Electrostatic environment of hemes in proteins: pK(a)s of hydroxyl ligands.
The pK(a)s of ferric aquo-heme and aquo-heme electrochemical midpoints (E(m)s) at pH 7 in sperm whale myoglobin, Aplysia myoblogin, hemoglobin I, heme oxygenase 1, horseradish peroxidase and cytochrome c oxidase were calculated with Multi-Conformation Continuum Electrostatics (MCCE). The pK(a)s span 3.3 pH units from 7.6 in heme oxygenase 1 to 10.9 in peroxidase, and the E(m)s range from -250 mV in peroxidase to 125 mV in Aplysia myoglobin. Proteins with higher in situ ferric aquo-heme pK(a)s tend to have lower E(m)s. Both changes arise from the protein stabilizing a positively charged heme. However, compared with values in solution, the protein shifts the aquo-heme E(m)s more than the pK(a)s. Thus, the protein has a larger effective dielectric constant for the protonation reaction, showing that electron and proton transfers are coupled to different conformational changes that are captured in the MCCE analysis. The calculations reveal a breakdown in the classical continuum electrostatic analysis of pairwise interactions. Comparisons with DFT calculations show that Coulomb's law overestimates the large unfavorable interactions between the ferric water-heme and positively charged groups facing the heme plane by as much as 60%. If interactions with Cu(B) in cytochrome c oxidase and Arg 38 in horseradish peroxidase are not corrected, the pK(a) calculations are in error by as much as 6 pH units. With DFT corrected interactions calculated pK(a)s and E(m)s differ from measured values by less than 1 pH unit or 35 mV, respectively. The in situ aquo-heme pK(a) is important for the function of cytochrome c oxidase since it helps to control the stoichiometry of proton uptake coupled to electron transfer [Song, Michonova-Alexova, and Gunner (2006) Biochemistry 45, 7959-7975].
Song
Yifan
Y
Physics Department J-419, City College of New York, 138th Street and Convent Avenue, New York, New York 10031, USA.
Mao
Junjun
J
Gunner
M R
MR
D000818:Animals; D001048:Aplysia; D006419:Heme Oxygenase (Decyclizing) / Q000737:chemistry* / Q000378:metabolism*; D006420:Hemeproteins / Q000737:chemistry* / Q000378:metabolism; D017665:Hydroxyl Radical / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D009211:Myoglobin / Q000737:chemistry; D011487:Protein Conformation; D055672:Static Electricity; D013816:Thermodynamics; D014907:Whales
D006420:Hemeproteins; D008024:Ligands; D009211:Myoglobin; D017665:Hydroxyl Radical; D006419:Heme Oxygenase (Decyclizing)