Family 3 CBM improves the biochemical properties, substrate hydrolysis and coconut oil extraction by hemicellulolytic and holocellulolytic chimeras

Abstract

To understand the influence of family 3 Carbohydrate Binding Module (hereafter CBM3), single (GH5 cellulase; CelB, CelBΔCBM), bi-chimeric [GH26 endo-mannanase (ManB-1601) and GH11 endo-xylanase (XynB); ManB-XynB [1], ManB-XynB-CBM] and tri-chimeric [ManB-XynB-CelB [1], ManB-XynB-CelBΔCBM] enzyme variants (fused or deleted of CBM) were produced and purified to homogeneity. CBM3 did not alter the pH and temperature optima of bi- and tri-chimeric enzymes but improved the pH and temperature stability of ManB in CBM variants of bi-/tri-chimeric enzymes. Truncation of CBM in CelB shifted the pH optimum and increased the melting temperature (Tm 65 ℃). CBM3 improved both substrate affinity (Km) and catalytic efficiency (kcat/Km) of fused enzymes in tri-chimera and CelB but only Km for bi-chimera. Far-UV CD of CelB and bi- and tri-chimeric enzymes suggested that CBM3 improved the α-helical content and compactness in the native state but did not prevent disintegration of secondary structural contents at acidic pH. Steady-state fluorescence studies suggested that under acidic conditions CBM3 prevented the exposure of hydrophobic patches in bi-chimeric protein but could not avert the opening up of chimeric enzyme structure. Aqueous enzyme assisted treatment of mature coconut kernel using single, bi- and tri-chimeric enzymes led to cracks, peeling and fracturing of the matrix and improved the oil yield by up to 22%.