Covalent immobilization of soybean seed hull urease on chitosan mini-spheres and the impact on their properties

Abstract

Urease (EC 3.5.1.5) is found in bacteria, fungi, algae, plants and some invertebrates, and in soils. Soil ureases are partly extracellular being liberated during microbial and plant root metabolism and death. In plants, urease is present in most of Leguminosae seeds, accounting for 0.2–0.3% of the extractable soybean seed protein, being also present in the hulls, in lesser amounts. Soybean hull is a low-cost agro-industrial by-product from the soybean manufacturing process. In a way of exploiting its possible applications, urease was covalently immobilized on thiolated chitosan mini-spheres through its sulphydryl groups. Almost quantitative (98%) immobilization from a crude extract of soybean seed hull was obtained. After 4 h at 70 °C, the activity of free soybean hull urease (SBU) was 50% lower than that of the immobilized enzyme. The pH of maximum activity shifted from 7 to 5 after immobilization, thus allowing its use in processes performed at acid pH (pH 3–5). The immobilization brought about an increase in the optimum activity temperature from 65 °C to 75 °C. An 84% of the activity of the immobilized SBU was retained after 25 cycles of utilization, and the activity was maintained after 90 days at 4 °C.