Contribution of the heme propionate groups to the electron transfer and electrostatic properties of myoglobin.

The role of the heme propionate groups in determining the electron transfer and electrostatic properties of myoglobin have been studied by thermodynamic, kinetic, and spectroscopic studies of horse heart myoglobin in which the heme propionate groups are esterified. Spectroelectrochemical analysis has established that the E(m,7) of dimethylester heme-substituted Mb (DME-Mb) (E(m,7)=100.2(2)mV vs. NHE (Normal Hydrogen Electrode) (25 degrees C) is increased approximately 40mV relative to that of the native protein with DeltaH degrees =-12.9(2) kcal/mol and DeltaS degrees =-51.0(8) cal/mol/deg (pH 7.0, mu=0.1M (phosphate)). The second order rate constant for reduction of DME-metMb by Fe(EDTA)(2-) is increased >400-fold relative to that for reduction of native metMb to a value of 1.34(2)x10(3)M(-1)s(-1) with DeltaS(double dagger)=-13(1) cal/mol/deg and DeltaH(double dagger)=9.2(3) (pH 7.0, micro=0.1M (phosphate)). Analysis of the pH dependences of the reduction potential and rate constant for reduction by Fe(EDTA)(2-) demonstrates that heme propionate esterification introduces significant changes into the electrostatic interactions in myoglobin. These changes are also manifested by differences in the pH dependences of the (1)H NMR spectra of native and DME-metMb that reveal shifts in pK(a) values for specific His residues as the result of heme propionate esterification. In sum, the current results establish that heme propionate esterification not only affects the electron transfer properties of myoglobin but also influences the titration behavior of specific His residues.