RESUMEN
Two thermostable phytases were identified from Thai isolates of Aspergillus japonicus BCC18313 (TR86) and Aspergillus niger BCC18081 (TR170). Both genes of 1404 bp length, coding for putative phytases of 468 amino acid residues, were cloned and transferred into Pichia pastoris. The recombinant phytases, r-PhyA86 and r-PhyA170, were expressed as active extracellular, glycosylated proteins with activities of 140 and 100 U mL(-1), respectively. Both recombinant phytases exhibited high affinity for phytate but not for p-nitrophenyl phosphate. Optimal phytase activity was observed at 50 degrees C and pH 5.5. High thermostability, which is partly dependent on glycosylation, was demonstrated for both enzymes, as >50% activity was retained after heating at 100 degrees C for 10 min. The recombinant phytases also exhibited broad pH stability from 2.0 to 8.0 and are resistant to pepsin. In vitro digestibility tests suggested that r-PhyA86 and r-PhyA170 are at least as efficient as commercial phytase for hydrolyzing phytate in corn-based animal feed and are therefore suitable sources of phytase supplement.
Asunto(s)
6-Fitasa/genética , 6-Fitasa/metabolismo , Pichia/enzimología , Alimentación Animal , Aspergillus/clasificación , Aspergillus/enzimología , Aspergillus/genética , Aspergillus niger/enzimología , Aspergillus niger/genética , Biotecnología , Estabilidad de Enzimas , Calor , Concentración de Iones de Hidrógeno , Ácido Fítico/metabolismo , Pichia/genética , Zea mays/química , Zea mays/metabolismoRESUMEN
A gene encoding a thermostable pullulan-hydrolyzing enzyme was isolated from environmental genomic DNA extracted from soil sediments of Bor Khleung hot spring in Thailand. Sequence comparison with related enzymes suggested that the isolated enzyme, designated Env Npu193A, was most likely a neopullulanase-like enzyme. Env Npu193A was expressed in Pichia pastoris as a monomeric recombinant protein. The purified Env Npu193A exhibited pH stability ranging from 3 to 9. More than 60% of enzyme activity was retained after incubation at 60 degrees C for 1 h. Env Npu193A was found to hydrolyze various substrates, including pullulan, starch, and gamma-cyclodextrin. The optimal working condition for Env Npu193A was at pH 7 at 75 degrees C with K(m) and V(max) toward pullulan of 1.22+/-0.3% and 23.24+/-1.7 U/mg respectively. Env Npu193A exhibited distinct biochemical characteristics as compared with the previously isolated enzyme from the same source. Thus, a culture-independent approach with sequence-basing was found to be an effective way to discover novel enzymes displaying unique substrate specificity and high thermostability from natural bioresources.