Protein- and peptide-modified synthetic polymeric biomaterials

Abstract

This review presents an overview on bio-hybrid approaches of integrating the structural and functional features of proteins and peptides with synthetic polymers and the resulting unique properties in such hybrids, with a focus on bioresponsive/bioactive systems with biomaterials applications. The review is divided in two broad sections. First, we describe several examples of bio-hybrids produced by combining versatile synthetic polymers with proteins/enzymes and drugs that have resulted in (1) hybrid materials based on responsive polymers, (2) responsive hydrogels based on enzyme-catalyzed reactions, protein-protein interactions and protein-drug sensing, and (3) dynamic hydrogels based on conformational changes of a protein. Next, we present hybrids produced by combining synthetic polymers with peptides, classified based on the properties of the peptide domain: (1) peptides with different conformations, such as alpha-helical, coiled-coil, and beta-sheet; (2) peptides derived from structural protein domains such as silk, elastin, titin, and collagen; and (3) peptides with other biofunctional properties such as cell-binding domains and enzyme-recognized degradation domains.

FIGURE 1

Schematic illustration of responsive hybrid hydrogels modulated by the activities of the protein domain. (a) Mechanism of shrinkage of a glucose-sensitive, pH-responsive polymer network consisting of poly(methacrylic acid-g-ethylene glycol). Glucose oxidase (GOD) immobilized in the polymer network catalyzes the oxidation of glucose (G) to gluconic acid (GlucA) in the presence of oxygen, which causes protonation of carboxylic acid groups, leading to the collapse of hydrogel. Reproduced with permission from Ref. . © 1997 American Chemical Society. (b) Swelling mechanism of hydrogel formed from polyacrylamide-based semi-interpenetrating network grafted with specific antigen and corresponding antibody. Upon competitive binding of the free antigen, preformed physical crosslinks in the hydrogels are disrupted, leading to reversible swelling. Reproduced with permission from Ref. . © 1999 Nature Publishing Group. (c) Gelation of polyacrylamide-based hydrogels containing a genetically engineered bacterial gyrase subunit B (GyrB) is triggered upon introduction of the GyrB dimerizing antibiotic coumermycin; gels are subsequently dissolved upon competitive binding of antibiotic novobiocin. Reproduced with permission from Ref. . © 2008 Nature Publishing Group.

FIGURE 2

Schematic illustration of responsive hybrid hydrogels based on conformational changes of protein domains. (a) Swelling mechanism of a polyacrylamide-based hydrogel coupled with genetically engineered calmodulin (CaM) and the corresponding ligand phenothiazine. Original state: hydrogel saturated with Ca²⁺ had phenothiazine-binding site accessible and immobilized phenothiazine bound to it forming noncovalent cross-links. Swollen state: Ca²⁺ removed from the CaM-binding site upon introduction of Ca²⁺ -chelating agent ethylene glycol-bis(β-aminoethyl ether)-N, N, N′, N′-tetraacetic acid leads to the disruption of cross-links and swelling of the hydrogel. Reproduced with permission from Ref. . © 2005 Nature Publishing Group. (b) Mechanism of deswelling of photopolymerized PEG-CaM-PEG hydrogel. Hydrogel underwent significant volume reduction upon the conformational change of CaM from an extended conformation (in Ca²⁺ buffer) to a closed conformation (in Ca²⁺ buffer with the ligand trifluoperazine). Reproduced with permission from Ref. . © 2007 Wiley. (c) HPMA-based hydrogels cross-linked with adenylate kinase (AKtm)/DTT. The expected nanoscale conformational change of AKtm upon substrate (ATP, adenosine triphosphate) binding, from open to closed form, concomitantly resulted in a macroscopic volume transition of the hydrogel. Reproduced with permission from Ref. . © 2008 American Chemical Society.

FIGURE 3

Schematic illustration of various hybrids generated via conjugation of α-helical coiled-coil peptides to synthetic polymers. (a) Alternating multiblock hybrids synthesized via conjugation of PEG with coiled-coil peptides through the f-position on the heptads. Top, blue-colored schematic of the coiled coil represents arginine-rich coiled-coil partner, whereas the bottom, green one represents glutamic acid-rich coiled-coil partner. The conjugation of PEG triggered homo-oligomer formation in arginine-rich coiled-coil-PEG conjugate, which is monomeric in the absence of PEG. (b) Fibrin-derived coiled coil conjugated to a central PEG block. The oligomeric association of the coiled-coil end blocks mediates cross-linking of the hybrids to form hydrogel. (c) A synthetic HPMA-based copolymer carrying pendant metal-chelating group of iminodiacetic acid is cross-linked upon the tetrameric association of the histidine-tagged coiled-coil peptides, leading to gelation. (d) Hybrid graft copolymers are produced by grafting coiled-coil peptides on HPMA-based copolymer. Homodimeric association of the coiled-coil peptides leads to cross-linking of the polymer chains and gelation.

FIGURE 4

Schematic illustration of various hybrids generated via conjugation of β-sheet peptides to synthetic polymers. PEG or HPMA-based copolymer conjugated to the β-sheet forming peptides (represented by blue arrow) at the (a) C terminus, (b) N terminus, and (c) both the termini. These peptide–polymer conjugates could form fibrils, mediated by the peptide domain. Schematic (d) represents a three-functional carbazole-based template, which was functionalized with a PEO chain on one side, and two strands of the short (Val-Thr)₂ tetrapeptide on the other side. This tetrapeptide organizer in water-formed interdigitated, antiparallel β-sheets as a core, with PEO chains extended as a shell. Schematic (e) represents poly(n-butyl acrylate) conjugated to β-sheet forming peptides. The peptide block had defect in the primary sequence, which restricted β-sheet formation. However, upon increasing the pH, O- N-acyl migration corrects the primary structure to generate intact β-sheet forming peptide and exhibits higher-order assembly to form fibrils and fibrillar network.

FIGURE 5

Schematic illustration of various hybrids generated via conjugation of silklike β-sheet peptides or polypeptides to synthetic polymers. PEG chains conjugated to (a) silk-inspired β-sheet peptide sequence (indicated by blue arrow) (b) silk-inspired β-sheet peptide sequence preorganized on an aromatic hairpin template to prepare segmented multiblock copolymer. These multiblock hybrid forms microphase-separated architecture displaying nanodomains of the aggregated peptide block interspread within the PEG phase. Schematic (c) represents a hybrid triblock system in which PEG chains were conjugated to central β-sheet forming silklike polypeptide. The peptide block permitted formation of fibrils with dimensions well correlated with the dimensions of the polypeptide chain. Schematic (d) represents a triblock hybrid system with the central block containing β-sheet peptide as pendant side chains. The conformational behavior of silk-inspired β-sheet peptide sequence was still retained in such a design.