Enzyme Immobilisation Using Porous Frameworks

Abstract

Enzymes are often sought after for applications in industry and synthetic chemistry due to their high reactivity and substrate selectivity, often surpassing their chemical counterparts. They are, however, limited by their structural instability and require restrictive environmental conditions that are often not compatible with industrial processing. As such, new technologies are required to protect enzymes from non-biological conditions. This thesis investigates enzyme immobilisation using porous frameworks including metal-organic frameworks (MOFs) and hydrogen-bonded organic frameworks (HOFs). The diverse nature of both the enzyme and MOF/HOF components offers great potential for creating a broad library of biocomposites with novel function. There are however inherent challenges in finding appropriate conditions for immobilisation in which the enzyme remains active, and where the overall biocomposite is stable. Initial studies utilised Zeolitic Imidazolate Framework 8 (ZIF-8), a subclass of MOFs, for protein immobilisation. The addition of biomacromolecules, such as proteins, can promote the self-assembly of ZIF-8 by a process known as “biomimetic mineralisation”. Systematic screening studies established that this process is biomacromolecule dependent, with a subset of proteins requiring the addition of organic solvent or increased ligand concentrations to promote ZIF-8 nucleation. These reaction conditions were also instrumental in controlling the topology, morphology, and particle size of the biocomposites. Investigations into the influence of the protein revealed that biomimetic mineralisation is governed by the surface chemistry of the biomacromolecules, with a more negative surface charge promoting rapid nucleation, resulting from enhanced zinc ion concentration at the surface. Chemical functionalisation can be implemented, to alter the electrostatic potential of the protein surface and control the biomimetic mineralisation process. The biocomposites from different immobilisation strategies for ZIF-8 were assessed for biocatalytic activity using two distinct enzymes, a lipase, and a dehalogenase. The activity was analysed relative to the free enzyme to interrogate the impact of immobilisation on the function and stability of the biocatalyst. Variation in support stability and biocomposite activity were observed. Each were dependent on the method of immobilisation with some strategies yielding inactive or unstable biocomposites. For lipase, the ZIF-8 framework provided enzymatic stability to organic solvent, whilst the framework itself was susceptible to degradation by phosphate buffer. In the case of the dehalogenase biocomposite, substrate dependent crystal degradation was observed that was deemed responsible for variations in the observed enzyme activity. These findings highlight the potential limitations of ZIF-8 for enzyme immobilisation and as such, alternative porous supports were targeted. Framework chemistry and porosity were further investigated utilising Zeolitic Imidazolate Framework-90 (ZIF-90) and a biologically compatible HOF (BioHOF-1) to immobilise the lipase and dehalogenase enzymes. Relative to ZIF-8, enhanced activity was observed for both enzymes upon immobilisation using these frameworks, with the lipase biocomposites demonstrating retention of enantioselectivity, comparable to the free enzyme. However, the metal based ZIF-90 material faced similar challenges to ZIF-8, being unstable towards phosphate buffer and the dehalogenation reaction conditions. The preliminary results for BioHOF-1 were promising, with both enzyme biocomposites maintaining high levels of activity, and enzyme stability. BioHOF-1 was capable of protecting the enzymes to denaturing conditions including thermal treatment (dehalogenase) and organic solvents (lipase). Additionally, both biocomposites could be recycled five times without a significant reduction in activity.