Potential new biocatalysts for biofuel production: The fungal lipases of Thermomyces lanuginosus and Rhizomucor miehei immobilized on zeolitic supports ion exchanged with transition metals

Nanozeolite NaX ion exchanged with different transition metals (Mn2+,Cu2+, Co2+, Zn2+, Ni2+) was used as a solid support for the immobilization of the lipases of Thermomyces lanuginosus (TLL) and Rhizomucor miehei (RML). The nanozeolite-enzyme complexes were used as heterogeneous catalysts for the transesterification reaction of palm oil to fatty acid ethyl esters (FAEEs). The most relevant results were obtained with the T lanuginosus enzyme immobilized on nanozeolitic supports ion exchanged with Ni2+. Although these zeolitic supports were able to immobilize a relatively small amount of the enzyme (43.7%) in comparison with the other nanozeolitic supports, the FAEE yields obtained with Nano-X/Ni/0.5 M-TLL complexes were above 94%. These results revealed an unusual synergistic effect between the T lanuginosus enzyme and the nickel ion-exchanged nanozeolitic support; this effect was not observed for the complexes prepared with the R. miehei enzyme. Bioinformatics calculations were performed for both enzymes by taking into consideration the crystallographic structures of the enzymes and the zeta potential of the surface of the nanozeolitic supports. By combining calculations of the protein electrostatic potential surface and normal mode analyses in a model, we were able to propose an explanation for the synergistic effect between the lipases and the nanozeolitic supports. The synergistic effect could be explained through an allosteric mechanism describing the interaction between aspartic acid residues 102 and 158 of the T lanuginosus lipase and the positively charged zeolitic support surface. This interaction results in the stabilization of the opening of the enzyme lid and leaves its catalytic triad permanently exposed to the reaction medium. (C) 2015 Elsevier Inc. All rights reserved. (AU)