Os(VI)temed modification results in destabilization of this surface-attached protein. Peak H was also used for detection of RNase B separated from a large excess of non-glycosylated proteins on a lectin (concanavalin A) monolithic flow-through column.

Voltammetric and chronopotentiometric protein structure-sensitive analysis

Previously we showed that constant current chronopotentiometric stripping (CPS) is convenient for protein analysis based on the ability of some amino acid residues to catalyze hydrogen evolution on mercury-containing electrodes. This method showed a remarkable sensitivity to changes in protein structures, including protein denaturation and even small protein damage. Here we used normal pulse voltammetric stripping (NPVS) with bare and dithiothreitol-modified hanging mercury drop electrode. We found that NPV pulses denatured the surface-attached protein but we showed conditions under which this method was able to distinguish native and denatured proteins with sensitivity approaching that of CPS. Using NPVS it was possible to follow bovine serum albumin (BSA) thermal denaturation as well as to investigate the effect of NPV pulses on the structure of the surface-attached protein. In addition to BSA we studied several proteins, such as human serum albumin, ovalbumin, urease, aldolase, concanavalin A, histone and vasopressin peptide. Our results suggest that CPS remains the method-of-choice for studies of changes in protein structure and of biochemical processes, such as DNA-protein specific binding, lectin-glycoprotein interactions, detection of protein damage, etc., while voltammetric methods, such as NPVS may suit better for investigation of the processes which proteins undergo at the electrode surface.

Protein structural transition at negatively charged electrode surfaces. Effects of temperature and current density

Earlier it was shown that surface-attached bovine serum albumin undergoes ionic strength-induced structural transition at mercury electrodes. Using constant current chronopotentiometric stripping (CPS) here we show that this electric field-driven transition is influenced also by other factors, such as temperature, current density, and particularly by the time of surface-attached protein exposure to the negative potentials. For example, 1s exposure to –1.6V (against Ag/AgCl/3M KCl) results in the protein denaturation. Exposures longer than 1s resulted in protein denaturation even at less negative potentials. On the other hand, in CPS much shorter exposure times can be used. Exposing the surface-attached protein to negative potentials for ms time intervals the protein stability can be tested at different current densities. We investigated several proteins such as bovine and human serum albumins, ovalbumin, α-macroglobulin and α-synuclein, which showed various profiles in dependence on different parameters, including time of the protein exposure to negative potentials, ionic strengths and temperature. We believe that using this approach, new methods for testing surface-attached protein stabilities in relation to the electric field effects and to other factors can be developed.

Enzymatic activity and catalytic hydrogen evolution in reduced and oxidized urease at mercury surfaces

It was originally shown [10] that urease retains its enzymatic activity when adsorbed at bare mercury and solid amalgam surfaces. However the opinion later prevailed that, when adsorbed at bare metal electrodes, proteins are irreversibly denatured. Here we confirm that urease is enzymatically active at a bare solid amalgam surface as found by Santhanam et al., and we show that this enzyme is equally active at a thiol-modified amalgam surface. We also show that it is the reduced form of urease, which is enzymatically active at Hg surfaces. Oxidation of the protein, resulting in formation of disulfide bonds, strongly decreases the enzyme activity. Using constant current chronopotentiometric stripping (CPS) we show that the exposure of surface-attached urease to negative potentials results in the protein unfolding. The extent of the unfolding depends upon the amount of time for which the protein is exposed to negative potentials, and at very short times this unfolding can be avoided. At thiol-modified Hg surfaces the protein is less vulnerable to the effects of the electric field. We conclude that the loss of enzymatic activity, resulting from a 10min exposure of the protein to −0.58V, is not due to reduction of the disulfide bonds as suggested by Santhanam et al. This loss is probably a result of protein reorientation, due to reduction of the Hg-S bonds (formed by accessible cysteines), followed by prolonged electric field effect on the surface-attached protein.

Plasma immersion ion implantation treatment of polyethylene for enhanced binding of active horseradish peroxidase

Robust attachment of active proteins to synthetic surfaces underpins the development of biosensors and protein arrays. This paper presents the results of experiments in which energetic ions, extracted from an inductively coupled nitrogen plasma, are used to modify the surface of ultrahigh molecular weight polyethylene (UHMWPE). The ability of the surface to bind active horseradish peroxidase (HRP) is significantly enhanced by the plasma treatment. The amide signal in infrared spectroscopy indicates an increased quantity of surface-attached protein on the modified surface. The activity of the bound HRP remains high compared with that of protein attached to the untreated surface, after repeated washing in buffer solution. Although Tween 20 was an effective blocking agent for the unmodified polyethylene surface, binding of HRP to the modified surface is not inhibited by its presence. We propose that the treatment produces new binding sites on the surface and that the function of the HRP is retained because the treated surface is substantially more hydrophilic.

