Hydroxyapatite: an eco-friendly material for enzyme immobilisation in the sustainable synthesis of value-added products

In this PhD thesis, the use of hydroxyapatite (HAP) as a support for enzyme immobilisation was studied. HAP is an eco-friendly inorganic material obtainable from waste-to-energy plant ashes. As a first approach, bare HAP was used for the direct adsorption of a gamma-glutamyltransferase (GGT) from E. coli. The stability and reusability of the obtained GGT-HAP were assessed, and the biocatalyst was integrated in a packed-bed reactor to perform continuous flow synthesis (Chapter 2). In Chapter 3, different strategies of HAP functionalisation were tested to allow covalent immobilisation of a mutant GGT. The biocatalyst properties were compared to those of the adsorbed GGT. Chapter 4 deals with the immobilisation, both via adsorption and covalent bonds, of an acid phosphatase from M. morganii with the aim of improving the biocatalyzed preparation of nucleotides. The results obtained during the period spent abroad in Professor Paradisi’s group at the University of Bern are presented in Chapter 5. The work aimed to expand the pool of immobilised enzymes through covalent interactions. In particular, a vanadium-dependent chloroperoxidase, a tyrosine decarboxylase and a transaminase were used. Finally, Chapter 6 underlines the importance of using biocatalysis to increase the sustainability of the Made in Italy productive system. The Italian industry provides chemicals needed in many other productive sectors of the Made in Italy. To make the chemical production more sustainable, the use of less energy-intensive methodologies, as well as of materials coming from renewable sources, is crucial. By diminishing the formation of side-products, the energy requirements and the use of organic solvents in the reaction, biocatalysis is an important tool to increase the eco-friendliness of the chemical industry, especially when the enzyme is immobilised on a solid support that increases its stability and allows its reuse in multiple reactions and its combination with continuous flow technologies.