Carolin
Bapp‡
a,
Ahmed Z.
Mustafa‡
b,
Cheng
Cao
b,
Erica J.
Wanless
a,
Martina H.
Stenzel
*b and
Robert
Chapman
*a
aSchool of Environmental and Life Sciences, University of Newcastle, Callaghan, NSW 2308, Australia. E-mail: robert.chapman@newcastle.edu.au
bCentre for Advanced Macromolecular Design, School of Chemistry, UNSW Sydney, Kensington, NSW 2052, Australia. E-mail: m.stenzel@unsw.edu.au
First published on 1st July 2025
Using polymers for protein encapsulation can enhance stability in processing environments and prolong activity and half-life in vivo. However, finding the best polymer structure for a target protein can be difficult, labour- and cost-intensive. In this study we introduce a high throughput screening approach to identify strong polymer–protein interactions by use of Förster Resonance Energy Transfer (FRET), enabling a rapid read out. We iteratively screened a total of 288 polymers containing varying hydrophilic, hydrophobic, anionic and cationic monomers against a panel of eight different enzymes (glucose oxidase, uricase, manganese peroxidase, bovine serum albumin, carbonic anhydrase, lysozyme, trypsin and casein). By optimisation of the assay conditions it was possible to read out strongly binding polymers at protein concentrations down to 0.1 μM. We were able to use the screening data to locate moderately selective polymer binders in most cases, and elucidate general trends in polymer design that lead to strong binding. Interestingly, these trends are not consistent across proteins, underscoring the value of a screening approach for identification of the best polymers. We applied this technique to identify lead polymers suitable for encapsulation of the important therapeutic protein TNF-related apoptosis-inducing ligand (TRAIL), at a concentration of 0.25 μM (5 μg mL−1). This approach should be valuable in the design of polymers for either selective protein binding, or for universal protein repulsion, particularly where the protein is too expensive to work with at high concentrations and large volumes.
While we lack the tools to achieve the sequence control of proteins in a synthetic polymer, statistical populations of copolymers (termed random heteropolymers, RHPs) have been shown to very effectively and selectively encapsulate and stabilize proteins,5,6,14,15 and fold and mimic their function.16–18 Key to this approach is the insight that although a random copolymer lacks precise sequence definition, within any given population of appropriately designed copolymers there will exist some sequences that complement the protein surface well enough to encapsulate the protein, or which match the primary sequence well enough to fold and recapitulate the activity of the protein. The group of Ting Xu has shown that even using only four different monomers reflecting the four classes of amino acids (methyl methacrylate (MMA) and 2-ethylhexyl methacrylate (EHMA) for the hydrophobic residues, oligo(ethylene glycol) methyl ether methacrylate (OEGMA) for the hydrophilic residues and 3-sulfopropyl methacrylate potassium salt (SPMA) for the anionic residues), very effective mimics of membrane transport proteins, for example, can be found.18 In their study, proton transfer comparable to natural proton channels was achieved with purely synthetic polymers. Likewise the group has stabilised a range of common enzymes such as horseradish peroxidase,6 lipase and proteinase K.19 Encapsulation of lipase and proteinase K enabled dispersion into poly(caprolactone) and polylactic acid (PLA), respectively, and direct the activity of the enzyme to chain end depolymerisation instead of random chain scission.
While computational approaches can help refine the best RHP combinations to try for any given application,20,21 high throughput experimental screening approaches have been very helpful in refining the design of similar materials.22–28 Recent innovations in high throughput polymer synthesis have enabled screening of polymer structures in the open atmosphere,29 but applying these methods to designing polymers for single protein encapsulation requires efficient read out mechanisms for binding. Using activity assays as the readout, Gormley and coworkers recently applied a high throughput approach to designing polyelectrolytes for glucose oxidase, lipase, and horseradish peroxidase encapsulation.15 By screening a total of over 500 automatically synthesised polymers over five “learn-design-build-test” cycles, polymers able to improve the retained enzyme activity after thermal stress by up to 90% in some cases were identified. Importantly, most polymer combinations were largely inactive, and so this kind of high throughput screen was critical to guiding the polymer design. Unfortunately, easy activity assays are not always readily available for a given protein, particularly in the case of therapeutic proteins, and do not provide a direct read-out of binding. In many cases encapsulation would ‘switch off’ or modulate the activity of the underlying protein. Furthermore, many therapeutic and engineered proteins are very expensive, and this rules out techniques which require large amounts of material such as isothermal calorimetry (ITC), X-ray (SAXS) or light scattering (MALS).
