RESUMO
Directed evolution strategy (error-prone PCR) was conducted to improve the activity of lipase from Rhizopus chinensis CCTCC M201021. Through two rounds of ep-PCR and pNPP top agar screening, two optimum mutant strains 1-11 and 2-28 were obtained with 2 and 4 fold of enzyme activity higher than that of parent strain, respectively. DNA sequencing of mutant lipase 2-28 revealed four amino acid substitutions: A129S, K161R, A230T, K322R. According to the simulated protein structure of Rhizopus chinensis lipase, A129S, K161R, A230T were located on the surface of the protein. A230T substitution improved the stability of the alpha-helix loop. K322R, near the catalytic center of lipase, located at a loop, formed a salt-bridge with a nearby aspartic acid (negative charged). Electrostatic force pulled the loop to the opposite direction of the substrate channel and made it easier for substrate to enter the lipase catalytic domain. Purified lipase was characterized and the result showed that Km of 2-28 lipase decreased by 10% compared with Km of the parent lipase, and Kcat was 2.75 fold improved than that of the original lipase.