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 (O-2), possibly due to stabilization of the heme Fe atom-bound O-2 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 O-2 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 (rho(Fe)) on the O-2 versus CO discrimination. The study demonstrated that the preferential binding of O-2 over CO by the protein was achieved through increasing rho(Fe), and the ordinary ligand-binding preference, that is, the preferential binding of CO over O-2, by the protein was achieved through decreasing rho(Fe). Thus, the O-2 and CO binding preferences of the L29F mutant protein could be controlled through electronic modulation of intrinsic heme Fe reactivity through a change in rho(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.