A Crucial Role for Exopolysaccharide Modification in Bacterial Biofilm Formation, Immune Evasion, and Virulence

Biofilms play an important role in many chronic bacterial infections. Production of an extracellular mixture of sugar polymers called exopolysaccharide is characteristic and critical for biofilm formation. However, there is limited information about the mechanisms involved in the biosynthesis and modification of exopolysaccharide components and how these processes influence bacterial pathogenesis. Staphylococcus epidermidis is an important human pathogen that frequently causes persistent infections by biofilm formation on indwelling medical devices. It produces a poly-N-acetylglucosamine molecule that emerges as an exopolysaccharide component of many bacterial pathogens. Using a novel method based on size exclusion chromatography-mass spectrometry, we demonstrate that the surface-attached protein IcaB is responsible for deacetylation of the poly-N-acetylglucosamine molecule. Most likely due to the loss of its cationic character, non-deacetylated poly-acetylglucosamine in an isogenic icaB mutant strain was devoid of the ability to attach to the bacterial cell surface. Importantly, deacetylation of the polymer was essential for key virulence mechanisms of S. epidermidis, namely biofilm formation, colonization, and resistance to neutrophil phagocytosis and human antibacterial peptides. Furthermore, persistence of the icaB mutant strain was significantly impaired in a murine model of device-related infection. This is the first study to describe a mechanism of exopolysaccharide modification that is indispensable for the development of biofilm-associated human disease. Notably, this general virulence mechanism is likely similar for other pathogenic bacteria and constitutes an excellent target for therapeutic maneuvers aimed at combating biofilm-associated infection.

Dynamic Alterations of Fibronectin Layers on Copolymer Substrates with Graded Physicochemical Characteristics

Desorption and exchange of preadsorbed fibronectin layers in pure buffer solution and solutions of human serum albumin or fibronectin, respectively, were studied in dependence on the physicochemical characteristics of maleic acid copolymer films used as substrates. Although the preadsorbed amount of fibronectin differed only slightly, the protein was found to exhibit a significantly enhanced anchorage at the more hydrophobic polymer surface as compared to the more hydrophilic and more negatively charged polymer surface. The preadsorbed fibronectin layer was most efficiently exchanged by fibronectin (i.e., in the homodisplacement process) while pure buffer solution and human serum albumin solutions induced desorption or exchange of fibronectin to lower and similar degrees. An increase of the total adsorbed amount of protein due to additional adsorption of fibronectin or human serum albumin accompanied the partial exchange of the preadsorbed fibronectin in the displacement experiments. Evaluation of the kinetics of desorption and exchange of fibronectin at any of the substrates revealed two kinds of surface-attached protein populations--a fast desorbing species and a species with a slow desorption and exchange rate. By a multivariate regression analysis the surface characteristics of the polymer substrate were confirmed to determine the degree of protein desorption and exchange while the dynamics of the layer alteration was found to solely depend on the diffusion behavior of the proteins.

Protein GRAB of Streptococcus pyogenes Regulates Proteolysis at the Bacterial Surface by Binding α2-Macroglobulin

In the molecular interplay between pathogenic microorganisms and their host, proteolytic mechanisms are believed to play a crucial role. Here we find that the important human pathogen Streptococcus pyogenes (group A Streptococcus) expresses a surface protein with high affinity (Ka = 2.0 x 10(8) M-1) for alpha2-macroglobulin (alpha2M), the dominating proteinase inhibitor of human plasma. The immunoglobulin-binding protein G of group C and G streptococci also contains an alpha2M-binding domain and a gene encoding protein GRAB (protein G-related alpha2M-binding protein) was identified in the S. pyogenes Genome Sequencing data base. The grab gene is present in most S. pyogenes strains and is well conserved. Protein GRAB has typical features of a surface-attached protein of Gram-positive bacteria. It also contains a region homologous to parts of the alpha2M-binding domain of protein G and a variable number of a unique 28-amino acid-long repeat. Using Escherichia coli-produced protein GRAB and synthetic GRAB peptides, the alpha2M-binding region was mapped to the NH2-terminal part of protein GRAB, which is the region with homology to protein G. An isogenic S. pyogenes mutant lacking surface-associated protein GRAB showed no alpha2M binding activity and was attenuated in virulence when injected intraperitoneally in mice. Finally, alpha2M bound to the bacterial surface via protein GRAB was found to entrap and inhibit the activity of both S. pyogenes and host proteinases, thereby protecting important virulence determinants from proteolytic degradation. This regulation of proteolytic activity at the bacterial surface should affect the host-microbe relation during S. pyogenes infections.

Competitive binding of thrombospondin, fibronectin, and fibrinogen to adsorbed proteins in multicomponent systems

The proteins thrombospondin, fibronectin, and fibrinogen are distributed among the plasma, the basement membrane of the vascular endothelium, and the storage pool of the unstimulated platelet. Upon platelet stimulation these proteins undergo redistribution and coexist both in solution and in bound states on the platelet and other surfaces. In our earlier studies of complex formation in systems involving these proteins in solution and in the surface-immobilized form we have shown that they interact with each other on the surface but that there is inhibition of binding to the surface when all three proteins are present. The concentration dependence of the interactions in such multicomponent systems was therefore studied. For reference, ovalbumin was included as a noninteracting protein. We have shown (a) that no three-component chemical complex is formed at the surface; (b) that the inhibition of binding to a surface-attached protein by any of the three proteins in solution is competitive; and (c) that proteins form complexes in solution, and that the dissociation constant of these complexes are on the order of 5 × 10 −8 M. The results suggest that thrombospondin secreted by the stimulated platelet will interact mainly with fibronectin and fibrinogen in the plasma with little involvement of the platelet or the extracellular matrix surface.