Kinetics and thermodynamics of thermal inactivation for recombinant Escherichia coli cellulases, cel12B, cel8C, and polygalacturonase, peh28; biocatalysts for biofuel precursor production.

Lignocellulosic biomass conversion using cellulases/polygalacturonases is a process that can be progressively influenced by several determinants involved in cellulose microfibril degradation. This article focuses on the kinetics and thermodynamics of thermal inactivation of recombinant Escherichia coli cellulases, cel12B, cel8C and a polygalacturonase, peh 28, derived from Pectobacterium carotovorum sub sp. carotovorum. Several consensus motifs conferring the enzymes' thermal stability in both cel12B and peh28 model structures have been detailed earlier, which were confirmed for the three enzymes through the current study of their thermal inactivation profiles over the 20-80°C range using the respective activities on carboxymethylcellulose and polygalacturonic acid. Kinetic constants and half-lives of thermal inactivation, inactivation energy, plus inactivation entropies, enthalpies and Gibbs free energies, revealed high stability, less conformational change and protein unfolding for cel12B and peh28 due to thermal denaturation compared to cel8C. The apparent thermal stability of peh28 and cel12B, along with their hydrolytic efficiency on a lignocellulosic biomass conversion as reported previously, makes these enzymes candidates for various industrial applications. Analysis of the Gibbs free energy values suggests that the thermal stabilities of cel12B and peh28 are entropy-controlled over the tested temperature range.

Molecular and biochemical characterization of recombinant cel12B, cel8C, and peh28 overexpressed in Escherichia coli and their potential in biofuel production.

BACKGROUND: The high crystallinity of cellulosic biomass myofibrils as well as the complexity of their intermolecular structure is a significant impediment for biofuel production. Cloning of celB-, celC-encoded cellulases (cel12B and cel8C) and peh-encoded polygalacturonase (peh28) from Pectobacterium carotovorum subsp. carotovorum (Pcc) was carried out in our previous study using Escherichia coli as a host vector. The current study partially characterizes the enzymes' molecular structures as well as their catalytic performance on different substrates which can be used to improve their potential for lignocellulosic biomass conversion. RESULTS: ß-Jelly roll topology, (α/α)6 antiparallel helices and right-handed ß-helices were the folds identified for cel12B, cel8C, and peh28, respectively, in their corresponding protein model structures. Purifications of 17.4-, 6.2-, and 6.0-fold, compared to crude extract, were achieved for cel12B and cel8C, and peh28, respectively, using specific membrane ultrafiltrations and size-exclusion chromatography. Avicel and carboxymethyl cellulose (CMC) were substrates for cel12B, whereas for cel8C catalytic activity was only shown on CMC. The enzymes displayed significant synergy on CMC but not on Avicel when tested for 3 h at 45 °C. No observed ß-glucosidase activities were identified for cel8C and cel12B when tested on p-nitrophenyl-ß-d-glucopyranoside. Activity stimulation of 130% was observed when a recombinant ß-glucosidase from Pcc was added to cel8C and cel12B as tested for 3 h at 45 °C. Optimum temperature and pH of 45 °C and 5.4, respectively, were identified for all three enzymes using various substrates. Catalytic efficiencies (kcat/Km) were calculated for cel12B and cel8C on CMC as 0.141 and 2.45 ml/mg/s respectively, at 45 °C and pH 5.0 and for peh28 on polygalacturonic acid as 4.87 ml/mg/s, at 40 °C and pH 5.0. Glucose and cellobiose were the end-products identified for cel8C, cel12B, and ß-glucosidase acting together on Avicel or CMC, while galacturonic acid and other minor co-products were identified for peh28 action on pectin. CONCLUSIONS: This study provides some insight into which parameters should be optimized when application of cel8C, cel12B, and peh28 to biomass conversion is the goal.