By retaining the native distal His64 in sperm whale myoglobin (Mb),a second distal histidine was engineered in Mb by mutating Leu29 to His29.The resultant mutant of L29H Mb exhibits an unusual enhanced peroxidase activity with a positive cooperativity in comparison to that of wild type Mb.The new enzyme with two cooperative distal histidines has not been found in native peroxidase,which emphasizes a creation of the rational protein design.