Examination of the role of tyrosine-174 in the catalytic mechanism of the Arabidopsis thaliana chitinase: comparison of variant chitinases generated by site-directed mutagenesis and expressed in insect cells using baculovirus vectors.

Using the catalytic mechanism of lysozyme as a paradigm for the mechanism of other enzymes that catalyze the hydrolysis of beta-1,4-glycosidic linkages, including chitinase, we have examined the effect of chemical modification with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) on the reaction catalyzed by Zea mays chitinase. Inactivation with EDC did not result in derivatization of essential carboxylic acid residues, but resulted in the selective modification of a single essential tyrosine residue (Verburg, J. G., Smith, C. E., Lisek, C. A., and Huynh, Q. K., 1991, J. Biol. Chem. 267, 3886-3893). Here, we examine the role of the homologous tyrosine residue in the catalytic mechanism of the Arabidopsis thaliana chitinase. Tyrosine-174 of the Arabidopsis chitinase was replaced, with phenylalanine, alanine, histidine, and methionine by site-directed mutagenesis, and the variant chitinases were expressed in insect cells using baculovirus transfer vectors. A comparison of the reaction catalyzed by each of the variant enzymes indicates that substitution of another amino acid for Tyr-174 alters, but does not eliminate, enzymatic activity. Estimates of the specific activities of the variant chitinases reveal that substitution of His for Tyr-174 has a minimal effect on catalysis, the specific activities of the Phe and Met variants are approximately equivalent to each other, but are 60% the specific activity of wild-type Arabidopsis chitinase, and the specific activity of the Ala variant is only 40% that of wild-type. The observation that the Arabidopsis chitinase is tolerant to mutagenesis at this position suggests that Tyr-174 does not participate directly in catalysis.