To address this, we recently introduced the use of Förster resonance energy transfer (FRET) as an alternative and complementary method for screening protein–polymer binding.30 In these assays we attached cyanine 3 (Cy3) to our model protein (glucose oxidase) via amide coupling, and cyanine 5 (Cy5) to the polymer via a functionalised monomer and measured the FRET ratio upon excitation of Cy3. Using ITC, SAXS, we demonstrated that the FRET ratio correlates well with binding strength in a small library of positively charged polymers. In this work, we advanced this technique to screen a large polymer library prepared using automated synthesis, against a range of proteins varying in size and surface characteristics including glucose oxidase, uricase, manganese peroxidase, bovine serum albumin, carbonic anhydrase, lysozyme, trypsin and casein. Since the target polymers are soluble in water/alcohol mixtures, their synthesis is compatible with enzyme-assisted RAFT,29,31,32 and so can be prepared in high throughput. The ability to both synthesise polymers and measure protein binding in this way opens up the possibility of automating the design of polyelectrolytes for expensive proteins, as well as those which are hard to find a suitable binding (or non-binding) polymer for. We were interested to understand the limits of this approach to polymer design and to what extent the protein sequence could be used to predict polymer binding. We also sought to understand what general features of polymer design control binding strength (see Fig. 1) and performed complementary activity assays. With this information in hand, we then apply the method to design a polymer capable of encapsulating TNF-related apoptosis-inducing ligand (TRAIL),33–35 an expensive and poorly circulating chemotherapeutic protein.
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Fig. 1 Layout of the two polymer libraries synthesised. Each polymer was made at a degree of polymerisation (DP) 100 with randomly copolymerised hydrophobic, hydrophobic, cationic and anionic monomers. The libraries are organised according to charge in the vertical axis (red-blue scale) and have varying log![]() |
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Any measurements that showed a standard deviation >0.1 in duplicate measurements were excluded, although this was very rare. In total, only 4% of data was excluded, which corresponds to less than 4 FRET ratios per 96 well plate. The entire database of FRET data is provided in csv form in the ESI.†Fig. 2 shows the FRET data for the polymer library at 146 eq. polymer to 0.1 μM GOx, 16 eq. polymer to 0.25 μM GOx, 16 and 32 eq. polymer to 0.5 μM GOx and 16 eq. polymer to 1 μM GOx. Reducing the overall protein concentration at a fixed equivalence of polymer (16 eq.) results in very little difference to the measured FRET ratio, until the GOx concentration drops to 0.25 μM, where the signal begins to fade. Likewise, doubling the polymer concentration from 6 to 32 eq. at a fixed GOx concentration (Fig. 2b) makes very little difference to the measured FRET ratios. This demonstrates the versatility of this readout mechanism. Provided the Cy3 and Cy5 signals are high enough to read, the background signals are generally low enough that the ratio of FRET signal to Cy3 emission doesn't greatly depend upon the assay conditions. At 0.1 μM GOx, it was still possible to read out strong binding polymers provided a large excess of the polymer was used, as shown in Fig. 2b for the 146 eq. polymer:
GOx case. It is noteworthy that the high excess of polymer in this case does not contribute to any significant background, and the very low concentration of Cy3 can still be accurately measured. By diluting a subset of both strong and weak binding polymers we observed that strong binding polymers could be differentiated from weak binding polymers at even ∼25 nM protein (4 μg mL−1, Fig. S2†).
