Endoxylanase II from Trichoderma reesei has several isoforms with different isoelectric points

Two minor xylanases present in Trichoderma reesei Rut C30 cultivation broth were purified as a mixture using ion-exchange, hydrophobic-interaction and gel chromatography. The purified enzyme preparation contained two active xylanases with pi values of 7.1 and 8.1. Both components had a molecular mass of 20 kDa. The purified xylanase preparation exhibited properties very similar to those of the previously isolated XYL II (pl 9.0) of T. reesei Rut C30. The activity and stability properties, apparent kinetic parameters as well as the titration curve forms were similar. The major difference in enzymic properties was the significantly lower specific activity of the p1-7.1+8.1 xylanase mixture (3350 nkat/mg) compared with the specific activity of XYL 11 (13 500 nkat/mg). Amino acid sequences of tryptic peptides (34% of the total amino acid sequence was determined) were identical to the amino acid sequence of XYL II. Furthermore, in vitro modification of the p1-9.0 form of XYL II to p1-8.1 and p1-7.1 forms was demonstrated. Thus the purified xylanase preparation most probably contained two modified forms of XYL II. The primary amino acid sequence of XYL II contains 28 glutamine and asparagine residues and theoretically deamination of one of them lowers the pi to 8.06 and deamination of two amino acids lowers the pi to 7.02.