High NaCl concentrations induce the resistance to thermal denaturation of an extremely halotolerant (salt-activated) β-mannanase from Bacillus velezensis H1.

Abstract

β-mannanase catalyzes the hydrolysis of mannans β-1,4-mannosidic linkages to produce industrially relevant oligosaccharides. These enzymes have numerous important applications in the detergent, food, and feed industries, particularly those that are resistant to harsh environmental conditions such as salts and heat. While, moderately salt-tolerant β-mannanases are already reported, existence of a high halotolerant β-mannanase is still elusive. This study aims to report the first purification and characterization of ManH1, an extremely halotolerant β-mannanase from the halotolerant B. velezensis strain H1. Electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-Q-TOF-MS) analysis revealed a single major peak with a molecular mass of 37.8kDa demonstrating its purity. The purified enzyme showed a good thermostability as no activity was lost after a 48h incubation under optimal conditions of 50°C and pH 5.5. The enzyme's salt activation nature was revealed when its maximum activity was obtained in the presence of 4M NaCl,it doubled compared to the no-salt condition. Moreover, NaCl strengthens its resistance to thermal denaturation, as its melting temperature (Tm) increased steadily with increasing NaCl concentrations reaching 75.5°C in the presence of 2.5M NaCl. The Km and Vmax values were 5.63mg/mL and 333.33 µmol/min/mL, respectively, using carob galactomannan (CG) as a substrate. The enzyme showed a significant ability to produce manno-oligosaccharides (MOS) from lignocellulosic biomass releasing 13mg/mL of reducing sugars from olive mill wastes (OMW)after 24h incubation. The results revealed that this enzyme may have significant commercial values for agro-waste treatment, and other potential applications.