In order to understand the effect of the heme environment on the electron transfer reaction of Mb, modification of distal histidine (His-64, E7) of native sperm whale (SW) and horse heart (HH) myoglobin (Mb) molecules by cyanated imidazole (CN-E7-Mb) was made by the reaction of native Mb with cyanogen bromide (BrCN), and its electrochemical behavior was investigated at an In2O3 electrode. The CN-E7-Mb showed a well-defined redox wave at an In2O3 electrode with a highly hydrophilic surface. The redox potentials of CN-E7-Mbs from SW and HI-I were - 0.035 and - 0.038 V (vs. Ag \ AgCl), respectively, which are ca. 100 mV more positive than those of corresponding native Mbs. This positive shift in redox potential is reasonable because no water molecule is present at the sixth coordination site of CN-E7-Mb. More interestingly, the heterogeneous electron transfer rate constants (k degrees') of CN-E7-Mb from SW and HH were 2.3 (+/- 0.2) x 10(-3) and ca. 2.5 (+/- 0.2) x 10(-3) cm s(-1), respectively, which are much faster than those of native Mbs (1.0 (+/- 0.1) x 10(-4) for Mb(SW) and 8 (+/- 0.5) x 10(-4) cm s(-1) for Mb(HH) under the best conditions). These values are large for Mb having a five-coordinated high-spin heme iron. CN-E7-Mb is an excellent example which indicates that the rapid electron transfer takes place when less displacement of the heme iron is required during the electron transfer and the sixth ligand of the heme iron is not always necessary for the rapid electron transfer. The k degrees' values obtained for CN-E7-Mbs were independent of the pH of the solution in the pH region tested (pH 5.5-7.5). Modification of distal histidine of Mbs by tetrazolyl imidazole was also carried out to give the modified Mb (Tet-E7-Mb) having a six-coordinated low-spin heme iron. Tet-E7-Mb showed a reduction peak around -0.5 V (vs. Ag \ AgCl) and slower electron transfer kinetics compared with those of native Mb and CN-E7-Mb. Also, voltammograms for Tet-E7-Mb suggested the de-coordination of the tetrazolyl imidazole moiety from the heme iron during electrochemical reduction.