Calbindin

{{Main|Calbindin 1}}

Calbindin 1 or simply calbindin was first shown to be present in the intestine in birds and then found in the mammalian kidney. It is also expressed in a number of neuronal and endocrine cells, particularly in the cerebellum. It is a 28 kDa protein encoded in humans by the CALB1 gene.

Calbindin contains 4 active calcium-binding domains, and 2 modified domains that have lost their calcium-binding capacity. Calbindin acts as a calcium buffer and calcium sensor and can hold four Ca²⁺ in the EF-hands of loops EF1, EF3, EF4 and EF5. The structure of rat calbindin was originally solved by nuclear magnetic resonance and was one of the largest proteins then to be determined by this technique. The sequence of calbindin is 263 residues in length and has only one chain. The sequence consists mostly of alpha helices but beta sheets are not absent. According to the NMR PDB (PDB entry 2G9B){{Cite journal|last1=Kojetin|first1=Douglas J|last2=Venters|first2=Ronald A|last3=Kordys|first3=David R|last4=Thompson|first4=Richele J|last5=Kumar|first5=Rajiv|last6=Cavanagh|first6=John|date = July 2006|title=Structure, binding interface and hydrophobic transitions of Ca2+-loaded calbindin-D28K|journal=Nature Structural & Molecular Biology|language=en|volume=13|issue=7|pages=641–647|doi=10.1038/nsmb1112|pmid=16799559|s2cid=29426332|issn=1545-9993}} it is 44% helical with 14 helices containing 117 residues, and 4% beta sheet with 9 strands containing 13 residues.

In 2018 the X-ray crystal structure of human calbindin was published (PDB entry 6FIE).{{Cite journal|last1=Noble|first1=J. W.|last2=Almalki|first2=R.|last3=Roe|first3=S. M.|last4=Wagner|first4=A.|last5=Duman|first5=R.|last6=Atack|first6=J. R.|date=2018|title=The X-ray structure of human calbindin-D28K: an improved model.|url=https://www.rcsb.org/structure/6FIE|journal=Acta Crystallogr D|language=en-US|volume=74|issue=Pt 10|pages=1008–1014|doi=10.2210/pdb6fie/pdb|pmid=30289411|pmc=6173056}}{{cite journal | vauthors = Noble JW, Almalki R, Roe SM, Wagner A, Duman R, Atack JR | title = The X-ray structure of human calbindin-D28K: an improved model | journal = Acta Crystallographica Section D | volume = 74 | issue = Pt 10 | pages = 1008–1014 | date = October 2018 | pmid = 30289411 | pmc = 6173056 | doi = 10.1107/S2059798318011610 | bibcode = 2018AcCrD..74.1008N }} There were differences observed between the nuclear magnetic resonance and crystal structure despite 98% sequence identity between the rat and human isoforms. Small angle X-ray scattering indicates that the crystal structure better predicts the properties of calbindin in solution compared with the structure determined by nuclear magnetic resonance.

Calbindin is a vitamin D–responsive gene in many tissues, in particular the chick intestine, where it has a clear function in mediating calcium absorption.{{cite book | vauthors = Wasserman RH, Fullmer CS | title = Mineral Absorption in the Monogastric GI Tract | chapter = On the Molecular Mechanism of Intestinal Calcium Transport | series = Advances in Experimental Medicine and Biology | volume = 249 | pages = 45–65 | year = 1989 | pmid = 2543194 | doi = 10.1007/978-1-4684-9111-1_5 | isbn = 978-1-4684-9113-5 }} In the brain, its synthesis is independent of vitamin-D.

{{Main|Calretinin}}

Calretinin, also known as calbindin 2, is a 29 kDa protein with 58% homology to calbindin 1 and principally found in nervous tissues.{{cite journal | vauthors = Rogers JH | title = Calretinin: a gene for a novel calcium-binding protein expressed principally in neurons | journal = The Journal of Cell Biology | volume = 105 | issue = 3 | pages = 1343–53 | date = September 1987 | pmid = 3654755 | pmc = 2114790 | doi = 10.1083/jcb.105.3.1343 }} It is encoded in humans by the CALB2 gene and was formerly known as calbindin-D29k.

