Rayani, Ekta Yogesh; (2023) Stabilisation of low temperature biocatalysts. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Cold marine environments represent a rich resource for the bioprospecting of low-temperature enzymes from the polar waters surrounding the Antarctic. Psychrophilic lipase candidates were identified using in silico bioprospecting methods from two different genomic resources. The Tara Ocean database for bacterial candidates using motif searches identified a 921bp lipase gene (PL001). The exploration of eukaryotic lipases from assembled genomes of the Salpa thompsoni, a marine tunicate, discovered a 1761bp pancreatic-like lipase sequence (PL002). Recombinant bacterial expression produced only PL002 at 10°C followed by affinity purification. Hydrolysis of the synthetic substrate ρ-nitrophenyl butyrate (PNPB) revealed that PL002 is a cold-active lipase with an optimal activity temperature and pH profile of 20°C and pH 7 and a specific activity of 3.16U/mg that was maintained at over 60% in temperatures from 15 to 25°C. A meta-analysis of lipase activities towards PNPB showed that PL002 displays a higher activity at lower temperatures relative to reported lipases. Site-directed covalent immobilisation, using chemically activated cellulose nanofiber membranes (CNF), was trialled to improve lipase stability and activity. Using the Lilly-Hornby equation to determine the Michaelis-Menten constant, KM, it was established that the KM in the free and immobilised systems were comparable and therefore the mass transfer properties exhibited in CNF are favourable with immobilised lipases. Lipase-mediated decomposition of polyurethane-polyesters (PUs) has potential to reduce waste accumulation. The greater ester cleaving potential demonstrated by positive control, Candida antarctica lipase B, presented further analysis to characterise the degradation capabilities by assessing enzyme loading on CNF. Over a 12-hour period, the lower loaded membranes degraded more of the 0.01 mg/ml PU substrate (56%), and at higher rate of 3.78 x10-03mg/ml/hr than the free enzyme in solution (33% and 3.53 x10-03mg/ml/hr). The findings underline the potential of a CNF immobilisation system for process intensification and monomer recycling from PU degradation.

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