The influence of a single amide group located in the heme active site of Pseudomonas aeruginosa cytochrome C-551 on its redox potential (E degrees') has been characterized through amino acid substitution of Asn64 with Gln or Ala. The E degrees' variation of ca. 60mV among the proteins is attributed mainly to the alteration of the thermodynamic stability of the oxidized protein by the dipole and hydrogen-bonding ability of the amide group.