Serpins (SERine Proteinase INhibitors) [14705960, 2690952, 8417965] belong to MEROPS inhibitor family I4, clan ID.
Serpins are proteins that are primarily known as irreversible serine protease inhibitors active against S1 , S8 and C14 peptidases. There are both extra- and intra-cellular serpins, which are found in all groups of organisms with the notable exception of fungi [11116082, 12411597]. In contrast to "rigid" proteinase inhibitors, such as those of the Kunitz or Kazal families, the serpins are metastable proteins (active-state proteins) which interact with their substrate and irreversibly trap the acyl intermediate as a result of a major conformational change ; they are best described as suicide substrate inhibitors. The common structure of these proteins is a multi-domain fold containing a bundle of 8 or 9 alpha helices and a beta sandwich formed by 3 beta sheets. The reactive centre loop (RCL) is found in the C-terminal part of these proteins.
Serpins and their homologues are a group of high molecular weight (40 to 50 kDa) structurally related proteins involved in a number of fundamental biological processes such as blood coagulation, complement activation, fibrinolysis, angiogenesis, inflammation, tumour suppression and hormone transport. All known serpins have been classified into 16 clades and 10 orphan sequences; the vertebrate serpins can be conveniently classified into six sub-groups . In human plasma they represent approximately 2% of the total protein, of which 70% is alpha-1-antitrypsin.
On the basis of strong sequence similarities, a number of proteins with no
known inhibitory activity also belong to this family, these include: angiotensinogen, corticosteroid-binding globulin and thyroxin-binding globulin .