Editing Human chorionic gonadotropin (section)

== Structure ==

Human chorionic gonadotropin is a [[glycoprotein]] composed of 237 [[amino acid]]s with a [[molecular mass]] of 36.7 [[kDa]], approximately 14.5kDa αhCG and 22.2kDa βhCG.<ref name="Canfield_1987">{{cite journal | vauthors = Canfield RE, O'Connor JF, Birken S, Krichevsky A, Wilcox AJ | title = Development of an assay for a biomarker of pregnancy and early fetal loss | journal = Environmental Health Perspectives | volume = 74 | pages = 57–66 | date = October 1987 | pmid = 3319556 | pmc = 1474496 | doi = 10.1289/ehp.877457 | bibcode = 1987EnvHP..74...57C }}</ref>

It is [[heterodimer]]ic, with an α (alpha) [[protein subunit|subunit]] identical to that of [[luteinizing hormone]] (LH), [[follicle-stimulating hormone]] (FSH), [[thyroid-stimulating hormone]] (TSH), and a β (beta) subunit that is unique to hCG.
* The [[chorionic gonadotropin alpha|α (alpha)]] [[protein subunit|subunit]] is 92 amino acids long.<ref name="urlGlycoprotein hormones alpha chain precursor - Homo sapiens (Human)">{{cite web | url = https://www.uniprot.org/uniprot/P01215 | title = Glycoprotein hormones alpha chain precursor - Homo sapiens (Human) | publisher = UniProt Consortium | work = UniProt accession number P01215 | quote = P01215[25-116] }}</ref>
* The β-subunit of hCG gonadotropin ('''beta-hCG''') contains 145 amino acids, encoded by six highly homologous [[gene]]s that are arranged in tandem and inverted pairs on [[chromosome 19]]q13.3 - ''CGB'' ([[CGB1|''1'']], [[CGB2 (gene)|''2'']], [[CGB3 (gene)|''3'']], [[CGB5|''5'']], [[CGB7|''7'']], [[CGB8|''8'']]). It is known that CGB7 has a sequence slightly different from that of the others.<ref name="urlChoriogonadotropin subunit beta precursor - Homo sapiens (Human)">{{cite web | url = https://www.uniprot.org/uniprot/P0DN86 | title = Choriogonadotropin subunit beta 3 - Homo sapiens (Human) | publisher = UniProt Consortium | work = UniProt accession number P01233 | quote = P0DN86[21-165]; Two specific hCGb proteins that differ by three amino acids in positions 2,4 and 117 have been described: type 1 (CGB7) and type 2 (CGB3, CGB5, CGB8).}}</ref>

The two subunits create a small [[hydrophobic]] core surrounded by a high surface area-to-volume ratio: 2.8 times that of a sphere.  The vast majority of the outer amino acids are [[hydrophilic]].<ref name="pmid8202136">{{PDB|1HRP}}; {{cite journal | vauthors = Lapthorn AJ, Harris DC, Littlejohn A, Lustbader JW, Canfield RE, Machin KJ, Morgan FJ, Isaacs NW | title = Crystal structure of human chorionic gonadotropin | journal = Nature | volume = 369 | issue = 6480 | pages = 455–461 | date = June 1994 | pmid = 8202136 | doi = 10.1038/369455a0 | s2cid = 4263358 | bibcode = 1994Natur.369..455L }}</ref>

beta-hCG is mostly similar to [[beta-LH]], with the exception of a Carboxy Terminus Peptide (beta-CTP) containing four glycosylated serine residues that is responsible for hCG's longer half-life.<ref>{{cite journal | vauthors = Furuhashi M, Shikone T, Fares FA, Sugahara T, Hsueh AJ, Boime I | title = Fusing the carboxy-terminal peptide of the chorionic gonadotropin (CG) beta-subunit to the common alpha-subunit: retention of O-linked glycosylation and enhanced in vivo bioactivity of chimeric human CG | journal = Molecular Endocrinology | volume = 9 | issue = 1 | pages = 54–63 | date = January 1995 | pmid = 7539107 | doi = 10.1210/mend.9.1.7539107 | doi-access = free }}</ref>