The catalytic role of the distal site asparagine‐histidine couple in catalase‐peroxidases

Catalase‐peroxidases (KatGs) are unique in exhibiting an overwhelming catalase activity and a peroxidase activity of broad specificity. Similar to other peroxidases the distal histidine in KatGs forms a hydrogen bond with an adjacent conserved asparagine. To investigate the catalytic role(s) of this potential hydrogen bond in the bifunctional activity of KatGs, Asn153 in Synechocystis KatG was replaced with either Ala (Asn153→Ala) or Asp (Asn153→Asp). Both variants exhibit an overall peroxidase activity similar with wild‐type KatG. Cyanide binding is monophasic, however, the second‐order binding rates are reduced to 5.4% (Asn153→Ala) and 9.5% (Asn153→Asp) of the value of native KatG [(4.8 ± 0.4) × 10 5 m −1 ·s −1 at pH 7 and 15 °C]. The turnover number of catalase activity of Asn153→Ala is 6% and that of Asn153→Asp is 16.5% of wild‐type activity. Stopped‐flow analysis of the reaction of the ferric forms with H 2 O 2 suggest that exchange of Asn did not shift significantly the ratio of rates of H 2 O 2 ‐mediated compound I formation and reduction. Both rates seem to be reduced most probably because (a) the lower basicity of His123 hampers its function as acid‐base catalyst and (b) Asn153 is part of an extended KatG‐typical H‐bond network, the integrity of which seems to be essential to provide optimal conditions for binding and oxidation of the second H 2 O 2 molecule necessary in the catalase reaction.