T. Takasawa et al., POLYGALACTURONASE ISOLATED FROM THE CULTURE OF THE PSYCHROPHILIC FUNGUS SCLEROTINIA-BOREALIS, Canadian journal of microbiology, 43(5), 1997, pp. 417-424
A polygalacturonase was isolated from the culture medium of Sclerotini
a borealis, a psychrophilic fungus that grows on lawn and wheat seedli
ng under the snow in winter and induces the snow mold disease. Pectic
acid was a better substrate of this enzyme than pectin when the activi
ty was determined by measuring the reducing sugar produced. However, w
hen the activity was measured by viscosity change, the viscosity of pe
ctin decreased more rapidly than that of pectic acid. The results of v
iscosity change apparently indicate that the polygalacturonase catalyz
es pectin hydrolysis as an endo-type enzyme. Highly methyl-esterified
pectin was a poor substrate, as determined by measurements of reducing
sugar production and viscosity change. It is suggested from the resul
ts that the methoxy group of pectin affects the polygalacturonase reac
tion. A reaction mechanism was proposed for the polygalacturonase reac
tion. Molecular mass of this enzyme was 40 kDa and its isoelectric poi
nt was pH 7.5. Optimum pH of the enzyme reaction was 4.5 and its optim
um temperature was 40-50 degrees C. Thirty percent of the maximum acti
vity was observed at 5 degrees C, but it was only slightly active abov
e 60 degrees C. The activity was preserved for more than 2 years at 5
degrees C and pH 4.5, but it was lost when kept at room temperature ov
ernight or heated at 50 degrees C for 30 min. The amino acid sequence
of the N-terminal region of the psychrophilic polygalacturonase of Scl
erotinia borealis is compared with those of polygalacturonases of meso
philic fungi. The function of this enzyme against the target plants is
discussed with reference to the reaction of polygalacturonases of mes
ophilic fungi.