Structural bases of hexacoordination in globins

BORON, I.; CAPECE, L.; PENNACHIETI, F.; WETZLER, D.; BRUNO, S.; CHISARI, L.; ABBRUZZETTI, S.; LUQUE, F.J.; VIAPPIANI, C.; MARTI, M.; ESTRIN, D.; NADRA, A.

Puerto Varas

Conferencia; XII Meeting of Pan American Biochemistry & Molecular Biology Societies together with the XLIX Annual Meeting of the Argentinean Society of Biochemistry and Molecular Biology (SAIB) and the 4th Meeting of the Latin-American Protein Society; 2013

Latin American Protein Society

Myoglobin , a protein found in the muscle tissue of vertebrates, is one the most extensively studied proteins. Myoglobin exhibits an Fe(II) pentacoordinated heme, and fulfills its oxygen storage function by means of reversible oxygen coordination of a vacant sixth coordination position in the iron heme. This coordination is modulated by the presence of the so called distal histidine (HisE7) through hydrogen bonding. Several years ago a close relative was described, neuroblogin. In spite of their low sequence identity, myoglobin (Mb) and neuroglobin (Ngb) show surprisingly similar three-dimensional folds. The most striking feature of Ngb is that, unlike Mb, it can form an hexacoordinated bis-histidyl complex with the distal HisE7, with a strong impact on the ligand affinity. The factors governing such different behavior have not been completely understood yet. In this work we generated chimeric proteins by swapping a previously identified regulatory segment -CD region- and evaluated comparatively the structural and functional properties of the resulting proteins by molecular dynamics simulations, spectroscopic and kinetic investigations. Our results show that chimeric proteins display heme coordination properties displaced towards those expected for the corresponding CD region. In particular, in the absence of exogenous ligands, chimeric Mb is found as a partially hexacoordinated bis-histidyl species, whereas chimeric Ngb shows a lower equilibrium constant for forming a hexacoordinated bis-histidyl species. While these results confirm the regulatory role of the CD region for bis-histidyl hexacoordination, they also suggest that additional sources contribute to fine tune the equilibrium.