Two hexacoordinate hemoglobins (hxHbs) have very recently been identified in humans. Neuroglobin (neuroHb) was the first human hxHb discovered, and is localized in the brain. We have identified a second human hxHb that is currently referred to as histoglobin (histoHb), and is found in nearly all tissues. HxHbs are a class of proteins found in a wide array of organisms including animals, plants, and photosynthetic microorganisms. The nomenclature """"""""hxHb"""""""" denotes a hemoglobin with intramolecular coordination of the ligand binding site similar to cytochrome b5. In spite of this obstruction (cytochrom b5 does not bind gaseous ligands), hxHbs bind ligands reversibly and with apparently high affinities. NeuroHb is thought to play a role in cell survival during hypoxia, but no physiological roles have yet been established with confidence for these proteins. ? ? Our long range goal is to deduce the physiological role(s) of human hxHbs, and to understand how their function is dictated by protein structure. The objective of this proposal is to measure the affinities of human hxHbs for ligands including 02 and nitric oxide (NO), to identify basic regulatory elements of these proteins, and to evaluate their capacity to scavenge NO both in vitro and in vivo. Our central hypothesis is that neuroHb and histoHb are high affinity Hbs that use reversible hexacoordination and protein conformational changes to regulate ligand binding. We further suggest that neuroHb serves to scavenge NO in neural tissues. The rationale underlying our objective is that fundamental examination of the reactions of these proteins with their biological ligands is crucial in ascertaining their potential physiological roles. ? ? Four specific aims are proposed to test our hypotheses. 1) Characterization of 02, CO, NO, and CN- binding to neuroHb. 2) Elucidation of the structure and mechanism of ligand binding for human neuroHb. 3) Examination of the NO dioxygenation activity of neuroHb, and evaluation of its physiological relevance. 4) Structural and biophysical characterization of histoHb, and its ability to function as an NO dioxygenase. ? ? The study of both hemoglobins and NO have been important topics in medical research for some time. But a large body of recent work has made it clear that we are only beginning to realize the physiological significance of their interactions with one another. Our work is critical as hxHbs are proving to be biologically ubiquitous molecules, and it is imperative that we assess their potential involvement in NO homeostasis in humans. Additionally, our experiments with neuroHb and histoHb will develop general principles that will be applicable to all hxHb. ? ? ?
