Thermo- and salt-tolerant chitosan cross-linked γ-glutamyl transpeptidase from Bacillus licheniformis ER15

Gamma-glutamyl transpeptidase enzyme, from Bacillus licheniformis ER15 (BLGGT), was produced extracellularly using a complex medium with high enzyme titers. Enzyme was concentrated and purified using ultra-filtration and ion exchange chromatography, respectively, with a purification fold of 4.6 and 50.11% yield. Enzyme was covalently immobilized onto chitosan microspheres (CMS). Immobilization was standardized with respect to pH, enzyme load and time. Immobilization efficiency of 11.9 U/ mg dry weight of microsphere was obtained in Tris-HCl buffer (pH 9.0) at 18 °C in 4 h. Immobilized enzyme (CMS-GGT) exhibited improved thermal stability (t1/2 of 70.7 min at 60 °C), activity in a broader pH range and improved salt stability in 18% (3 M) sodium chloride solution as compared to free enzyme. Both free and immobilized enzymes specifically converted glutamine to glutamic acid in a mixture of amino acids. CMS-GGT had a better shelf life and high recyclability retaining 90% catalytic efficiency upto 10 reaction cycles. For long-term storage, CMS-GGT can be disinfected using either sodium azide or sodium hypochlorite solution without affecting enzyme activity. Thus, the present study provides an easy and efficient method for GGT enzyme immobilization that results in an improved and robust enzyme preparation.