<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> Leghaemoglobins are haem-proteins, first identified in root nodules of leguminous plants, where they are crucial for supplying sufficient oxygen to root nodule bacteria for nitrogen fixation to occur [<cite idref="PUB00035889"/>, <cite idref="PUB00006066"/>]. Although leghaemoglobin and myoglobin both share a common fold, and both regulate the facilitated diffusion of oxygen, leghemoglobins regulate oxygen affinity through a mechanism different from that of myoglobin using a novel combination of haem pocket amino acids that lower the oxygen affinity [<cite idref="PUB00035890"/>, <cite idref="PUB00035891"/>]. The structure of leghaemoglobins is similar to that of haemoglobins and myoglobins, although there is little sequence conservation [<cite idref="PUB00006066"/>]. The protein is largely alpha-helical, eight helices providing the scaffold for a well-defined haem-binding pocket [<cite idref="PUB00006066"/>]. By contrast with the tetrameric mammalian globin assembly, the plant form is monomeric [<cite idref="PUB00004012"/>]. </p> <p> The structural similarity of leghaemoglobins and haemoglobins has suggested a common evolutionary origin. It was thought that haemoglobins may be found in plants other than legumes [<cite idref="PUB00004012"/>], and indeed globins have now been identified in the roots of non-leguminous plants, where they have a role in respiratory metabolism in the root cells [<cite idref="PUB00004012"/>]. </p><p> This entry also represents Non-symbiotic haemoglobins (NsHb) which play important roles in a variety of cellular processes. A class I NsHb from cotton plants can be induced in plant roots as a defence mechanism against pathogen invasions, possibly by modulating nitric oxide (NO) levels [<cite idref="PUB00035892"/>]. Several NsHbs appear to play a role NO scavenging in plants, indicating that the primordial function of haemoglobins may well be to protect against nitrosative stress and to modulate NO signalling functions [<cite idref="PUB00035893"/>].</p>