Identification of the aspartic acid residue located at or near substrate-binding site of rye seed chitinase-c

Carboxyl groups of rye seed chitinase-c (RSC-c) were modified with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and glycine ethyl ester (GEE) at pH 5.5 and 5°C in the presence and absence of (GlcNAc)₄. In the absence of (GlcNAc)₄, 5.2 carboxyl groups were modified by 90 min-reaction and the chitinase activity was reduced to 2.0%, while in the presence of (GlcNAc)₄, 4.6 carboxyl groups were modified and 72% of the activity was retained. To identify the carboxyl group protected by (GlcNAc)₄ from the modification, RSC-c was first modified with EDC and GEE in the presence of (GlcNAc)₄ and then radiolabeled with EDC and [¹⁴C]GEE in the absence of (GlcNAc)₄. Analyses of the radioactive peptides from the tryptic and chymotryptic digests of radiolabeled RSC-c showed that the main radiolabeled carboxyl group is that of Asp95, suggesting that Asp95 is located at or near substrate-binding site of RSC-c.