Complement component 1q: Difference between revisions

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{{cs1 config|name-list-style=vanc}}

{{short description|Protein complex}}

{{Infobox protein family

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===C1q chains A, B and C===

C1q is composed of 18 polypeptide chains: six A-chains, six B-chains, and six C-chains. Each chain contains a collagen-like region located near the N terminus and a C-terminal globular region. The A-, B-, and C-chains are arranged in the order A-C-B on chromosome 1.<ref name="entrez ciqa">{{cite web | title = Entrez Gene: C1QA complement component 1, q subcomponent, A chain| url = https://www.ncbi.nlm.nih.gov/sites/entrezgene?Db=gene&Cmd=ShowDetailView&TermToSearch=712}}</ref>

== Domain ==

The C1q ___domain is a conserved [[protein ___domain]]. [[C1Q complex|C1q]] is a subunit of the C1 [[enzyme]] [[Protein complex|complex]] that activates the [[blood serum|serum]] [[complement system]]. C1q comprises 6 A, 6 B and 6 C [[polymer|chain]]s. These share the same topology, each possessing a small, globular [[N-terminal]] ___domain, a collagen-like Gly/Pro-rich central region, and a [[conserved sequence|conserved]] C-terminal region, the C1q ___domain.<ref name="pmid1706597">{{cite journal |vauthors=Sellar GC, Blake DJ, Reid KB | title = Characterization and organization of the genes encoding the A-, B- and C-chains of human complement subcomponent C1q. The complete derived amino acid sequence of human C1q | journal = Biochem. J. | volume = 274 | issue = 2| pages = 481–90 |date=March 1991 | pmid = 1706597 | pmc = 1150164 | doi = 10.1042/bj2740481}}</ref> The C1q protein is produced in collagen-producing [[cell (biology)|cell]]s and shows [[sequence (biology)|sequence]] and structural similarity to [[collagen]]s VIII and X.<ref name="pmid2591537">{{cite journal |vauthors=Petry F, Reid KB, Loos M | title = Molecular cloning and characterization of the complementary DNA coding for the B-chain of murine Clq | journal = FEBS Lett. | volume = 258 | issue = 1 | pages = 89–93 |date=November 1989 | pmid = 2591537 | doi = 10.1016/0014-5793(89)81622-9| s2cid = 44986344 | doi-access = }}</ref><ref name="pmid2019595">{{cite journal |vauthors=Muragaki Y, Jacenko O, Apte S, Mattei MG, Ninomiya Y, Olsen BR | title = The alpha 2(VIII) collagen gene. A novel member of the short chain collagen family located on the human chromosome 1 | journal = J. Biol. Chem. | volume = 266 | issue = 12 | pages = 7721–7 |date=April 1991 | doi = 10.1016/S0021-9258(20)89508-8 | pmid = 2019595 | doi-access = free }}</ref>

==Function==

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It is potentially [[Valence (chemistry)|multivalent]] for attachment to the complement fixation sites of [[immunoglobulin]].

The sites are on the CH2 ___domain of [[Immunoglobulin G|IgG]] and, it is thought, on the CH4 ___domain of [[IgM]]. IgG4 cannot bind C1q, but the other three IgG typessubclasses can.

The appropriate [[peptide sequence]] of the complement fixing site might become exposed following complexing of the immunoglobulin, or the sites might always be available, but might require multiple attachment by C1q with critical geometry in order to achieve the necessary [[avidity]].