CHARACTERIZATION OF A XYLANOLYTIC AMYLOGLUCOSIDASE OF TERMITOMYCES-CLYPEATUS

A xylanolytic amyloglucosidase of Termitomyces clypeatus was character ised with respect to other amyloglucosidases. The enzyme contained hig h alpha-helix destabilising amino acids but no sulphur amino acid. It contained high threonine and serine, analogous to other raw starch hyd rolysing enzymes. Both xylanase and amyloglucosidase activities were g radually lost with the progress of tryptophan oxidation by NBS and tot al inactivation occurred after oxidation of 4-5 tryptophan residues, I n the presence of substrates (either starch or xylan), complete inacti vation of either activities was not noticed even after oxidation of 7. 7 mol of tryptophan residues. Inactivation by HNBB was not possible in the absence of any denaturant, Only 4.9 mol of tryptophan could be mo dified in the presence of 5 M urea which resulted in only 42% inhibiti on of activity. Thus modified enzyme had higher V-m/K-m and lower pH o ptima in comparison to those of native enzyme. It was suggested that t ryptophan was present at the substrate binding site and not at the act ive site. No such change in activity was noticed after modification of tyrosine, lysine or arginine residues. HPGPLC analysis of both dilute and concentrated enzyme solution indicated that the enzyme existed as an equilibrium mixture of protomer-oligomer. Perhaps for this reason molar mass of NAI modified enzyme appeared to be almost half of that m odified by NAI in presence of substrate. Arrhenius plot of the enzyme also indicated reversible oligomerisation as a function of temperature .