The L29F mutant of sperm whale myoglobin (Mb), where the leucine 29 residue was replaced by phenylalanine (Phe), was shown to exhibit remarkably high affinity to oxygen (O2), possibly due to stabilization of the heme Fe atom-bound O2 in the mutant protein through a proposed unique electrostatic interaction with the introduced Phe29, in addition to well-known hydrogen bonding with His64 [Carver, T. E.; Brantley, R. E.; Singleton, E. W.; Arduini, R. M.; Quillin, M. L.; Phillips, G. N., Jr.; Olson, J. S. J. Biol. Chem., 1992, 267, 14443-14450]. We analyzed the O2 and carbon monoxide (CO) binding properties of the L29F mutant protein reconstituted with chemically modified heme cofactors possessing a heme Fe atom with various electron densities, to determine the effect of a change in the electron density of the heme Fe atom (ρ(Fe)) on the O2 versus CO discrimination. The study demonstrated that the preferential binding of O2 over CO by the protein was achieved through increasing ρ(Fe), and the ordinary ligand-binding preference, that is, the preferential binding of CO over O2, by the protein was achieved through decreasing ρ(Fe). Thus, the O2 and CO binding preferences of the L29F mutant protein could be controlled through electronic modulation of intrinsic heme Fe reactivity through a change in ρ(Fe). The present study highlighted the significance of the tuning of the intrinsic heme Fe reactivity through the heme electronic structure in functional regulation of Mb.