Structural characterization of mutants of human cytoglobin

Cytoglobin (Cygb), a heme protein recently added to the family of vertebrate globins, is an intracellular protein endowed with hexacoordinated heme-iron atom in its ferrous and ferric forms, displaying medium oxygen affinity. It is expressed in many different tissues, although at different levels. The physiological roles of cytoglobin are not completely understood. The crystal structure of hexacoordinated human Cygb displays a core region of about 150 residues, structurally related to hemoglobin and myoglobin, and two extra segments, about 20 residues each, at the N- and C-termini, disordered in both molecules of the crystal asymmetric unit. Cygb displays a large apolar protein matrix cavity next to the heme, which may provide a heme ligand diffusion pathway, and connected to the external space through a narrow tunnel nestled between the globin G- and H-helices. Despite the conserved globin fold, the cavity found in Cygb is structured differently from those recognized to play a functional role in myoglobin, neuroglobin, 2-on-2 hemoglobins, and Cerebratulus lacteus mini-hemoglobin. Here, we present the X-ray three-dimensional structures of three point mutants of human Cygb at topological positions E7, E10 and G12. All the three data sets were collected at synchrotron ESRF (beamline ID23-2, Grenoble, France), at 2.50 Å, 2.50 Å and 2.80 Å resolution, respectively.