The fibrinolytic pathway is activated by fibrin clots. Coagulation, anticoagulant systems and fibrinolysis pathways coordinate to maintain proper hemostasis. The coagulation cascade, tightly controlled by anticoagulation and intimately connected with the complement system of innate immunity, culminates in the activation of thrombin (F2) - the last protease of the cascade, and the subsequent formation of insoluble fibrin clots. Fibrin, in turn, is a substrate for the initiation of the fibrinolyti
c pathway whose end result is the dissolution of clots. The main component of fibrinolysis is plasmin - the active form of plasminogen (PLG). Plasminogen consists of seven domains: an N-terminal cleavable domain, five kringle domains (KR1-5) and the serine protease domain (click to access the PFAM entry for KR). A characteristic feature of KR is the ability of binding lysine residues via 'lysine binding sites' (LBS) that most KRs, though not all, possess. Plasminogen adopts two distinct conformations: a closed conformation which is not easily activated (click to access the crystal structure page, and an open conformation when bound to fibrin or cell surfaces. Removal of the N-terminal domain produces an alternative zymogen that also adopts an open conformation. The open conformation exposes the activation loop for cleavage by PLG activators. Plasmin processing of fibrin generates C-terminal lysines that promote enhanced plasminogen binding, thus providing a positive feedback mechanism. C-terminal and also internal lysines can be bound by the LBS found in KR. The two glycoform types of PLG - type I which has both N- and O-linked moieties (click to see type I structure), and type II which only has O-linked (click to see type II structure), have distinct conformations and activities. A major activator of PLG is 'tissue plasminogen activator' - tPA (PLAT). Fibrin harbors binding sites for Plg as well as Plat. Colocalization of Plat with Plg on fibrin stimulates Plat activity by several fold. Plat contains five modules: an N-terminal finger domain followed by an epidermal growth factor-like (EGF) domain, two KRs (K1 and K2) and the C-terminal serine protease domain. The finger domain is involved in fibrin binding. The secreted single chain Plat (Sc-tPA), unlike other members of the chymotrypsin family, has intrinsic catalytic activity. However, it can also be processed to a two-chain form (tc-tPA) by plasmin or kallikrein. There are three major N-glycosylation sites in PLAT, of which two are constitutive and a third is alternative. Depending on whether all sites or only the constitutive ones are glycosylated, the enzyme is referred to as type I or type II tPA, respectively. Another PLG activator is 'urokinase plasminogen activator' - uPA (PLAU), usually involved in cell-specific fibrinolysis and in association with a specific receptor (PLAUR).
Fibrinolysis is tightly regulated by several inhibitory systems. Circulating plasmin and plasminogen activators are targeted by serpin serine proteases which form irreversible, covalent complexes with their targets that are then cleared from circulation. Two major inhibitors are Serpnf2 and Serpine1. The interaction of SERPINF2, (known as alpha-2 antiplasmin) with plasmin is one of the fastest, close to the diffusion limit. SERPINE1 (known as PAI-1) inhibits both the tissue (PLAT) and urokinase (PLAU) plasminogen activators. Other serpins include SERPINB2 (PAI-2), SERPINA5 (PAI-3), SERPINE2 (nexin). On the other hand, fibrinolysis can be inhibited within a clot by carboxypeptidase B2 (CPB2, known as TAFI), activated by thrombin (F2) in conjunction with its receptor thrombomodulin (Thbd). Cpb2 removes C-terminal lysine residues in fibrin, thus reducing plasmin binding and fibrinolysis. Plasmin and PLG activators are highlighted in green; the main inhibitors are highlighted in pink.
PLG, PLAT and PLAUR, besides their well-known roles in the vascular system, through their substrates and/or interactors play a range of other roles in the nervous and immune systems, in cell adhesion, migration and proliferation, and in wound healing and signaling. For instance, besides fibrin, substrates of plasmin include components of the complement system and of the coagulation cascade, vitronectin (involved in cell adhesion), osteocalcin (bone formation), tenascin C (cell migration and proliferation), and others. Plasminogen can bind to several receptors such as the heterotetrameric ANXA2 (annexin A2)-S100A10 complex or the plasminogen receptor PLGRKT, with roles in the immune and vascular systems, among others (both receptors also increase the rate of PLG activation). PLG interaction with the metalloenzyme enolase-1 on the other hand, promotes cell migration in pathophysiological processes. Tissue plasminogen activator PLAT plays many roles in the nervous system where it exerts both neurotrophic and neurotoxic effects. A major serpin inhibitor of PLAT in the brain is SERPINI1 (neuroserpin). PLAUR can also play a role in adhesion as a receptor for vitronectin and is involved in signaling via G protein coupled receptors (GPCR), integrins, and receptor tyrosine kinases (RTK). The fibrinolytic system is targeted by bacteria which hijack the host plasminogen. Several pathogens have been shown to recruit plasminogen to facilitate invasion of the host. Defects in fibrinolysis are associated with several conditions.