As should be expected from the low isoelectric point of GOx (pI = 4.55), positively charged polymers were required for any binding. Using H or A as the hydrophilic component made little difference to binding, as can be seen by comparing the 1st and 2nd column of each data set in Fig. 2 which are identical in composition but for this difference. The 3rd and 4th columns in each data set show the H bearing polymers and A bearing polymers with a 5 kDa PEG block included and show somewhat stronger binding than the comparable PEG-free polymers in the first two columns. Most strikingly, however, incorporation of the quarternized amine (Q) over the tertiary amine D, and of ∼10% of the phenylalanine mimicking monomer F over other hydrophobic monomers lead to the strongest binding polymers to GOx (Fig. 3). Interestingly, the amino acid make-up of the protein (see the pie chart insert in Fig. 5) was not a very strong predictor of the composition of the strongest binding polymers, and some positively charged polymers which might be expected to bind well did not bind as well as others. We expect that this is in part because the total amino acid make-up of a protein is not necessarily representative of what is presented on the surface – hydrophobic amino acids will tend to be buried. However, it is also because even two different proteins with the same surface amino acid composition can display vastly different distributions. This can be seen by looking at the crystal structures of a panel of proteins used later in this study, obtained from the Protein Data Bank (PDB). Table S1† shows snapshots of the surface potential of these proteins, analysed using UCSF's ChimeraX program10 in order to calculate and visualise the protein's surface coulombic electrostatic potential (ESP) and hydrophobicity, also known as molecular lipophilicity potential (MLP). It is for this reason that screening methodologies are valuable – it is very difficult to predict from first principles which polymers will bind strongly and selectively to a given protein, but relatively easy to find by iterative screening.
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Fig. 3 Detail from Fig. 2, showing the FRET hits (FRET ratio >0.375) of library 1 and corresponding polymers at a GOx concentration of 0.50 μM at 16 eq. polymer to enzyme. In each row the composition of monomers is the same, except for the base monomer m (H or A), plus PEG block for the last two columns. The results show high FRET binding for polymers with benzyl acrylate (F) and monomer Q. |
To test whether the length of the polymer is influencing the binding strength, a strongly binding polymer (P(H50-co-N40-co-Q10)) and a weakly binding polymer (P(H90-co-F10)) were synthesized at 5 different chain lengths (DP = 25, 50, 100, 200 and 400). We fixed the total Cy5 content in each polymerisation such that the concentration of dye is equivalent in each sample, but this means that longer polymers will contain more Cy5 monomers/chain. Because of this, we investigated the FRET from experiments in which the [Cy5] was held constant (resulting in different molar equivalences of the polymer) and from experiments in which the molar ratio of polymer was held constant (resulting in different concentrations of Cy5). Both were conducted at a GOx concentration of 1 μM. When the [Cy5] was held constant the FRET ratios were the same for all polymers (see Fig. S4, ESI†). Our results from the DP100 screens (Fig. 2) demonstrated that the molar equivalences of polymer has little influence on the FRET result, so the lack of increase in FRET as a function of chain length here demonstrates that this variable has little (if any) impact on binding. When the polymer concentration was fixed, the FRET ratio did increase with increasing chain length, but we believe this is due to the commensurate increase in Cy5 concentration and not due to any stronger binding. Longer polymers have a greater chance of bearing a Cy5 monomer than shorter polymers (as the ratio of Cy5:
total monomer is fixed in our polymerisations, but not the ratio of Cy5
:
chains). This means that if the [Cy5] in the FRET experiment is fixed, there is a higher chance of the polymer bound to the protein bearing a Cy5 in the case of longer polymers. Further evidence for this is given by the fact that the increase in FRET ratio as a function of chain length in this experiment is greater for the more strongly binding polymer. The amount of Cy5 per chain in the weakly binding polymer doesn't influence the FRET result by as much, because fewer binding events occur.
