Diferenzas entre revisións de «Fibrinóxeno»

Low levels of fibrinogen can indicate a systemic activation of the clotting system, with consumption of clotting factors faster than synthesis. This excessive clotting factor consumption condition is known as [[disseminated intravascular coagulation]] or "DIC." DIC can be difficult to diagnose, but a strong clue is low fibrinogen levels in the setting of prolonged clotting times ([[Prothrombin time|PT]] or [[Partial thromboplastin time|aPTT]]), in the context of acute critical illness such as [[sepsis]] or trauma. Besides low fibrinogen level, fibrin polymerization disorders that can be induced by several factors, including plasma expanders, can also lead to severe bleeding problems.<ref name="Lang"/> Fibrin polymerization disorders can be detected by [[viscoelastic]] methods such as thrombelastometry.<ref name="Lang"/>
== Fixioloxía ==
Fibrinogen is a 340 [[kDa]] [[glycoprotein]] synthesised in the [[liver]] by [[hepatocyte]]s. The concentration in [[blood plasma]] is 1.5-4.0 g/L (normally measured using the [[Clauss method]]) or about 7 µM. In its natural form, fibrinogen can form bridges between [[platelet]]s, by binding to their [[GpIIb]]/[[GpIIIa|IIIa]] surface membrane [[protein]]s; however, its major function is as the precursor to fibrin.
Fibrinogen, the principal protein of vertebrate blood clotting, is a [[hexamer]] containing two sets of three different chains (α, β, and γ), linked to each other by [[disulfide]] bonds. The [[N-terminus|N-terminal]] sections of these three chains contain the [[cysteine]]s that participate in the cross-linking of the chains. The [[C-terminus|C-terminal]] parts of the α, β and γ chains contain a domain of about 225 [[amino acid|amino-acid]] residues, which can function as a molecular recognition unit. In fibrinogen as well as in [[angiopoietin]], this domain is implicated in protein-protein interactions. In [[lectin]]s, such as [[mammal]]ian [[ficolin]]s and invertebrate [[tachylectin 5A]], the fibrinogen C-terminal domain binds [[carbohydrate]]s. On the fibrinogen α and β chains, there is a small [[peptide]] sequence (called a [[fibrinopeptide]]). These small peptides are what prevent fibrinogen from spontaneously forming [[polymer]]s with itself.<ref>[http://www.expasy.org/prosite/PDOC00445 PDOC00445] Fibrinogen C-terminal domain in [[PROSITE]]</ref>
The conversion of fibrinogen to fibrin occurs in several steps. First, thrombin cleaves the [[N-terminus]] of the fibrinogen alpha and beta chains to fibrinopeptide A and B respectively.<ref name="pmid692730">{{cite journal | author = Blombäck B, Hessel B, Hogg D, Therkildsen L | title = A two-step fibrinogen--fibrin transition in blood coagulation | journal = Nature | volume = 275 | issue = 5680 | pages = 501–5 | year = 1978 | month = October | pmid = 692730 | doi = 10.1038/275501a0| url = | issn = }}</ref> The resulting fibrin monomers polymerize end to end to from protofibrils, which in turn associate laterally to form fibrin fibers.<ref name="pmid7036348">{{cite journal | author = Hermans J, McDonagh J | title = Fibrin: structure and interactions | journal = Semin. Thromb. Hemost. | volume = 8 | issue = 1 | pages = 11–24 | year = 1982 | month = January | pmid = 7036348 | doi = 10.1055/s-2007-1005039 | url = | issn = }}</ref> In a final step, the fibrin fibers associate to form the fibrin gel.<ref name="isbn0-250-40160-6">{{cite book | editor = Mann KG, Lundblad RL, Fenton J | authorlink = | author = Lorand L, Credo RB | others = | title = Chemistry and Biology of Thrombin | chapter = hrombin and fibrin stabilization | language = | publisher = Ann Arbor Science Publishers | location = Ann Arbor, Mich | year = 1977 | origyear = | pages = 311–323 | quote = | isbn = 0-250-40160-6 | oclc = | doi = | url = | accessdate = }}</ref>