Influence of PEGylation on Domain Dynamics of Phosphoglycerate Kinase: PEG Acts Like Entropic Spring for the Protein

Molecular Dynamics Simulations for Rationalizing Polymer Bioconjugation Strategies: Challenges, Recent Developments, and Future Opportunities

The covalent modification of proteins with polymers is a well-established method for improving the pharmacokinetic properties of therapeutically valuable biologics. The conjugated polymer chains of the resulting hybrid represent highly flexible macromolecular structures. As the dynamics of such systems remain rather elusive for established experimental techniques from the field of protein structure elucidation, molecular dynamics simulations have proven as a valuable tool for studying such conjugates at an atomistic level, thereby complementing experimental studies. With a focus on new developments, this review aims to provide researchers from the polymer bioconjugation field with a concise and up to date overview of such approaches. After introducing basic principles of molecular dynamics simulations, as well as methods for and potential pitfalls in modeling bioconjugates, the review illustrates how these computational techniques have contributed to the understanding of bioconjugates and bioconjugation strategies in the recent past and how they may lead to a more rational design of novel bioconjugates in the future.

Instability Challenges and Stabilization Strategies of Pharmaceutical Proteins

Maintaining the structure of protein and peptide drugs has become one of the most important goals of scientists in recent decades. Cold and thermal denaturation conditions, lyophilization and freeze drying, different pH conditions, concentrations, ionic strength, environmental agitation, the interaction between the surface of liquid and air as well as liquid and solid, and even the architectural structure of storage containers are among the factors that affect the stability of these therapeutic biomacromolecules. The use of genetic engineering, side-directed mutagenesis, fusion strategies, solvent engineering, the addition of various preservatives, surfactants, and additives are some of the solutions to overcome these problems. This article will discuss the types of stress that lead to instabilities of different proteins used in pharmaceutics including regulatory proteins, antibodies, and antibody-drug conjugates, and then all the methods for fighting these stresses will be reviewed. New and existing analytical methods that are used to detect the instabilities, mainly changes in their primary and higher order structures, are briefly summarized.

Neutron Scattering Techniques and Complementary Methods for Structural and Functional Studies of Biological Macromolecules and Large Macromolecular Complexes

Abstract

The review describes the application of small-angle scattering (SAS) of neutrons and complementary methods to study the structures of biomacromolecules. Here we cover SAS techniques, such as the contrast variation, the neutron spin-echo, and the solution of direct and inverse problems of three-dimensional reconstruction of the structures of macromolecules from SAS spectra by means of molecular modeling. A special section is devoted to specific objects of research, such as supramolecular complexes, influenza virus nucleoprotein, and chromatin.

Poly(propylene imine) dendrimers can bind to PEGylated albumin at PEG and albumin surface: Biophysical examination of a PEGylated platform to transport cationic dendritic nanoparticles

Cationic dendrimers are considered one of the best drug transporters in the body. However, in order to improve their biocompatibility, modification of them is required to reduce toxicity. In this way, many dendrimers may lose their original properties, for example, anticancer. To improve biocompatibility of dendrimers, it is possible to complex them with albumin, as is done very often in drug delivery. However, the interaction of dendrimers with albumin can lead to protein structure disruption or no complexation at all. Therefore, the investigation of the interaction between cationic poly-(propylene imine) dendrimers and polyethylene glycol (PEG)-albumin by fluorescence, circular dichroism, small angle X-ray scattering (SAXS), and transmission electron microscopy was carried out. Results show that cationic dendrimers bind to PEGylated albumin at PEG and albumin surfaces. The obtained results for 5k-PEG indicate a preferential binding of the dendrimers to PEG. For 20k-PEG binding of dendrimers to PEG and protein could induce a collapse of the PEG chain onto the protein surface. This opens up new possibilities to the use of PEGylated albumin as a platform to carry dendrimers without changing the albumin structure and improve the pharmacokinetic properties of dendrimers without further modification.

Transition between protein-like and polymer-like dynamic behavior: Internal friction in unfolded apomyoglobin depends on denaturing conditions

Equilibrium dynamics of different folding intermediates and denatured states is strongly connected to the exploration of the conformational space on the nanosecond time scale and might have implications in understanding protein folding. For the first time, the same protein system apomyoglobin has been investigated using neutron spin-echo spectroscopy in different states: native-like, partially folded (molten globule) and completely unfolded, following two different unfolding paths: using acid or guanidinium chloride (GdmCl). While the internal dynamics of the native-like state can be understood using normal mode analysis based on high resolution structural information of myoglobin, for the unfolded and even for the molten globule states, models from polymer science are employed. The Zimm model accurately describes the slowly-relaxing, expanded GdmCl-denaturated state, ignoring the individuality of the different aminoacid side chain. The dynamics of the acid unfolded and molten globule state are similar in the framework of the Zimm model with internal friction, where the chains still interact and hinder each other: the first Zimm relaxation time is as large as the internal friction time. Transient formation of secondary structure elements in the acid unfolded and presence of α-helices in the molten globule state lead to internal friction to a similar extent.

PEGylation within a confined hydrophobic cavity of a protein

The conjugation of polyethylene glycol (PEG) to proteins, known as PEGylation, has increasingly been employed to expand the efficacy of therapeutic drugs. Recently, research has emphasized the effect of the conjugation site on protein-polymer interactions. In this study, we performed atomistic molecular dynamics (MD) simulations of lysine 116 PEGylated bovine serum albumin (BSA) to illustrate how conjugation near a hydrophobic pocket affects the conjugate's dynamics and observed altered low mode vibrations in the protein. MD simulations were performed for a total of 1.5 μs for each PEG chain molecular mass from 2 to 20 kDa. Analysis of preferential PEG-BSA interactions showed that polymer behavior was also affected as proximity to the attractive protein surface patches promoted interactions in small (2 kDa) PEG chains, while the confined environment of the conjugation site reduced the expected BSA surface coverage when the polymer molecular mass increased to 10 kDa. This thorough analysis of PEG-BSA interactions and polymer dynamics increases the molecular understanding of site-specific PEGylation and enhances the use of protein-polymer conjugates as therapeutics.

Measuring the Impact of PEGylation on a Protein-Polysaccharide Interaction

PEGylation is the most widely used half-life extension strategy for protein therapeutics. While it imparts a range of attractive attributes PEGylation can impede protein binding and reduce efficacy. A model system to probe the effects of PEGylation on protein binding has practical applications. Here, we present a system based on complex formation between a hexavalent lectin (RSL) and the globular polysaccharide Ficoll PM70 (a type of glycocluster). Mutants of the lectin were used to generate conjugates with 3, 6, or 12 PEG (1 kDa) chains. Using NMR spectroscopy we monitored how the degree of PEGylation impacted the lectin-Ficoll interaction. The binding propensity was observed to decrease with increasing polymer density. Apparently, the extended PEG chains sterically impede the lectin-Ficoll binding. This deduction was supported by molecular dynamics simulations of the protein-polymer conjugates. The implications for protein-surface interactions are discussed.

A guide to maximizing the therapeutic potential of protein-polymer conjugates by rational design

Proteins are an important class of therapeutics that have advantages including high target specificity, but challenges to their use include rapid clearance and low physical stability. Conjugation of synthetic polymers is an effective approach to address the drawbacks and enhance other properties such as solubility. In this review, we present various considerations in synthesizing protein-polymer conjugates for therapeutic applications with a focus on the choice of polymer, protein, and conjugation method, as well as characterization and evaluation of the resulting conjugate in order to maximize the therapeutic potential of the protein drug.