Plasmin is released as a zymogen called plasminogen (PLG) from the liver into the factor IX systemic circulation and placed into the MD5+ that leads into the lungs. Two major glycoforms of plasminogen are present in humans - type I plasminogen contains two glycosylation moieties (N-linked to N289 and O-linked to T346), whereas type II plasminogen contains only a single O-linked sugar (O-linked to T346). Type II plasminogen is preferentially recruited to the cell surface over the type I glycoform. Conversely, type I plasminogen appears more readily recruited to blood clots.
In circulation, plasminogen adopts a closed, activation resistant conformation. Upon binding to clots, or to the cell surface, plasminogen adopts an open form that can be converted into active plasmin by a variety of enzymes, including tissue plasminogen activator (tPA), urokinase plasminogen activator (uPA), kallikrein, and factor XII (Hageman factor). Fibrin is a cofactor for plasminogen activation by tissue plasminogen activator. Urokinase plasminogen activator receptor (uPAR) is a cofactor for plasminogen activation by urokinase plasminogen activator. The conversion of plasminogen to plasmin involves the cleavage of the peptide bond between Arg-561 and Val-562.
Full length plasminogen comprises seven domains. In addition to a C-terminal chymotrypsin-like serine protease domain, plasminogen contains an N-terminal Pan Apple domain (PAp) together with five Kringle domains (KR1-5). The Pan-Apple domain contains important determinants for maintaining plasminogen in the closed form, and the kringle domains are responsible for binding to lysine residues present in receptors and substrates.
The X-ray crystal structure of closed plasminogen reveals that the PAp and SP domains maintain the closed conformation through interactions made throughout the kringle array . Chloride ions further bridge the PAp / KR4 and SP / KR2 interfaces, explaining the physiological role of serum chloride in stabilizing the closed conformer. The structural studies also reveal that differences in glycosylation alter the position of KR3. These data help explain the functional differences between the type I and type II plasminogen glycoforms.
In closed plasminogen, access to the activation bond (R561/V562) targeted for cleavage by tPA and uPA is blocked through the position of the KR3/KR4 linker sequence and the O-linked sugar on T346. The position of KR3 may also hinder access to the activation loop. The Inter-domain interactions also block all kringle ligand-binding sites apart from that of KR-1, suggesting that the latter domain governs pro-enzyme recruitment to targets. Analysis of an intermediate plasminogen structure suggests that plasminogen conformational change to the open form is initiated through KR-5 transiently peeling away from the PAp domain. These movements expose the KR5 lysine-binding site to potential binding partners, and suggest a requirement for spatially distinct lysine residues in eliciting plasminogen recruitment and conformational change respectively.
Plasmin is inactivated by proteins such as α2-macroglobulin and α2-antiplasmin. The mechanism of plasmin inactivation involves the cleavage of an α2-macroglobulin at the bait region (a segment of the aM that is particularly susceptible to proteolytic cleavage) by plasmin. This initiates a conformational change such that the α2-macroglobulin collapses about the plasmin. In the resulting α2-macroglobulin-plasmin complex, the active site of plasmin is sterically shielded, thus substantially decreasing the plasmin's access to protein substrates. Two additional events occur as a consequence of bait region cleavage, namely (i) a h-cysteinyl-g-glutamyl thiol ester of the α2-macroglobulin becomes highly reactive and (ii) a major conformational change exposes a conserved COOH-terminal receptor binding domain. The exposure of this receptor binding domain allows the α2-macroglobulin protease complex to bind to clearance receptors and be removed from circulation.
Deficiency in plasmin may lead to thrombosis, as clots are not degraded adequately. Plasminogen deficiency in mice leads to defective liver repair, defective wound healing, reproductive abnormalities.
