PROTON-TRANSFER IN THE CATALYTIC MECHANISM OF CARBONIC-ANHYDRASE .2. EFFECTS OF PLACING HISTIDINE-RESIDUES AT VARIOUS POSITIONS IN THE ACTIVE-SITE OF HUMAN ISOENZYME-II

The maximal rate of CO2 hydration catalyzed by human carbonic anhydrase II (carbonate hydro-lyase, EC 4.2.1.1) is limited by proton transfer steps involving the acid-base function of His-64. To test whether or not the precise location of this proton transfer group is critical, histidine residues were placed in various positions in the active site of the enzyme. Thus, four double mutants were made, all with His-64 replaced by Ala-64, and with a histidine residue replacing Asn-62, Ala-65, Asn-67 or Thr-200. The results show that the mutants with His-62, His-67 and His-200, but not the mutant with His-65, yield significantly higher k(cat) values for CO2 hydration than the single mutant with Ala-64, indicating that His-62, His-67 and His-200 can contribute to proton transfer between the metal center and the reaction medium. However, the average proton transfer efficiency of these histidines is only about 5% of that of His-64 in the unmodified enzyme.