Hydroxyapatite: a renewable and biocompatible support for enzyme immobilization

High efficiency and selectivity, mild operational conditions and low toxicity are some of the main features that make enzymes efficient catalysts and important tools in making chemical processes greener. However, to render the process economically viable, enzymes should be recovered and reused and sometimes their operational stability should be improved. A common strategy to tackle these problems is the immobilization of the enzyme on a solid support. Hydroxyapatite (HAP) is a biocompatible inorganic material which is suitable for this task thanks to its structural stability, non-toxicity, large surface area and ease of surface modification. Moreover, it can be obtained from waste such as ashes from waste-to-energy plants, the fish supply chain, the avian supply chain, etc., in agreement with circular economy principles. Gamma-glutamyl transferases from both Bacillus subtilis (BsGGT) and Escherichia coli (EcGGT) were chosen as model enzymes to study the immobilization process on HAP. HAP was synthesised according to a literature-reported procedure. Enzyme immobilization was carried out exploiting the electrostatic interactions between the charged groups on the surfaces of both HAP and the enzyme, simply by mixing an enzyme solution and a hydroxyapatite suspension under controlled conditions (pH, temperature). Different particle sizes and experimental set-up were investigated and, after assessing that the enzyme did not desorb under reaction conditions, the supported GGTs were tested as biocatalysts in the gamma-glutamylation of L-methionine as a model reaction. Moreover, the enzyme reusability was tested in five consecutive reaction cycles and its storage stability was verified.