Redox Capacity of an Extracellular Matrix Protein Associated with Adhesion in Mytilus californianus.
ABSTRACT: Adhesive mussel foot proteins (Mfps) rely in part on DOPA (3,4-dihydroxyphenyl-l-alanine) side chains to mediate attachment to mineral surfaces underwater. Oxidation of DOPA to Dopaquinone (Q) effectively abolishes the adsorption of Mfps to these surfaces. The thiol-rich mussel foot protein-6 (Mfp-6) rescues adhesion compromised by adventitious DOPA oxidation by reducing Q back to DOPA. The redox chemistry and kinetics of foot-extracted Mfp-6 were investigated by using a nonspecific chromogenic probe to equilibrate with the redox pool. Foot-extracted Mfp-6 has a reducing capacity of ~17 e(-) per protein; half of this comes from the cysteine residues, whereas the other half comes from other constituents, probably a cohort of four or five nonadhesive, redox-active DOPA residues in Mfp-6 with an anodic peak potential ~500 mV lower than that for oxidation of cysteine to cystine. At higher pH, DOPA redox reversibility is lost possibly due to Q scavenging by Cys thiolates. Analysis by one- and two-dimensional proton nuclear magnetic resonance identified a pronounced ?-sheet structure with a hydrophobic core in foot-extracted Mfp-6 protein. The structure endows redox-active side chains in Mfp-6, i.e., cysteine and DOPA, with significant reducing power over a broad pH range, and this power is measurably diminished in recombinant Mfp-6.
Project description:Mussel adhesion is mediated by foot proteins (mfps) rich in a catecholic amino acid, 3,4-dihydroxyphenylalanine (dopa), capable of forming strong bidentate interactions with a variety of surfaces. A tendency toward facile auto-oxidation, however, often renders dopa unreliable for adhesion. We demonstrate that mussels limit dopa oxidation during adhesive plaque formation by imposing an acidic, reducing regime based on the thiol-rich mfp-6, which restores dopa by coupling the oxidation of thiols to dopaquinone reduction.
Project description:Mussel (Mytilus californianus) adhesion to marine surfaces involves an intricate and adaptive synergy of molecules and spatio-temporal processes. Although the molecules, such as mussel foot proteins (mfps), are well characterized, deposition details remain vague and speculative. Developing methods for the precise surveillance of conditions that apply during mfp deposition would aid both in understanding mussel adhesion and translating this adhesion into useful technologies. To probe the interfacial pH at which mussels buffer the local environment during mfp deposition, a lipid bilayer with tethered pH-sensitive fluorochromes was assembled on mica. The interfacial pH during foot contact with modified mica ranged from 2.2 to 3.3, which is well below the seawater pH of ~ 8. The acidic pH serves multiple functions: it limits mfp-Dopa oxidation, thereby enabling the catecholic functionalities to adsorb to surface oxides by H-bonding and metal ion coordination, and provides a solubility switch for mfps, most of which aggregate at pH ? 7-8.
Project description:The underwater adhesion of marine mussels relies on mussel foot proteins (mfps) rich in the catecholic amino acid 3,4-dihydroxyphenylalanine (Dopa). As a side chain, Dopa is capable of strong bidentate interactions with a variety of surfaces, including many minerals and metal oxides. Titanium is among the most widely used medical implant material and quickly forms a TiO2 passivation layer under physiological conditions. Understanding the binding mechanism of Dopa to TiO2 surfaces is therefore of considerable theoretical and practical interest. Using a surface forces apparatus, we explored the force-distance profiles and adhesion energies of mussel foot protein 3 (mfp-3) to TiO2 surfaces at three different pHs (pH 3, 5.5 and 7.5). At pH 3, mfp-3 showed the strongest adhesion force on TiO2, with an adhesion energy of ?-7.0 mJ/m(2). Increasing the pH gives rise to two opposing effects: (1) increased oxidation of Dopa, thus, decreasing availability for the Dopa-mediated adhesion, and (2) increased bidentate Dopa-Ti coordination, leading to the further stabilization of the Dopa group and, thus, an increase in adhesion force. Both effects were reflected in the resonance-enhanced Raman spectra obtained at the three deposition pHs. The two competing effects give rise to a higher adhesion force of mfp-3 on the TiO2 surface at pH 7.5 than at pH 5.5. Our results suggest that Dopa-containing proteins and synthetic polymers have great potential as coating materials for medical implant materials, particularly if redox activity can be controlled.