{{See also|S100G}}

S100G, formerly calbindin 3 and calbindin-D9k, is present in mammalian enterocytes (epithelial cells of the intestine). S100G can also be found in the kidney and uterus in some mammalian species. It is encoded in humans by the S100G gene which has also been termed CALB3. Nonetheless, there is no homology between calbindin 1 and S100G, apart from their calcium binding domains (EF-hands): S100G has two EF-hands, and calbindin 1 has six. Unlike calbindin 1 and 2, S100G is a member of the S100 family of calcium-binding proteins.

S100G mediates the transport of calcium across the enterocytes from the apical side, where entry is regulated by the calcium channel TRPV6, to the basolateral side, where calcium pumps such as PMCA1 utilize intracellular adenosine triphosphate to pump calcium into the blood.{{cite journal | vauthors = Wasserman RH, Chandler JS, Meyer SA, Smith CA, Brindak ME, Fullmer CS, Penniston JT, Kumar R | display-authors = 6 | title = Intestinal calcium transport and calcium extrusion processes at the basolateral membrane | journal = The Journal of Nutrition | volume = 122 | issue = 3 Suppl | pages = 662–71 | date = March 1992 | pmid = 1311756 | doi = 10.1093/jn/122.suppl_3.662 | doi-access = free }} The transport of calcium across the enterocyte cytoplasm appears to be rate-limiting for calcium absorption in the intestine; the presence of calbindin increases the amount of calcium crossing the cell without raising the free concentration.{{cite journal | vauthors = Feher JJ, Fullmer CS, Wasserman RH | title = Role of facilitated diffusion of calcium by calbindin in intestinal calcium absorption | journal = The American Journal of Physiology | volume = 262 | issue = 2 Pt 1 | pages = C517-26 | date = February 1992 | pmid = 1539638 | doi = 10.1152/ajpcell.1992.262.2.C517 }} S100G may also stimulate the basolateral calcium-pumping ATPases. Expression of S100G, like that of calbindin 1, is stimulated by the active vitamin D metabolite, calcitriol although the precise mechanisms are still controversial.{{cite journal | vauthors = Barley NF, Prathalingam SR, Zhi P, Legon S, Howard A, Walters JR | title = Factors involved in the duodenal expression of the human calbindin-D9k gene | journal = The Biochemical Journal | volume = 341 ( Pt 3) | issue = 3 | pages = 491–500 | date = August 1999 | pmid = 10417310 | pmc = 1220384 | doi = 10.1042/0264-6021:3410491 }} In mice in which the vitamin D receptor is not expressed, S100G is less abundant, but not absent.{{citation needed|date=August 2019}}

Vitamin D-dependent calcium binding proteins were discovered in the cytosolic fractions of chicken intestine, and later in mammalian intestine and kidney, by researchers including Robert Wasserman of Cornell University.{{cite journal | vauthors = Wasserman RH, Taylor AN | title = Vitamin d3-induced calcium-binding protein in chick intestinal mucosa | journal = Science | volume = 152 | issue = 3723 | pages = 791–3 | date = May 1966 | pmid = 17797460 | doi = 10.1126/science.152.3723.791 | bibcode = 1966Sci...152..791W | s2cid = 8221178 }}{{cite journal | vauthors = Wasserman RH, Corradino RA, Taylor AN | title = Binding proteins from animals with possible transport function | journal = The Journal of General Physiology | volume = 54 | issue = 1 | pages = 114–37 | date = July 1969 | pmid = 19873640 | pmc = 2225897 | doi = 10.1085/jgp.54.1.114 }} Such proteins bound calcium in the micromolar range and were greatly reduced in vitamin D-deficient animals. Expression could be induced by treating these animals with vitamin D metabolites such as calcitriol.

They were found to exist in two distinct sizes with a molecular weight of approximately 9 kDa and 28 kDa, and they were renamed calbindins.

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{{Calcium-binding proteins}}

Category:Protein families