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- Description:
- 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. 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. Several functionally different haemoglobins can coexist in the same species. The major types of globins include:Haemoglobin (Hb): tetramer 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. Hb proteins are also present in unicellular organisms where they act as enzymes or sensors.Myoglobin (Mb): monomeric protein responsible for oxygen storage in vertebrate muscle. Neuroglobin: a myoglobin-like haemprotein expressed in vertebrate brain and retina, where it is involved in neuroprotection from damage due to hypoxia or ischemia. Neuroglobin belongs to a branch of the globin family that diverged early in evolution. Cytoglobin: an oxygen sensor expressed in multiple tissues. Related to neuroglobin.Erythrocruorin: highly cooperative extracellular respiratory proteins found in annelids and arthropods that are assembled from as many as 180 subunit into hexagonal bilayers.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.Non-symbiotic haemoglobin (NsHb): occurs in non-leguminous plants, and can be over-expressed in stressed plants.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.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. Protoglobin: a single domain globin found in archaea that is related to the N-terminal domain of globin-coupled sensors.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.Lampreys have haemoglobins with self-association and ligand-binding properties that are very different from those characteristic of the tetrameric Hbs of higher vertebrates. Monomeric, ligated lamprey Hb self-associates to dimers and tetramers on deoxygenation; dissociation to monomers on oxygenation accounts for the cooperative binding of O(2) and its pH dependence. Adult erythrocytes of Mordacia mordax (Southern hemisphere lamprey), a southern hemisphere lamprey, contain three monomeric haemoglobins that are closely related to hag-fish haemoglobins.The 3D structures of a great number of vertebrate Hbs in various states are known. The protein is largely alpha-helical, eight conserved helices (A to H) providing the scaffold for a well-defined haem-binding pocket. The imidazole ring of the "proximal" His residue provides the fifth haem iron ligand; the other axial haem iron position remains essentially free for O(2) coordination.The crystal structure of deoxygenated lamprey haemoglobin V has been determined by molecular replacement to 2.7A resolution, in a crystal form with twelve protomers in the asymmetric unit. The subunits are arranged as identical dimers, their interface comprising non-polar interactions and a cluster of four glutamate residues contributed by the E helices and the AB corner - the Bohr effect seems to result from proton uptake by the inter- facial glutamate residues. By contrast with human and mollusc Hbs, where modulation of function results primarily from proximal effects, regulation of oxygen affinity in lamprey Hb V seems to depend on changes at the distal (ligand-binding) side of the haem group.
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