More than 8 billion tons of plastic waste has been generated, posing severe environmental consequences and health risks. Due to prolonged exposure, microplastic particles are found in human blood and other bodily fluids. Despite a lack of toxicity studies regarding microplastics, harmful effects for humans seem plausible and cannot be excluded. As small plastic particles readily translocate from the gut to body fluids, enzyme-based treatment of serum could constitute a promising future avenue to clear synthetic polymers and their corresponding oligomers via their degradation into monomers of lower toxicity than the material they originate from. Still, whereas it is known that the enzymatic depolymerization rate of synthetic polymers varies by orders of magnitude depending on the buffer and media composition, the activity of plastic-degrading enzymes in serum was unknown. Here, we report how an engineered PETase, which we show to be generally trans-selective via induced fit docking, can depolymerize two different microplastic-like substrates of the commodity polymer polyethylene terephthalate (PET) into its non-toxic monomer terephthalic acid (TPA) alongside mono(2-hydroxyethyl)terephthalate (MHET) in human serum at 37 °C. We show that the application of PETase does not influence cell viability in vitro. Our work highlights the potential of applying biocatalysis in biomedicine and represents a first step towards finding a future solution to the problem that microplastics in the bloodstream may pose.

We report the application of ancestral sequence reconstruction on coronavirus spike protein, resulting in stable and highly soluble ancestral scaffold antigens (AnSAs). The AnSAs interact with plasma of patients recovered from COVID-19 but do not bind to the human angiotensin-converting enzyme 2 (ACE2) receptor. Cryo-EM analysis of the AnSAs yield high resolution structures (2.6-2.8 angstrom) indicating a closed pre-fusion conformation in which all three receptor-binding domains (RBDs) are facing downwards. The structures reveal an intricate hydrogen-bonding network mediated by well-resolved loops, both within and across monomers, tethering the N-terminal domain and RBD together. We show that AnSA-5 can induce and boost a broad-spectrum immune response against the wild-type RBD as well as circulating variants of concern in an immune organoid model derived from tonsils. Finally, we highlight how AnSAs are potent scaffolds by replacing the ancestral RBD with the wild-type sequence, which restores ACE2 binding and increases the interaction with convalescent plasma. Development of vaccines remains challenging because viral antigens can be unstable or aggregate. Here, authors present ancestral sequence reconstruction as a method to generate stable and soluble antigens using exclusively available sequence information.

Enzymes are natures catalysts that increase the rate of a chemical reaction. The increased rate of a reaction is required to be able to sustain life. Despite the huge impact of enzymes, they are not perfect catalysts. Enzyme and protein engineering is the discipline in which proteins are characterized and engineered to have improved inherent properties. Interesting properties of an enzyme to improve include stability and activity. The aim of this work is to understand how proteins and enzymes function and use a variety of different protein engineering techniques to enhance the properties of different proteins. In this work proteins and enzymes are engineered to increase our knowledge of the target proteins for downstream biomedical applications. A mix between rational and semi-rational engineering is applied in this work. In paper I and paper II, the method used is ancestral sequence reconstruction. A method that utilizes the evolutionary relationship between homologous sequences. In paper I the method was applied to a terpene cyclase, which cyclizes a precursor terpene into potential interesting drug leads. The result was a hyperstable enzyme variant. In paper II the technique was applied to the SARS-CoV-2 Spike protein. The protein is responsible for the virus SARS-CoV-2 to enter human cells. The work yielded a stable spike protein that readily expresses and can be utilized as a vaccine lead. In paper III, the aim was to understand human oxidosqualene cyclase (hOSC). A terpene cyclase essential in cholesterol synthesis. The enzyme hOSC was rationally engineered to change the driving force of the reaction. Through targeted mutations the reaction changed from entropy driven to enthalpy driven. Finally, in paper IV, a rationally engineered PETase, which is capable of degrading PET polymers into monomers, was proven to be active in human serum and verifies the proof-of-concept of degrading plastic in human blood. To summarize, the results in this thesis show the applicability of different enzyme engineering techniques to stabilize or change the function of proteins and the potential of engineered proteins in medical applications.

Enzymer är naturens katalysatorer vilka höjer hastigheten av kemiska reaktioner. En ökad hastighet av en reaktion är nödvändig för att upprätthålla liv. Trots enzymers och proteiners stora påverkan på reaktionshastigheter så är de inte perfekta katalysatorer. Enzym- och proteindesign är en vetenskap där enzymer och proteiner karaktäriseras och designas för att förbättra vissa egenskaper hos dem. Egenskaper hos enzymer så som stabilitet och aktivitet är intressanta att förbättra. Syftet med det här arbetet är att förstå hur proteiner och enzymer fungerar, samt deras egenskaper för olika applikationer i ett senare skede, exempelvis inom biomedicin. En blandning av rationell och semi-rationell enzym- och proteindesign används i det här arbetet. I artikel I och II används metoden ancestral sekvensrekonstruering. Metoden nyttjar de evolutionära sambanden mellan homologa sekvenser. I artikel I användes metoden på ett terpencyklas, ett enzym som skapar ringstrukturer hos en terpenföregångare, vilket resulterar i molekyler som kan vara intressanta för användning i läkemedel. Resultatet blev en hyperstabil enzymvariant. I artikel II användes metoden på SARS-CoV-2 Spike protein. Proteinet är ansvarigt för att viruset SARS-CoV-2 kan ta sig in i mänskliga celler. Arbetet resulterade i ett stabilt spike protein som lätt uttrycks med potentiell användning i vaccintillverkning. I artikel III var syftet att förstå humant oxidoskvalencyklas (hOSC). hOSC är ett terpencyklas som är nödvändig i syntesen av kolesterol. Enzymet designades för att ändra den drivande kraften hos reaktionen. Genom riktade mutationer ändrades reaktionen från att vara entropidriven till att vara entalpidriven. Slutligen, i artikel IV visades hur ett designat PETase, som bryter ned PET polymerer till monomerer, är aktivt i mänskligt serum och bekräftar att nedbrytning av plast i mänskligt blod är möjligt. Sammanfattningsvis, resultaten i den här avhandlingen visar hur olika enzymdesignstekniker kan appliceras för att stabilisera eller ändra funktioner hos proteiner och potentialen av designade enzymer i medicinska applikationer.