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1b2i: KRINGLE 2 DOMAIN OF HUMAN PLASMINOGEN: NMR SOLUTION STRUCTURE OF TRANS-4-AMINOMETHYLCYCLOHEXANE-1-CARBOXYLIC ACID (AMCHA) COMPLEX
1bml: COMPLEX OF THE CATALYTIC DOMAIN OF HUMAN PLASMIN AND STREPTOKINASE
1bui: STRUCTURE OF THE TERNARY MICROPLASMIN-STAPHYLOKINASE-MICROPLASMIN COMPLEX: A PROTEINASE-COFACTOR-SUBSTRATE COMPLEX IN ACTION.
1cea: THE STRUCTURE OF THE NON-COVALENT COMPLEX OF RECOMBINANT KRINGLE 1 DOMAIN OF HUMAN PLASMINOGEN WITH EACA (EPSILON-AMINOCAPROIC ACID)
1ceb: THE STRUCTURE OF THE NON-COVALENT COMPLEX OF RECOMBINANT KRINGLE 1 DOMAIN OF HUMAN PLASMINOGEN WITH AMCHA (TRANS-4-AMINOMETHYLCYCLOHEXANE-1-CARBOXYLIC ACID)
1ddj: CRYSTAL STRUCTURE OF HUMAN PLASMINOGEN CATALYTIC DOMAIN
1hpj: SOLUTION NMR STRUCTURE OF THE HUMAN PLASMINOGEN KRINGLE 1 DOMAIN COMPLEXED WITH 6-AMINOHEXANOIC ACID AT PH 5.3, 310K, DERIVED FROM RANDOMLY GENERATED STRUCTURES USING SIMULATED ANNEALING, 12 STRUCTURES
1hpk: SOLUTION NMR STRUCTURE OF THE HUMAN PLASMINOGEN KRINGLE 1 DOMAIN COMPLEXED WITH 6-AMINOHEXANOIC ACID AT PH 5.3, 310K, DERIVED FROM RANDOMLY GENERATED STRUCTURES USING SIMULATED ANNEALING, MINIMIZED AVERAGE STRUCTURE
1i5k: STRUCTURE AND BINDING DETERMINANTS OF THE RECOMBINANT KRINGLE-2 DOMAIN OF HUMAN PLASMINOGEN TO AN INTERNAL PEPTIDE FROM A GROUP A STREPTOCOCCAL SURFACE PROTEIN
1ki0: The X-ray Structure of Human Angiostatin
1krn: STRUCTURE OF KRINGLE 4 AT 4C TEMPERATURE AND 1.67 ANGSTROMS RESOLUTION
1l4d: CRYSTAL STRUCTURE OF MICROPLASMINOGEN-STREPTOKINASE ALPHA DOMAIN COMPLEX
1l4z: X-RAY CRYSTAL STRUCTURE OF THE COMPLEX OF MICROPLASMINOGEN WITH ALPHA DOMAIN OF STREPTOKINASE IN THE PRESENCE CADMIUM IONS
1pk4: CRYSTAL AND MOLECULAR STRUCTURE OF HUMAN PLASMINOGEN KRINGLE 4 REFINED AT 1.9-ANGSTROMS RESOLUTION
1pkr: THE STRUCTURE OF RECOMBINANT PLASMINOGEN KRINGLE 1 AND THE FIBRIN BINDING SITE
1pmk: KRINGLE-KRINGLE INTERACTIONS IN MULTIMER KRINGLE STRUCTURES
1qrz: CATALYTIC DOMAIN OF PLASMINOGEN
1rjx: Human PLASMINOGEN CATALYTIC DOMAIN, K698M MUTANT
2doh: The X-ray crystallographic structure of the angiogenesis inhibitor, angiostatin, bound a to a peptide from the group A streptococcal surface protein PAM
2doi: The X-ray crystallographic structure of the angiogenesis inhibitor, angiostatin, bound to a peptide from the group A streptococcus protein PAM
2pk4: THE REFINED STRUCTURE OF THE EPSILON-AMINOCAPROIC ACID COMPLEX OF HUMAN PLASMINOGEN KRINGLE
5hpg: STRUCTURE AND LIGAND DETERMINANTS OF THE RECOMBINANT KRINGLE 5 DOMAIN OF HUMAN PLASMINOGEN