Project description:Catecholic 3,4-dihydroxyphenyl-l-alanine (Dopa) residues in mussel foot proteins (mfps) contribute critically to mussel (Mytilus californianus) plaque adhesion, but only if protected from oxidation at the adhesive-substratum interface. Dopa oxidation is thermodynamically favorable in seawater yet barely detectable in plaques; therefore, we investigated how plaques insulate Dopa-containing mfps against oxidation. Seawater sulfate triggers an mfp3 and mfp6 liquid-liquid phase separation (LLPS). By combining plaque cyclic voltammetry with electrophoresis, mass spectrometry, and redox-exchange chemistry, we show that Dopa-containing mfp3 and mfp6 in phase-separated droplets remain stable despite rapid oxidation in the surrounding equilibrium solution. The results suggest that a cohort of oxidation-prone proteins is endowed with phase-dependent redox stability. Moreover, in forming LLPS compartments, Dopa proteins become reservoirs of chemical energy.
Project description:Understanding the interactions between collagen and adhesive mussel foot proteins (mfps) can lead to improved medical and dental adhesives, particularly for collagen-rich tissues. Here we investigated interactions between collagen type-1, the most abundant load-bearing animal protein, and mussel foot protein-3 (mfp-3) using a quartz crystal microbalance and surface forces apparatus (SFA). Both hydrophilic and hydrophobic variants of mfp-3 were exploited to probe the nature of the interaction between the protein and collagen. Our chief findings are: 1) mfp-3 is an effective chaperone for tropocollagen adsorption to TiO2 and mica surfaces; 2) at pH 3, collagen addition between two mfp-3 films (Wc = 5.4 ± 0.2 mJ/m(2)) increased their cohesion by nearly 35%; 3) oxidation of Dopa in mfp-3 by periodate did not abolish the adhesion between collagen and mfp-3 films, and 4) collagen bridging between both hydrophilic and hydrophobic mfp-3 variant films is equally robust, suggesting that hydrophobic interactions play a minor role. Extensive H-bonding, ?-cation and electrostatic interactions are more plausible to explain the reversible bridging of mfp-3 films by collagen.
Project description:The biochemistry of mussel adhesion has inspired the design of surface primers, adhesives, coatings and gels for technological applications. These mussel-inspired systems often focus on incorporating the amino acid 3,4-dihydroxyphenyl-L-alanine (Dopa) or a catecholic analog into a polymer. Unfortunately, effective use of Dopa is compromised by its susceptibility to auto-oxidation at neutral pH. Oxidation can lead to loss of adhesive function and undesired covalent cross-linking. Mussel foot protein 5 (Mfp-5), which contains ? 30 mole % Dopa, is a superb adhesive under reducing conditions but becomes nonadhesive after pH-induced oxidation. Here we report that the bidentate complexation of borate by Dopa to form a catecholato-boronate can be exploited to retard oxidation. Although exposure of Mfp-5 to neutral pH typically oxidizes Dopa, resulting in a>95% decrease in adhesion, inclusion of borate retards oxidation at the same pH. Remarkably, this Dopa-boronate complex dissociates upon contact with mica to allow for a reversible Dopa-mediated adhesion. The borate protection strategy allows for Dopa redox stability and maintained adhesive function in an otherwise oxidizing environment.
Project description:The adhesion of mussel foot proteins (Mfps) to a variety of specially engineered mineral and metal oxide surfaces has previously been investigated extensively, but the relevance of these studies to adhesion in biological environments remains unknown. Most solid surfaces exposed to seawater or physiological fluids become fouled by organic conditioning films and biofilms within minutes. Understanding the binding mechanisms of Mfps to organic films with known chemical and physical properties therefore is of considerable theoretical and practical interest. Using self-assembled monolayers (SAMs) on atomically smooth gold substrates and the surface forces apparatus, we explored the force-distance profiles and adhesion energies of three different Mfps, Mfp-1, Mfp-3, and Mfp-5, on (i) hydrophobic methyl (CH3)- and (ii) hydrophilic alcohol (OH)-terminated SAM surfaces between pH 3 and pH 7.5. At acidic pH, all three Mfps adhered strongly to the CH3-terminated SAM surfaces via hydrophobic interactions (range of adhesive interaction energy = -4 to -9 mJ/m(2)) but only weakly to the OH-terminated SAM surfaces through H- bonding (adhesive interaction energy ? -0.5 mJ/m(2)). 3, 4-Dihydroxyphenylalanine (Dopa) residues in Mfps mediate binding to both SAM surface types but do so through different interactions: typical bidentate H-bonding by Dopa is frustrated by the longer spacing of OH-SAMs; in contrast, on CH3-SAMs, Dopa in synergy with other nonpolar residues partitions to the hydrophobic surface. Asymmetry in the distribution of hydrophobic residues in intrinsically unstructured proteins, the distortion of bond geometry between H-bonding surfaces, and the manipulation of physisorbed binding lifetimes represent important concepts for the design of adhesive and nonfouling surfaces.
