Purification and characterization of thermostable cellulase from Enterobacter cloacae IP8 isolated from decayed plant leaf litter

Cellulases are important in the hydrolysis of lignocellulosic materials and thereby contribute to biomass conversion into fuels and chemicals. A cellulase-producing bacterium was isolated from decayed plant leaf litter in soil of a botanical garden. Based on morphological, biochemical and 16S rRNA gene sequencing, it was identified as Enterobacter cloacae IP8, with gene bank accession number NR118568.1. The bacterial cellulase was purified in a three-step procedure using lyophilization, ion exchange chromatography (QAE Sephadex A-50) and gel filtration (Biogel P100). Two isoforms of the enzyme were purified 1.21 and 1.23 folds, respectively, with yields of 30 and 29% for isoforms A and B, respectively. Apparent molecular weights of 36.61 +/- 1.40 and 14.1 +/- 0.10 kDa were obtained for isoforms A and B, respectively, using gel filtration chromatography. Kinetic parameters Km and Vmax were 0.13 +/- 0.04mg/ml and 3.84 +/- 0.05 U/ml/min, respectively, for isoform A and 0.58 +/- 0.06mg/ml and 13.8 +/- 0.10 U/ml/min, respectively, for isoform B. Optimum pH (7.0) and temperature (60 degrees C) of cellulase activity were determined for both isoforms A and B. Na+ and Ca2+ enhanced the activities of both isoforms. Mg2+ inhibited the enzyme activity at concentrations 4-15mM but, while it stimulated the activity of isoform A at concentrations 15-200mM, it inhibited that of isoform B at same concentration range. The strong inhibition of the enzyme by ethylenediaminetetraacetic acid (EDTA) confirmed the enzyme as a metalloenzyme. These results reveal the purified cellulase from E. cloacae IP8 as a thermostable, acidic to neutral metalloenzyme, suggesting that it has good potential for biotechnological applications.