<p>Globins are haem-containing proteins involved in binding and/or transporting oxygen. They belong to a very large and well studied family that is widely distributed in many organisms [<cite idref="PUB00035866"/>]. Globins have evolved from a common ancestor and can be divided into three groups: single-domain globins, and two types of chimeric globins, flavohaemoglobins and globin-coupled sensors. Bacteria have all three types of globins, while archaea lack flavohaemoglobins, and eukaryotes lack globin-coupled sensors [<cite idref="PUB00035865"/>]. Several functionally different haemoglobins can coexist in the same species. The major types of globins include:</p><p> <ul><li>Haemoglobin (Hb): trimer of two alpha and two beta chains, although embryonic and foetal forms can substitute the alpha or beta chain for ones with higher oxygen affinity, such as gamma, delta, epsilon or zeta chains. Hb transports oxygen from lungs to other tissues in vertebrates [<cite idref="PUB00035870"/>]. Hb proteins are also present in unicellular organisms where they act as enzymes or sensors [<cite idref="PUB00035871"/>].</li><li>Myoglobin (Mb): monomeric protein responsible for oxygen storage in vertebrate muscle [<cite idref="PUB00035872"/>].</li><li> Neuroglobin: a myoglobin-like haemprotein expressed in vertebrate brain and retina, where it is involved in neuroprotection from damage due to hypoxia or ischemia [<cite idref="PUB00029465"/>]. Neuroglobin belongs to a branch of the globin family that diverged early in evolution. </li><li>Cytoglobin: an oxygen sensor expressed in multiple tissues. Related to neuroglobin [<cite idref="PUB00035873"/>].</li><li>Erythrocruorin: highly cooperative extracellular respiratory proteins found in annelids and arthropods that are assembled from as many as 180 subunit into hexagonal bilayers [<cite idref="PUB00035877"/>].</li><li>Leghaemoglobin (legHb or symbiotic Hb): occurs in the root nodules of leguminous plants, where it facilitates the diffusion of oxygen to symbiotic bacteriods in order to promote nitrogen fixation.</li><li>Non-symbiotic haemoglobin (NsHb): occurs in non-leguminous plants, and can be over-expressed in stressed plants [<cite idref="PUB00055462"/>].</li><li>Flavohaemoglobins (FHb): chimeric, with an N-terminal globin domain and a C-terminal ferredoxin reductase-like NAD/FAD-binding domain. FHb provides protection against nitric oxide via its C-terminal domain, which transfers electrons to haem in the globin [<cite idref="PUB00035867"/>].</li><li>Globin-coupled sensors: chimeric, with an N-terminal myoglobin-like domain and a C-terminal domain that resembles the cytoplasmic signalling domain of bacterial chemoreceptors. They bind oxygen, and act to initiate an aerotactic response or regulate gene expression [<cite idref="PUB00035868"/>, <cite idref="PUB00035869"/>]. </li><li>Protoglobin: a single domain globin found in archaea that is related to the N-terminal domain of globin-coupled sensors [<cite idref="PUB00016016"/>].</li><li>Truncated 2/2 globin: lack the first helix, giving them a 2-over-2 instead of the canonical 3-over-3 alpha-helical sandwich fold. Can be divided into three main groups (I, II and II) based on structural features [<cite idref="PUB00055463"/>].</li></ul> </p><p>This entry represents a group of haemoglobin-like proteins found in eubacteria, cyanobacteria, protozoa, algae and plants, but not in animals or yeast. These proteins have a truncated 2-over-2 rather than the canonical 3-over-3 alpha-helical sandwich fold [<cite idref="PUB00035871"/>]. This entry includes:</p><p> <ul><li>HbN (or GlbN): a truncated haemoglobin-like protein that binds oxygen cooperatively with a very high affinity and a slow dissociation rate, which may exclude it from oxygen transport. It appears to be involved in bacterial nitric oxide detoxification and in nitrosative stress [<cite idref="PUB00035878"/>].</li><li>Cyanoglobin (or GlbN): a truncated haemoprotein found in cyanobacteria that has high oxygen affinity, and which appears to serve as part of a terminal oxidase, rather than as a respiratory pigment [<cite idref="PUB00035880"/>].</li><li>HbO (or GlbO): a truncated haemoglobin-like protein with a lower oxygen affinity than HbN. HbO associates with the bacterial cell membrane, where it significantly increases oxygen uptake over membranes lacking this protein. HbO appears to interact with a terminal oxidase, and could participate in an oxygen/electron-transfer process that facilitates oxygen transfer during aerobic metabolism [<cite idref="PUB00035879"/>].</li><li>Glb3: a nuclear-encoded truncated haemoglobin from plants that appears more closely related to HbO than HbN. Glb3 from <taxon tax_id="3702">Arabidopsis thaliana</taxon> (Mouse-ear cress) exhibits an unusual concentration-independent binding of oxygen and carbon dioxide [<cite idref="PUB00035881"/>].</li></ul> </p><p>These proteins in this entry contain a conserved histidine which could be involved in heme-binding. This signature represents the conserved region that ends with this histidine residue.</p> Globin, bacterial-like, conserved site