In these studies, we substitute electron-withdrawing (diacetyl) or - donating (diethyl) groups at the 2- and 4-positions of the heme in sperm whale Mb and HRP, and examine the structural and biochemical consequences. X- ray absorption spectroscopy shows that increased electron density at the heme results in an increased iron-pyrrole nitrogen average distance in both HRP and Mb, while decreased electron density results in shorter average distances. In HRP, the proximal ligand is constrained by a H-bonding network, and axial effects are manifested entirely at the distal site. Conversely, in Mb, where the proximal ligand is less constrained, axial effects are seen at the proximal side. In HRP, electron density at the heme iron depends linearly on pK3, a measure of the basicity of the porphyrin pyrrole nitrogens [Yamada, H., Makino, R., and Yamazaki, I. (1975) Arch. Biochem. Biophys. 169, 344-353]. Using diethyl substitution (pK3 = 5.8) and diacetyl substitution (pK3 = 3.3) in HRP and Mb, we measured the one-electron reduction potentials (E(o)') of HRP compounds I and II and ferryl Mb. Compound I showed a decreased E(o)' with increasing electron density at the heme (pK3), similar to E(o)' of ferric HRP. E(o)' of HRP compound II and ferryl Mb showed an opposite dependence. This behavior of E(o)', while initially surprising, can be explained by the apparent net positive charge on the iron porphyrin in each oxidation state of the hemoproteins.
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