Gene Cloning and Enzymatic Characterization of an Alkali-Tolerant Endo-1,4-β-mannanase from Rhizomucor miehei

An endo-1,4-β-mannanase gene (RmMan5A) was cloned from the thermophilic fungus Rhizomucor miehei for the first time and expressed in Escherichia coli . The gene had an open reading frame of 1330 bp encoding 378 amino acids and contained four introns. It displayed the highest amino acid sequence identity (42%) with the endo-1,4-β-mannanases from glycoside hydrolase family 5. The purified enzyme was a monomer of 43 kDa. RmMan5A displayed maximum activity at 55 °C and an optimal pH of 7.0. It was thermostable up to 55 °C and alkali-tolerant, displaying excellent stability over a broad pH range of 4.0-10.0, when incubated for 30 min without substrate. The enzyme displayed the highest specificity for locust bean gum (K(m) = 3.78 mg mL⁻¹), followed by guar gum (K(m) = 7.75 mg mL⁻¹) and konjac powder (K(m) = 22.7 mg mL⁻¹). RmMan5A hydrolyzed locust bean gum and konjac powder yielding mannobiose, mannotriose, and a mixture of various mannose-linked oligosaccharides. It was confirmed to be a true endo-acting β-1,4-mannanase, which showed requirement of four mannose residues for hydrolysis, and was also capable of catalyzing transglycosylation reactions. These properties make RmMan5A highly useful in the food/feed, paper and pulp, and detergent industries.