Electrostatic environment of hemes in proteins: pKas of hydroxyl ligands

• Main Authors: Yifan Song, Junjun Mao, M. R. Gunner
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2006
• Subjects: Sperm whale
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.504.8482; http://www.sci.ccny.cuny.edu/~gunner/Pages-lab/papers/bi06Song.heme.pdf

ABSTRACT: The pKas of ferric aquo-heme and aquo-heme electrochemical midpoints (Ems) 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 pKas span 3.3 pH units from 7.6 in heme oxygenase 1 to 10.9 in peroxidase, and the Ems range from-250 mV in peroxidase to 125 mV in Aplysia myoglobin. Proteins with higher in situ ferric aquo-heme pKas tend to have lower Ems. Both changes arise from the protein stabilizing a positively charged heme. However, compared with values in solution, the protein shifts the aquo-heme Ems more than the pKas. 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 CuB in cytochrome c oxidase and Arg 38 in horseradish peroxidase are not corrected, the pKa calculations are in error by as much as 6 pH units. With DFT corrected interactions calculated pKas and Ems differ from measured values by less than 1 pH unit or 35 mV, respectively. The in situ aquo-heme pKa is important for the function of cytochrome c oxidase since it helps to control the