Project description:Mussel foot protein-1 (mfp-1) is an essential constituent of the protective cuticle covering all exposed portions of the byssus (plaque and the thread) that marine mussels use to attach to intertidal rocks. The reversible complexation of Fe(3+) by the 3,4-dihydroxyphenylalanine (Dopa) side chains in mfp-1 in Mytilus californianus cuticle is responsible for its high extensibility (120%) as well as its stiffness (2 GPa) due to the formation of sacrificial bonds that help to dissipate energy and avoid accumulation of stresses in the material. We have investigated the interactions between Fe(3+) and mfp-1 from two mussel species, M. californianus (Mc) and M. edulis (Me), using both surface sensitive and solution phase techniques. Our results show that although mfp-1 homologues from both species bind Fe(3+), mfp-1 (Mc) contains Dopa with two distinct Fe(3+)-binding tendencies and prefers to form intramolecular complexes with Fe(3+). In contrast, mfp-1 (Me) is better adapted to intermolecular Fe(3+) binding by Dopa. Addition of Fe(3+) did not significantly increase the cohesion energy between the mfp-1 (Mc) films at pH 5.5. However, iron appears to stabilize the cohesive bridging of mfp-1 (Mc) films at the physiologically relevant pH of 7.5, where most other mfps lose their ability to adhere reversibly. Understanding the molecular mechanisms underpinning the capacity of M. californianus cuticle to withstand twice the strain of M. edulis cuticle is important for engineering of tunable strain tolerant composite coatings for biomedical applications.
Project description:The mussel byssus has long been a source of inspiration for the adhesion community. Recently, adhesive synergy between flanking lysine (Lys, K) and 3,4-Dihydroxyphenylalanine (DOPA, Y) residues in the mussel foot proteins (Mfps) has been highlighted. However, the complex topological relationship of DOPA and Lys as well as the interfacial adhesive roles of other amino acids have been understudied. Herein, we study adhesion of Lys and DOPA-containing peptides to organic and inorganic substrates using single-molecule force spectroscopy (SMFS). We show that a modest increase in peptide length, from KY to (KY)3, increases adhesion strength to TiO2. Surprisingly, further increase in peptide length offers no additional benefit. Additionally, comparison of adhesion of dipeptides containing Lys and either DOPA (KY) or phenylalanine (KF) shows that DOPA is stronger and more versatile. We furthermore demonstrate that incorporating a nonadhesive spacer between (KY) repeats can mimic the hidden length in the Mfp and act as an effective strategy to dissipate energy.
Project description:Mussels use a variety of 3, 4-dihydroxyphenyl-l-alanine (DOPA) rich proteins specifically tailored to adhering to wet surfaces. Synthetic polypeptide analogues of adhesive mussel foot proteins (specifically mfp-3) are used to study the role of DOPA in adhesion. The mussel-inspired peptide is a random copolymer of DOPA and N(5) -(2-hydroxyethyl)-l-glutamine synthesized with DOPA concentrations of 0-27 mol% and molecular weights of 5.9-7.1 kDa. Thin films (3-5 nm thick) of the mussel-inspired peptide are used in the surface forces apparatus (SFA) to measure the force-distance profiles and adhesion and cohesion energies of the films in an acetate buffer. The adhesion energies of the mussel-inspired peptide films to mica and TiO(2) surfaces increase with DOPA concentration. The adhesion energy to mica is 0.09 ?J m(-2) mol(DOPA) (-1) and does not depend on contact time or load. The adhesion energy to TiO(2) is 0.29 ?J m(-2) mol(DOPA) (-1) for short contact times and increases to 0.51 ?J m(-2) mol(DOPA) (-1) for contact times >60 min in a way suggestive of a phase transition within the film. Oxidation of DOPA to the quinone form, either by addition of periodate or by increasing the pH, increases the thickness and reduces the cohesion of the films. Adding thiol containing polymers between the oxidized films recovers some of the cohesion strength. Comparison of the mussel-inspired peptide films to previous studies on mfp-3 thin films show that the strong adhesion and cohesion in mfp-3 films can be attributed to DOPA groups favorably oriented within or at the interface of these films.