A novel multifunctional cellulolytic enzyme screened from metagenomic resources representing ruminal bacteria

Metagenomic resources representing ruminal bacteria were screened for novel exocellulases using a robotic, high-throughput screening system, the novel CelEx-BR12 gene was identified and the predicted CelEx-BR12 protein was characterized. The CelEx-BR12 gene had an open reading frame (ORF) of 1140 base pairs that encoded a 380-amino-acid-protein with a predicted molecular mass of 41.8 kDa. The amino acid sequence was 83% identical to that of a family 5 glycosyl hydrolase from Prevotella ruminicola 23. Codon-optimized CelEx-BR12 was overexpressed in Escherichia coli and purified using Ni–NTA affinity chromatography. The Michaelis–Menten constant (K_m value) and maximal reaction velocity (V_max values) for exocellulase activity were 12.92 μM and 1.55 × 10⁻⁴ μmol min⁻¹, respectively, and the enzyme was optimally active at pH 5.0 and 37 °C. Multifunctional activities were observed against fluorogenic and natural glycosides, such as 4-methylumbelliferyl-β-d-cellobioside (0.3 U mg⁻¹), CMC (105.9 U mg⁻¹), birch wood xylan (132.3 U mg⁻¹), oat spelt xylan (67.9 U mg⁻¹), and 2-hydroxyethyl-cellulose (26.3 U mg⁻¹). Based on these findings, we believe that CelEx-BR12 is an efficient multifunctional enzyme as endocellulase/exocellulase/xylanase activities that may prove useful for biotechnological applications.