Isozymes of Ipomoea batatas catechol oxidase differ in catalase-like activity

The amino acid sequences of two isozymes of catechol oxidase from sweet pot atoes (Ipomoea batatas) were determined by Edman degradation of BrCN cleava ge fragments of the native protein and by sequencing of amplified cDNA frag ments. Sequence alignment and phylogenetic analysis of plant catechol oxida ses revealed about 80% equidistance between the two I. batatas catechol oxi dases and approximately 40-60% to catechol oxidases of other plants. When H 2O2 was applied as substrate the 39 kDa isozyme, but not the 40 kDa isozyme , showed catalase-like activity. The structure of the 40 kDa isozyme was mo deled on the basis of the published crystal structure of the 39 kDa isozyme [T, Klabunde et al., Nat. Struct. Biol, 5 (1998) 1084]. The active site mo del closely resembled that of the 39 kDa isozyme determined by crystallogra phy, except for a mutation of Thr243 (40 kDa isozyme) to Ile241 (39 kDa iso zyme) close to the dimetal center. This residue difference affects the orie ntation of the Glu238/236 residue, which is thought to be responsible for t he catalase-like activity of the 39 kDa isozyme for which a catalytic mecha nism is proposed. (C) 2001 Elsevier Science B.V, All rights reserved.