Immobilization of tyrosinase on chitosan - An optimal approach to enhance the productivity of L-DOPA from L-tyrosine

Tyrosinase was immobilized on Chitosan (CTS) beads to produce 3,4-dihydroxy-L-phenylaianine (L-DOPA) from L-tyrosine. Epichlorohydrin (ECH), ethylene glycol diglycidyl ether (EGDE), and glutaraldehyde (GLU) were used as coupling agents, respectively. Ultraviolet/visible measurements on CTS films showed that the reaction intermediate (L-dopaquinone) attacked the amino groups on CTS, so the amine residues on chitosan were capped by acetic acid anhydride (Ac) or formaldehyde (Fm) to avoid the deactivation of the immobilized tyrosinase. The pH and temperature of the maximal rate to produce L-DOPA were investigated. GLU (coupling agent) and Ac (capping agent) were selected for practical utility. A 7.5% (w/v) concentration of GLU was found to attain maximal activity of the immobilized enzyme. The thermal stability of tyrosinase immobilized on CTS-GLU-Ac, and after treatment with sodium borohydride, was enhanced to a great extent. The L-DOPA converting efficiency in the environmental conditions of this study decreased from 45.1% to 39.9% (between 1(st) and 30(th) batch). This immobilized tyrosinase can be used practically in the production of L-DOPA from L-tyrosine.