Towards an Understanding of Ice and Hydrate Adsorption by an Antifreeze Protein from Lolium Perenne
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Lolium perenne (Lp), the perennial ryegrass, produces an antifreeze protein (AFP) when seasonal temperatures and levels of available sunlight drop in the autumn. This protein, along with other low-temperature induced stress proteins with distinct functions, allows the plant to survive sub-zero temperatures. LpAFP adsorbs to developing ice crystals, marginally lowering the freezing point of solutions but shows a more impressive capacity to prevent ice recrystallization. Its ability to inhibit ice recrystallization has resulted in interest in industrial applications of LpAFP, from its use as a frozen food additive and as a potential useful sequence for transgenic crop enhancement and even as a model to understand the inhibition of gas hydrates, which have a crystal structure distinct from ice. This thesis shows that the wild-type LpAFP sequence when placed in a suitable expression vector can be displayed on the bacterial membrane and increased the incorporation of the host bacterium into polycrystalline ice. Extensive in vitro mutagenesis has allowed the investigation of the relative importance of particular amino acids in LpAFP for adsorption to both ice and a model gas hydrate, tetrahydrofuran hydrate. Notably, certain steric mutations that disrupted ice affinity retained appreciable hydrate binding. These experiments have generated a greater understanding of LpAFP through the selective characterization of its recombinant mutants and wild-type states, and have prompted some suggestions and strategies to work towards the development of gas hydrate inhibitors modeled on this protein.