Kinetic and thermodynamic behavior of partially purified cellobiase from <em>Humicola fuscoatra</em> MTCC 1409

Abstract

The study was undertaken with the objective to purify and characterize cellulases from thermophilic fungus <em>Humicola fuscoatra</em> in order to find out its potential to increase hydrolysis of lignocellulosic biomass especially paddy straw. Cellobiase, an important component of cellulase enzyme complex, was produced by solid-state fermentation on Mandel media under optimized conditions using rice straw as substrate and was partially purified by ammonium sulphate saturation followed by DEAE-cellulose chromatography. Two isoforms of cellobiase, C-I and C-II, which were most kinetically efficient at their optimum pH of 6.0 and temperature of 50°C were identified after purification,. These isoforms were thermostable at a temperature range of 30–70°C. Co²⁺, Zn²⁺, and Mn²⁺ activated whereas, EDTA and Hg²⁺ inhibited the activity of cellobiase. Na⁺, Mg²⁺, K⁺, and Fe²⁺ did not influence the activity of isoforms. The molecular weight of isoforms C-I & C-II was 48 and 44 kDa, respectively. Activation energy (E_a) values for C-I and C-II isoforms were 7.65 and 9.57 KJ/mol and corresponding enthalpy change (ΔH) values were 16.27 and 11.49 KJ/mol, respectively. Change in entropy (ΔS) values at 50°C for C-I and C-II were 0.098 KJ/K/mol and 0.085 KJ/K/mol, respectively. The pK values of ionizing groups in free enzyme and enzyme-substrate complex were between 4.3 and 6.8, indicating the possible precipitation of carboxyl groups of aspartate and glutamate and imidazolium group of histidine in the cellobiase catalyzed the hydrolysis of cellobiose by both isoforms.