Optimization of Aspergillus niger NRC1ami Pectinase Using Citrus Peel Pectin, Purification, and Thermodynamic Characterization of the Free and Modified Enzyme

Abstract

Enzyme cost and stability are the main problems facing industrial applications. Consequently, Aspergillus nigerNRC1ami was isolated from rotten orange and recorded a promising pectinase activity (13.8 U/ml). The enzyme was optimized using citrus peel pectin as the sole carbon source and recorded (40 U/ml). It was purified by two steps purifications and recorded 632 purification folds. The pure enzyme showed 14.7% carbohydrate content and consists of 15 amino acids. Glutamic acid was the major (22%) followed by leucine (10.67%) and threonine was the minor (2.70%). A. niger NRC1ami pectinase was conjugated by covalent coupling to sodium periodate (NaIO4) activated polysaccharides. Galactomannan showed the highest recovered activity (85%) and 94.34% reduction in viscosity. The optimum temperature for the pure enzyme shifted from 40 to 45 °C after the conjugation process. Also, the free enzyme showed its optimum activity at pH 5 compared to pH 4, 5 in the conjugated form case. The thermal stability of the free enzyme greatly improved after the modification process. The conjugated process reduced the activation energy to 36%, prolonged the enzyme half-life 5.6-fold at 60 °C, and increase the deactivation energy (Ed) by about 19% in comparison to the free form. The D value of the conjugated enzyme increased to 13.2-fold at 50 °C compared to the free form. Gibbs's free energy (ΔG) of the enzyme increased after the modification process, while the enthalpy (ΔH) and entropy (ΔS) decreased. Na+ and Zn2+ had a stimulating effect on both enzyme forms. Graphical Abstract: [Figure not available: see fulltext.]