Thermostability is the key to maintain the structural integrity and catalytic activity of enzymes in industrial biotechnological processes, such as terpene cyclase-mediated generation of medicines, chiral synthons, and fine chemicals. However, affording a large increase in the thermostability of enzymes through site directed protein engineering techniques can constitute a challenge. In this paper, we used ancestral sequence reconstruction to create a hyperstable variant of the ent-copalyl diphosphate synthase PtmT2, a terpene cyclase involved in the assembly of antibiotics. Molecular dynamics simulations on the its timescale were performed to shed light on possible molecular mechanisms contributing to activity at an elevated temperature and the large 40 degrees C increase in melting temperature observed for an ancestral variant of PtmT2. In silico analysis revealed key differences in the flexibility of a loop capping the active site, between extant and ancestral proteins. For the modern enzyme, the loop collapses into the active site at elevated temperatures, thus preventing biocatalysis, whereas the loop remains in a productive conformation both at ambient and high temperatures in the ancestral variant. Restoring a Pro loop residue introduced in the ancestral variant to the corresponding Gly observed in the extant protein led to reduced catalytic activity at high temperatures, with only moderate effects on the melting temperature, supporting the importance of the flexibility of the capping loop in thermoadaptation. Conversely, the inverse Gly to Pro loop mutation in the modern enzyme resulted in a 3-fold increase in the catalytic rate. Despite an overall decrease in maximal activity of ancestor compared to wild type, its increased thermostability provides a robust backbone amenable for further enzyme engineering. Our work cements the importance of loops in enzyme catalysis and provides a molecular mechanism contributing to thermoadaptation in an ancestral enzyme.

The pandemic caused by Severe acute respiratory syndrome coronavirus 2 has had devastating consequences on global health and economy. Despite the success of vaccination campaigns emerging variants are of concern and novel viruses with the potential to drive future pandemics are circulating in nature. Development of vaccines can be challenging, as key viral protein antigens can be unstable or aggregate. In this study, we present the application of ancestral sequence reconstruction on coronavirus spike protein, resulting in stable and highly soluble ancestral scaffold antigens (AnSAs). The AnSAs interacted with plasma of patients recovered from COVID-19 but did not bind to the human angiotensin-converting enzyme 2 (ACE2) receptor. Cryo-EM analysis of the AnSAs yielded high resolution structures (2.6-2.8 Å) indicating a closed pre-fusion conformation in which all three receptor-binding domains (RBDs) are facing downwards. This captured closed state is stabilised by an intricate hydrogen‑bonding network mediated by well-resolved loops, both within and across monomers, tethering the N‑terminal domain and RBD together, which determines their relative spatial orientation. Finally, we show how AnSAs are potent scaffolds by replacing the ancestral RBD with the Wuhan wild-type sequence, which restored ACE2 binding and increased the interaction with convalescent plasma. In contrast to rational antigen design depending on prior structural knowledge, our work highlights how stable and potentially interesting antigens can be generated using exclusively available sequence information.

The formation of tetracyclic lanosterol from (S)-2,3-oxidosqualene is catalyzed by oxidosqualenecyclase (OSC). Lanosterol is of high interest due to its essential role in steroid metabolism. Therefore,understanding how the inherent high entropic cost of forming a multicyclic core from a flexible linearsubstrate is energetically driven is of high interest. Enzyme mechanisms can involve a reducedhydration state of rearranging transient charges in intermediates and transition states. Often thesereactions have an unusually low entropy barrier. We studied the activation enthalpy and entropy inrelation to solvent tunnels accessing the active site in the carbocationic polycyclization cascadecatalyzed by human OSC (hOSC). We applied Eyring transition state analysis of lanosterol formationby hOSC at different temperatures, alongside Molecular Dynamics simulations and CAVER analysis.hOSC showed a high favorable entropy of activation (+6.4 kcal mol-1 at 310 K) at ambienttemperatures. The introduction of bulky residues at the interface of several water tunnels, resulted inenzyme variants with altered thermodynamic properties. One of the variants was enthalpy-driven andshowed an inversed temperature dependence of cyclization. We further biochemically characterizeddifferent enzyme libraries, in which rational tunnel-mutations combined with mutations suggested byphylogeny-guided protein design were introduced in different combinations. This approach yieldedseveral highly active hOSC variants (5-6x increased activity at 37 °C), as well as a highly active variantat colder temperature with inversed temperature dependence. In summary, the present workhighlights the importance of activation entropy in enzymes, which is often considered negligible, aswell as the challenges associated with rational protein design aiming to modify activationthermodynamic parameters.