The effect of replacement of the highly conserved Lys45 residue in pig myoglobin (Mb) with His, Ser, Glu, and Arg has been investigated. Rate constants/M-1 s-1 at 25 degrees C and pH 8.0, I = 0.100 M (NaCl), for the oxidation of deoxyMb with [Fe(CN)6]3- have been determined, and are for wild-type Lys45 (2.83 x 10(6)), His45 (1.02 x 10(6)), Ser45 (1.12 x 10(6)), Glu45 (0.87 x 10(6)), and Arg45 (3.06 x 10(6)). It is concluded that charge on the residue at position 45 has only a mild effect on reactivity, and that this is unlikely to be the site for electron transfer.