pH‐Responsive Self‐Organization of Metal‐Binding Protein Motifs from Biomolecular Junctions in Mussel Byssus

Abstract

Mussels rapidly fabricate tough and self-healing biopolymeric fibers called byssal threads that provide an excellent role model for bio-inspired design. The remarkable tensile properties arise from a collagenous protein family known as preCols, which self-assemble into a semicrystalline array in the distal thread core. Histidine-rich domains (HRDs) at the preCol ends are critical both for the self-healing capacity and for the thread assembly process due to their propensity for coordinating transition metal ions; however, very little is understood about the molecular relationship between HRD sequence, structure, and function. Here, a comprehensive spectroscopic investigation of two model peptides based on conserved repetitive motifs in the HRDs is performed to elucidate molecular level details of their role in thread assembly and function. It is observed that environmental factors relevant to the natural assembly process (e.g., concentration, pH, and metal content) trigger dramatic changes in HRD nanostructure and higher order assembly, leading to the formation of highly defined backbone conformation and metal binding geometry

Histidine–Zinc Interactions Investigated by Isothermal Titration Calorimetry (ITC) and their Application in Self‐Healing Polymers

“…Presumably, the transition metals copper, nickel, and zinc play a key role in complexing histidine moieties in the histidine rich domains of the hierarchically structured proteins (PreCols) of the distal regions of threads . In fact, recent studies based on X‐ray absorption spectroscopy provided new insights into the structure of the histidine–zinc(II) bonds in byssal threads and their dynamic behavior during mechanical deformation and healing, while studies investigating peptides based on histidine‐rich protein sequences from the byssal thread proteins revealed the ability to form highly organized hierarchical structures and zinc(II) coordination complexes …”

Mussel Byssus Structure‐Function and Fabrication as Inspiration for Biotechnological Production of Advanced Materials

“…It is still unclear how the liquid crystal protein phase transitions to a solid semi‐crystalline fiber. One strong hypothesis supported by studies of HRD peptides (section 3.3) is that the pH increase going from secretory vesicle to seawater induces a conformational transition of the HRDs leading to aggregation and assembly, which is initiated by the deprotonation of Histidine (pKa ≈6.5) (Figure C) . Notably, histidine deprotonation also promotes metal binding, which is well‐established to contribute to the mechanical performance of the thread (section 2.2).…”

“…For example, several peptide‐based studies investigated the preCol histidine‐rich domains (HRDs), with a specific focus on their role in pH‐dependent self‐assembly and mechanical performance of threads . Inspired by the natural assembly process, HRD peptides based on the N‐terminal histidine‐rich domain of preCol‐D (HRD‐DN) were investigated and formed into free‐standing films both in the absence and presence of metal ions, demonstrating tunability of both the higher order structure and mechanical properties of the films (Figure C) . Similarly, short, but representative regions of mfp‐3 were harnessed to elucidate the role of amino acids other than DOPA in coacervation and adhesion .…”

“…[11,17,20] Inspired by the natural assembly process, HRD peptides based on the N-terminal histidine-rich domain of preCol-D (HRD-DN) were investigated and formed into free-standing films both in the absence and presence of metal ions, demonstrating tunability of both the higher order structure and mechanical properties of the films ( Figure 2C). [65,66] Similarly, short, but representative regions of mfp-3 were harnessed to elucidate the role of amino acids other than DOPA in coacervation and adhesion. [67] Although shorter, the resulting peptide possessed the main characteristics with abundant positively charged and aromatic residues.…”

“…One strong hypothesis supported by studies of HRD peptides (section 3.3) is that the pH increase going from secretory vesicle to seawater induces a conformational transition of the HRDs leading to aggregation and assembly, which is initiated by the deprotonation of Histidine (pKa %6.5) ( Figure 3C). [65,66] Notably, histidine deprotonation also promotes metal binding, which is well-established to contribute to the mechanical performance of the thread [11,17] (section 2.2). It has been further hypothesized that evolutionary conserved Tyr residues at the ends of preCol proteins are converted to DOPA, which becomes oxidized at the high pH of seawater, forming covalent crosslinks between the ends of neighboring preCols.…”