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Computational studies on imidazole heme conformations.
Artur S Galstyan1, Snezana D Zarić, Ernst-Walter Knapp
1Institute of Chemistry, Department of Biology, Chemistry, and Pharmacy, Free University of Berlin, Takustrasse 6, 14195 Berlin, Germany.
Summary
Density functional theory reveals imidazole orientation in heme proteins. Electrostatic interactions, influenced by the environment, dictate preferred ligand conformations, with charged propionate groups playing a key role.
Area of Science:
- Computational Chemistry
- Biophysical Chemistry
- Quantum Chemistry
Background:
- Heme proteins utilize imidazole ligands for axial coordination.
- Understanding ligand orientation is crucial for protein function.
- Previous studies suggest electrostatic interactions influence heme-ligand binding.
Purpose of the Study:
- To investigate the preferred orientations of imidazole ligands axially coordinated to heme.
- To elucidate the role of propionic acid groups and environmental factors on imidazole orientation.
- To explore the impact of charged propionates on ligand conformation.
Main Methods:
- Density functional theory (DFT) computations.
- Solving the Poisson equation for continuum dielectric models (water and protein environments).
- Geometry optimization and conformational free-energy calculations.
Main Results:
- In vacuum, imidazoles orient towards heme propionic groups, matching crystal structures.
- Environmental dielectric media (protein, water) reduce the energy difference between preferred and opposite imidazole orientations.
- Intramolecular electrostatics and reaction field interactions are key determinants of orientation.
Conclusions:
- Imidazole orientation is governed by direct propionate-imidazole interactions and the complex's dipole moment in a dielectric medium.
- Environmental polarity weakens orientational preference.
- Neutralizing propionate charges enhances imidazole orientational preference, suggesting salt bridges or protonation stabilize specific conformations.