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RESEARCH PRODUCT

Neuroglobin and Cytoglobin

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Neuroglobin and cytoglobin are two recent additions to the family of heme-containing respiratory proteins of man and other vertebrates. Here, we summarize the current state of knowledge of the structures, ligand-binding kinetics, evolution, and expression patterns of these two proteins. These data provide working hypotheses with regard to the possible physiological roles of these globins in the animal's metabolism. Both neuroglobin and cytoglobin are structurally similar to myoglobin, but they contain distinct features like extraordinarily high temperature resistances and unusual cavities inside the molecules. Kinetic and structural studies show that neuroglobin and cytoglobin belong to the class of hexacoordinated globins with a biphasic ligand-binding kinetics. Nevertheless, their oxygen affinities resemble that of myoglobin. While neuroglobin is evolutionarily related to one lineage of invertebrate nerve globins, cytoglobin shares a more recent common ancestry with myoglobin. Neuroglobin expression is confined to neurons of the central and peripheral nervous system and to endocrine tissues, with the highest expression observed in the retina. Present evidence points to an important role of neuroglobin in neuronal oxygen homeostasis and hypoxia protection, although other or additional functions are conceivable. Cytoglobin is predominantly expressed in fibroblasts and related cell types, but also in distinct nerve cell populations. Much less is known about its function; cytoglobin may be involved in oxygen transfer to enzymes like collagen prolyl-hydroxylase or NO synthase, ROS protection, or signaling.