Mannan-degrading enzymes from Sclerotium rolfsii: Characterisation and synergism of two endo β-mannanases and a β-mannosidase

For the degradation of α-mannans and β-mannans Sclerotium rolfsii produces multi-enzyme systems. When cultivated in the presence of glucomannan this fungus secrets at least two β-mannosidases (EC 3.2.1.25) and five β-mannanases (EC 3.2.1.78). The degradation of different mannans by two purified β-mannanases (61.2 kDa, pI 3.5; 41.9 kDa, pI 3.2) and a mannosidase (57.5 kDa, pI 4.5) is discussed in this paper. All three enzymes were most stable at pH 4.5 and when they were incubated for 1 h at this pH and 65°C they retained about 50% of their activity. Mannans, gluco- and galactomannans were completely hydrolysed by the 58 kDa β-mannosidase, which only liberated monomers from the mannans. The activity of this enzyme was enhanced by addition of either one of the β-mannanases. The two β-mannanases randomly cleaved fragments larger than mannobiose from the mannans. The KM values of the β-mannanases showed that galactose substituents of galactomannans seem to be required for the enzyme substrate binding, whilst acetyl groups inhibited both enzymes.