In most cases we screened at multiple polymer equivalences. While the overall trends did not change as the amount of polymer was varied, the optimised screens for each protein shown in Fig. 6 were all performed at 3 to 7 equivalents of polymer to protein. As can be seen in this figure, different proteins result in different ‘polymer hits’. A general trend corresponding to the pI of the enzymes is observed, with the best binding occurring at net lower polymer charge as the pI increased. This is seen most dramatically for the highly cationic protein lysozyme, where hits for good binding are only found for polymers with negative charge. However, while there are some common hits, screening in this way enables selection of a polymer in each case which is at least moderately selective towards the target protein. Such selectivity is still a long way from that which could be achieved by phage display for peptides,39 but is none the less notable given these polymers are simply copolymerised structures with no particular attention to sequence control. Interestingly different proteins seemed to prefer different polymer architectures, and this can be seen most clearly in the statistical analysis in Fig. 7a–c. This analysis compares the FRET ratio achieved for comparable polymers with and without PEG in the side chain (a), as a block (b) and comparing Hvs.A monomers (c) across all the proteins in the screen. Positive values in this analysis therefore represent an increase in binding strength results from each attribute in the polymer, while values close to zero indicate that this modification has negligible effect on the polymer binding strength. As expected from the GOx screen above, there is little effect of changing the hydrophilic monomer H to A in the polymer on binding to any protein. However, for carbonic anhydrase and uricase, binding is preferred for the PEG block copolymers, whereas GOx also shows good hits for polymers without any PEG components. By contrast, manganese peroxidase shows good binding with polymers with grafted PEG. As only a selection of polymers from the library has been screened against manganese peroxidase, it is unknown if PEG block copolymers would have resulted in even higher FRET ratios. These proteins have vastly different molecular weights, pI (Fig. 6) and surface characteristics (Table S1†), so these trends would be hard to predict from first principles.
We then proceeded to apply this assay to find polymer structures capable of encapsulating a therapeutically relevant protein, TNF-related apoptosis-inducing ligand (TRAIL). TRAIL is a promising chemotherapeutic protein, which our group has worked on extensively,26,40,41 but very expensive to purchase in any significant quantity. TRAIL works by clustering death receptor proteins (DR4 and DR5) which are selectively present on the surface of many cancer cells, which drives production of caspase 8 and ultimately cell death. While it is an effective drug in vitro it has shown poor performance in vivo, at least in part due to its very low circulation half-life (∼30 min in humans).42 We reasoned that encapsulation in a weak binding polymer could improve circulation half-life, without restricting its activity. TRAIL has a similar molecular weight and distribution of amino acids in its primary sequence to carbonic anhydrase (see ESI Fig. S6†). As a result, we selected 18 polymers from our library which had bound well to this protein, along with some negative controls, and screened them in duplicate against a Cy3-labelled TRAIL. As previously, analysis of the primary sequence provides only a rough starting point for determining the optimal polymer, but from this FRET screen we were able to identify three potential lead polymers with stronger binding than the others, even at only 5 μg mL−1 protein in a 40 μL sample well (see ESI† for the polymer compositions associated with each code). Fig. 7d shows each replicate, ordered by average FRET. The best leads (polymers #1–3 in this figure) show significantly elevated FRET (p < 0.0001) than the weakest 10 polymers and correspond to anionic PEG block copolymers with acrylamide (2A22 = PEG-b-P(A80-co-C20), 2A24 = PEG-b-P(A80-co-C50), and 2H28 = PEG-b-P(C100)). The pI of this ∼20 kDa TRAIL fragment is 8.89, so the fact that anionic polymers perform the best is not surprising. This data is also consistent with the finding from the protein panel screening that PEG-blocked polymers tend to work the best. We propose that these structures could be valuable leads to pursue in improving the delivery of this protein as a polyion complex.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5sc04391c |
‡ These two authors contributed equally. |
This journal is © The Royal Society of Chemistry 2025 |