Extremely thermoactive archaeal endoglucanase from a shallow marine hydrothermal vent from Vulcano Island

Already-characterized microbial cellulases have proven to be highly useful for industrial processes, since they can withstand harsh industrial conditions with characteristics such as high thermo- and acid stability. These properties provide promising features for the process of plant biomass degradation and biofuel generation. Nevertheless, the number of known extremely thermoactive archaeal cellulases is low. Hence, the discovery of archaeal cellulases with different characteristics is crucial for the development of efficient and sustainable biorefinery. In this work, the metagenome of a high-temperature enrichment culture from marine environment of Vulcano Island was screened for the presence of novel endoglucanase-encoding genes of archaeal origin. The ORF vul_cel5A was detected, and the deduced protein was characterized as the most thermoactive endoglucanase described to date. Vul_Cel5A was identified as a thermoactive glycoside hydrolase family 5 endoglucanase, with the highest sequence identity (72-75%) to putative endoglucanases from archaeal genera. Vul_Cel5A showed the highest activity at notable 115 °C towards barley β-glucan (210.7 U/mg), and lichenan (209.9 U/mg), and further towards carboxymethyl cellulose (38.6 U/mg) and locust bean gum (83.0 U/mg). The endoglucanase exhibited a half-life time of 46 min at 100 °C and did not show any loss of activity after incubation for 48 h at 75 °C. Furthermore, Vul_Cel5A showed high affinity to barley β-glucan with a K_m of 0.52 mg/mL and showed tolerance against various chemical reagents. Due to the outstanding high thermoactivity and thermostability and tolerance to acidic conditions, Vul_Cel5A represents a promising novel archaeal endo-β-glucanase for application in biorefineries for an efficient biomass pre-treatment.