Using synthetically modified proteins to make new materials - PubMed (original) (raw)

. 2011 Sep 20;44(9):774-83.

doi: 10.1021/ar2001292. Epub 2011 Aug 3.

Affiliations

Using synthetically modified proteins to make new materials

Leah S Witus et al. Acc Chem Res. 2011.

Abstract

The uniquely diverse structures and functions of proteins offer many exciting opportunities for creating new materials with advanced properties. Exploiting these capabilities requires a set of versatile chemical reactions that can attach nonnatural groups to specific locations on protein surfaces. Over the years, we and others have developed a series of new techniques for protein bioconjugation, with a particular emphasis on achieving high site selectivity and yield. Using these reactions, we have been able to prepare a number of new materials with functions that depend on both the natural and the synthetic components. In this Account, we discuss our progress in protein bioconjugation over the past decade, focusing on three distinct projects. We first consider our work to harness sunlight artificially by mimicking features of the photosynthetic apparatus, with its beautifully integrated system of chromophores, electron transfer groups, and catalytic centers. Central to these photosystems are light-harvesting antennae having hundreds of precisely aligned chromophores with positions that are dictated by the proteins within the arrays. Our approach to generating similar arrangements involves the self-assembly of tobacco mosaic virus coat proteins bearing synthetic chromophore groups. These systems offer efficient light collection, are easy to prepare, and can be used to build complex photocatalytic systems through the modification of multiple sites on the protein surfaces. We then discuss protein-based carriers that can deliver drugs and imaging agents to diseased tissues. The nanoscale agents we have built for this purpose are based on the hollow protein shell of bacteriophage MS2. These 27 nm capsids have 32 pores, which allow the entry of relatively large organic molecules into the protein shell without requiring disassembly. Our group has developed a series of chemical strategies that can install dyes, radiolabels, MRI contrast agents, and anticancer drugs on the inside surface of these capsids. We have also developed methods to decorate the external surfaces with binders for specific proteins on cancer cells. As a third research area, our group has developed protein-polymer hybrid materials for water remediation. To reduce the toxicity of heavy metals in living cells, Nature has evolved metallothioneins, which are sulfur-rich polypeptides that bind mercury, cadmium, and other toxic ions at sub-parts-per-billion concentrations. Unfortunately, these proteins are very difficult to incorporate into polymers, largely because typical protein modification reactions target the very cysteine, lysine, and carboxylate-containing residues that are required for their proper function. To address this challenge, we developed a new way to attach these (and many other) proteins to polymer chains by expressing them as part of an N- and C-terminal modification "cassette". The resulting materials retain their selectivity and can remove trace amounts of toxic metal ions from ocean water. Each of these examples has presented a new set of protein bioconjugation challenges that have been met through the development of new reaction methodology. Future progress in the generation of protein-based materials will require scalable synthetic techniques with improved yields and selectivities, inexpensive purification methods for bioconjugates, and theoretical and dynamical treatments for designing new materials through protein self-assembly.

© 2011 American Chemical Society

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Figures

Figure 1

Figure 1

Self-assembling light harvesting systems based on the coat protein of the tobacco mosaic virus (TMVP). (a) S123C TMVP monomers covalently labeled with either OG or AF were combined and self-assembled under disk and rod-forming conditions. (b) Excitation spectra for rod assemblies indicated light harvesting behavior through the increase in sensitivity at 500 nm with increasing donor fraction. TEM images of chromophore-labeled TMVP disks (c) and rods (d) are shown.

Figure 2

Figure 2

Energy transfer in multidimensional light harvesting arrays. (a) A model of the chromophore distribution in TMVP is provided. The calculated positions and orientations of the chromophores are shown in (b), indicating that the transition dipoles (green) align along the long axis of the rod. (c) In a 1D array or a ring, defects can block transfer pathways. (d) In multidimensional systems, redundant energy transfer pathways can circumvent these sites. (e) Electron transfer groups (blue) can be attached to the N-termini through transamination followed by oxime formation., These positions are shown for the native sequence in (f) and a cyclic permuatant in (g).

Figure 3

Figure 3

Synthesis of targeted delivery agents from bacteriophage MS2 capsids. (a) The interior surface can be modified by targeting a native tyrosine or an introduced cysteine residue. (b) Several types of cargo molecules (90–180 copies of each) have been attached to these sites.– (c) Targeting groups can be installed on the external surface by modifying an artificial amino acid using a new oxidative coupling reaction., These strategies can be combined to yield targeted particles for therapeutic applications, such as the structure in (d), which combines protein tyrosine kinase 7 (PTK7) binding aptamers with porphyrins for singlet oxygen generation.

Figure 4

Figure 4

Modification of metallothioneins (MTs) for the creation of metal-binding hydrogels. These small proteins have 12 to 20 cysteine residues (orange in a). A dual modification strategy was used to crosslink poly(2-hydroxypropyl methacrylamide) (HPMA) polymers with the MT sequences. (b) The intitial gels (left) shrink considerably in the presence of contaminant ions (right). (c) The gels respond to a number of toxic ions. The blue bars indicate the percentage of initial gel volume after the binding and contraction have occurred. In most cases the gels can be recycled using inexpensive chelators, such as EDTA (red bars). (d) Water from a saline marsh was spiked with four contaminant ions at ~50 ppb each. Upon addition of the MT hydrogel, the concentrations of all four toxic species were simultaneously reduced to below 10 ppb (left graph). In contrast, the concentrations of much more abundant Na+, K+, Ca2+, and Mg2+ ions were unchanged (right graph).

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