Iron-histidine bonding in bishistidyl hemoproteins-A local vibrational mode study

Abstract

We investigated the intrinsic strength of distal and proximal FeN bonds for both ferric and ferrous oxidation states of bishistidyl hemoproteins from bacteria, animals, human, and plants, including two cytoglobins, ten hemoglobins, two myoglobins, six neuroglobins, and six phytoglobins. As a qualified measure of bond strength, we used local vibrational force constants k ${}^a$ (FeN) based on local mode theory developed in our group. All calculations were performed with a hybrid QM/MM ansatz. Starting geometries were taken from available x-ray structures. k ${}^a$ (FeN) values were correlated with FeN bond lengths and covalent bond character. We also investigated the stiffness of the axial NFeN bond angle. Our results highlight that protein effects are sensitively reflected in k ${}^a$ (FeN), allowing one to compare trends in diverse protein groups. Moreover, k ${}^a$ (NFeN) is a perfect tool to monitor changes in the axial heme framework caused by different protein environments as well as different Fe oxidation states.

Keywords: QM/MM; bishistidyl hemoproteins; intrinsic bond strength; local mode analysis; local mode force constants.