Sperm whale myoglobin Mutant, F43Y Mb, with Tyr43 forming a novel crosslink to the heme 4-vinyl group, was found to exhibit dramatically enhanced dehaloperoxidase activity compared to that of wild-type Mb (ca. 115-fold), which is even beyond the catalytic efficiency of the native dehaloperoxidase from Amphitrite ornata (ca. 9-fold), indicating a potential application for bioremediation.