Degradation of cell wall materials from sweetpotato, cassava, and potato by a bacterial protopectinase and terminal sugar analysis of the resulting solubilized products

Abstract

Cell wall materials (CWMs) from sweetpotato, cassava, and potato starch residues were degraded using a crude enzyme solution from the culture filtrate of a Bacillus sp. isolated from soil, Bacillus sp. M4. This organism has been found to secrete polygalacturonic acid lyase (PGL) and glycan depolymerase activities, especially arabinanase, but cellulase activity was nearly absent. Sugar analysis of the solubilized product after enzyme treatment at pH 7.0 revealed that it is mainly composed of galacturonic acid, galactose, and arabinose, the sugars found commonly in the pectin fraction. This suggested the presence of a protopectinase (PPase) activity in the culture filtrate. The presence of EDTA completely inhibited PGL but PPase activity was almost retained, suggesting that the PGL is not the primary activity responsible for pectin solubilization. The mode of action of the crude enzyme was determined by terminal sugar analysis using HPAEC-PAD after hydrolysis of the reduced products. Results revealed that galactose is the main neutral sugar at the reducing terminal of the products, although rhamnose was also present in the higher molecular weight component. This suggested that at neutral pH, the primary activity in the culture filtrate of Bacillus sp. M4 is a B-type PPase, which attacked the galactan as well as rhamnogalacturonan moieties of the protopectin, resulting in the release of a soluble pectin fraction.