Genetic PEGylation

Therapeutic peptides: current applications and future directions

Peptide drug development has made great progress in the last decade thanks to new production, modification, and analytic technologies. Peptides have been produced and modified using both chemical and biological methods, together with novel design and delivery strategies, which have helped to overcome the inherent drawbacks of peptides and have allowed the continued advancement of this field. A wide variety of natural and modified peptides have been obtained and studied, covering multiple therapeutic areas. This review summarizes the efforts and achievements in peptide drug discovery, production, and modification, and their current applications. We also discuss the value and challenges associated with future developments in therapeutic peptides.

PEGylation near a Patch of Nonpolar Surface Residues Increases the Conformational Stability of the WW Domain

Many proteins have one or more surface-exposed patches of nonpolar residues; our observations here suggest that PEGylation near such locations might be a useful strategy for increasing protein conformational stability. Specifically, we show that conjugating a PEG-azide to a propargyloxyphenylalanine via the copper(I)-catalyzed azide-alkyne cycloaddition can increase the conformational stability of the WW domain due to a favorable synergistic effect that depends on the hydrophobicity of a nearby patch of nonpolar surface residues.

Influence of PEGylation on the Strength of Protein Surface Salt Bridges

Conjugation of polyethylene glycol (PEGylation) is a well-known strategy for extending the serum half-life of protein drugs and for increasing their resistance to proteolysis and aggregation. We previously showed that PEGylation can increase protein conformational stability; the extent of PEG-based stabilization depends on the PEGylation site, the structure of the PEG-protein linker, and the ability of PEG to release water molecules from the surrounding protein surface to the bulk solvent. The strength of a noncovalent interaction within a protein depends strongly on its microenvironment, with salt-bridge and hydrogen-bond strength increasing in nonpolar versus aqueous environments. Accordingly, we wondered whether partial desolvation by PEG of the surrounding protein surface might result in measurable increases in the strength of a salt bridge near a PEGylation site. Here we explore this possibility using triple-mutant box analysis to assess the impact of PEGylation on the strength of nearby salt bridges at specific locations within three peptide model systems. The results indicate that PEG can increase the nearby salt-bridge strength, though this effect is not universal, and its precise structural prerequisites are not a simple function of secondary structural context, of the orientation and distance between the PEGylation site and salt bridge, or of salt-bridge residue identity. We obtained high-resolution X-ray diffraction data for a PEGylated peptide in which PEG enhances the strength of a nearby salt bridge. Comparing the electron density map of this PEGylated peptide versus that of its non-PEGylated counterpart provides evidence of localized protein surface desolvation as a mechanism for PEG-based salt-bridge stabilization.

How PEGylation influences protein conformational stability

PEGylation is an important strategy for enhancing the pharmacokinetic properties of protein therapeutics. The development of chemoselective side-chain modification reactions has enabled researchers to PEGylate proteins with high selectivity at defined locations. However, aside from avoiding active sites and binding interfaces, there are few guidelines for the selection of optimal PEGylation sites. Because conformational stability is intimately related to the ability of a protein to avoid proteolysis, aggregation, and immune responses, it is possible that PEGylating a protein at sites where PEG enhances conformational stability will result in PEG-protein conjugates with enhanced pharmacokinetic properties. However, the impact of PEGylation on protein conformational stability is incompletely understood. This review describes recent advances toward understanding the impact of PEGylation on protein conformational stability, along with the development of structure-based guidelines for selecting stabilizing PEGylation sites.

Two site genetic incorporation of varying length polyethylene glycol into the backbone of one peptide

Polyethylene glycol (PEG) of different lengths was genetically incorporated into the backbone of a polypeptide using stop-anticodon and frameshift anticodon-containing tRNAs, which were acylated with PEG-containing amino acids.

Repurposing the translation apparatus for synthetic biology

The translation system (the ribosome and associated factors) is the cell's factory for protein synthesis. The extraordinary catalytic capacity of the protein synthesis machinery has driven extensive efforts to harness it for novel functions. For example, pioneering efforts have demonstrated that it is possible to genetically encode more than the 20 natural amino acids and that this encoding can be a powerful tool to expand the chemical diversity of proteins. Here, we discuss recent advances in efforts to expand the chemistry of living systems, highlighting improvements to the molecular machinery and genomically recoded organisms, applications of cell-free systems, and extensions of these efforts to include eukaryotic systems. The transformative potential of repurposing the translation apparatus has emerged as one of the defining opportunities at the interface of chemical and synthetic biology.

Protein synthesis using a reconstituted cell-free system

Most cell-free protein-synthesis systems are based on cell extracts, which often contain undesirable activities. Reconstituted systems, by contrast, are composed of a defined number of purified and recombinant components with minimal nuclease and protease activities. This unit describes the use of a particular commercial reconstituted system, PURExpress. This system allows in vitro synthesis of proteins from mRNA and circular and linear DNA templates, as well as co-translational labeling of proteins. Unique to this system, all recombinant protein components of the system are His-tagged, allowing purification of the synthesized untagged protein by removing the rest of the system's components. Newly synthesized proteins can often be visible on an SDS-PAGE gel and directly assayed for their functions without labeling and purification. Certain components of the system, such as ribosomes or release factors, can be omitted for specific applications. Such "delta" versions of the system are well suited for studies of bacterial translation, assays of ribosome function, incorporation of unnatural amino acids, and ribosome display of protein libraries.

The emerging age of cell-free synthetic biology

The engineering of and mastery over biological parts has catalyzed the emergence of synthetic biology. This field has grown exponentially in the past decade. As increasingly more applications of synthetic biology are pursued, more challenges are encountered, such as delivering genetic material into cells and optimizing genetic circuits in vivo. An in vitro or cell-free approach to synthetic biology simplifies and avoids many of the pitfalls of in vivo synthetic biology. In this review, we describe some of the innate features that make cell-free systems compelling platforms for synthetic biology and discuss emerging improvements of cell-free technologies. We also select and highlight recent and emerging applications of cell-free synthetic biology.

In vitro selection of a peptide aptamer that potentiates inhibition of cyclin-dependent kinase 2 by purvalanol

To increase the inhibitory activity of purvalanol against cyclin-dependent kinase 2, we increased the extent of interaction between the inhibitor and the target by coupling a peptide aptamer to purvalanol.

Bio-orthogonal and combinatorial approaches for the design of binding growth factors

Merrifield chemistry enables the convenient synthesis of oligonucleotides and peptides, while recombinant DNA technology has facilitated protein engineering. Recently, protein engineering has been extended into bio-orthogonal protein engineering by the development of specific chemical or enzymatic modification technologies. The combinatorial approach of molecular evolutionary engineering (or in vitro selection) has also provided a new design tool for functional peptides. These methodologies have enabled the development of various new proteinaceous materials for biological and medical applications. Here, we will discuss recent progress in the molecular design of proteins with respect to the preparation of binding growth factors, which are of increasing importance in the biomaterials field.