Serine proteases plasmin

The natural process of thrombosis functions to plug a damaged blood vessel, thus maintaining haemostasis until the damaged vessel can be repaired. Subsequent to this repair, the clot is removed via an enzymatic degradative process known as fibrinolysis. Fibrinolysis normally depends upon the serine protease plasmin, which is capable of degrading the fibrin strands present in the clot. 

Fibrinolysis refers to the process of fibrin digestion by the fibrin-specific protease, plasmin. The fibrinolytic system is similar to the coagulation system in that the precursor form of the serine protease plasmin circulates in an inactive form as plasminogen. In response to injury, endothelial cells synthesize and release tissue plasminogen activator (t-PA), which converts plasminogen to plasmin (Figure 34-3). Plasmin remodels the thrombus and limits its extension by proteolytic digestion of fibrin. 

Ng et al. (154) reported the identification of several antimicrobial peptoids from an 845-member SP pool library organized as 65 pools of 13 compounds inspired by previous, larger combinatorial efforts (155). A 400-member SP library based on a 1,3-disubstimted cyclohexanone scaffold, prepared as 20 pools of 20 compounds, was reported by Abato et al. (156) a selective inhibitor of a serine protease, plasmin, was identified from iterative deconvolution of the library. 

For many enzymes of clinical or commercial importance, chromogenic substrates or cofactors have been developed to allow enzyme assays to be analysed photometrically. For example the compounds tosyl-glycyl-prolyl-lysyl-4-nitroacetanilidoacetate, or tosyl-glycyl-arginyl-4-nitroacetanilide can be used to assay the dinically important serine proteases plasmin and thrombin. 

Plasminogen is the precursor of the serine protease plasmin which is involved in dissolution of fibrin clots and, hence, in removal of thrombi. The plasma concentration is about 12mg/100ml and the molecular mass 81 kDa. It contains 17.1% carbohydrate on two sites, one N-linked and one O-linked oligosaccharide. A variant is also known that is only 0-glycosylated. The structures of the oligosaccharides have been determined [71-73], and are shown in Fig. 11. 

Receptor-bound uPA efficiently cleaves and thus activates colocalized plasminogen at the cell surface, which reveals the broad spectrum serine protease plasmin. Plasmin activates downstream extracellular proteases (e.g., matrix met-alloproteases) and latent growth factors and together these lead to pericellular... 

The carbohydrate sequence of some coagulation factors has been determined (prothrombin or F II, thromboplastin or F X, fibrinogen or F I, and the fibrinolytic serine protease, plasmin), but only a few investigations have been reported on the functional significance of the carbohydrate residues (Mizuochi et al 1979, Mizuochi et al 1980, Gati and Straub 1978, Hayes and Castellino 1979, Lijinen et al 1981). 

Coagulation Factors II, III, VII, IX, X, XI, and Xlla fragments, thrombin, and plasmin are classified as serine proteases because each possesses a serine residue with neighboring histidine and asparagine residues at its enzymatically active site (Table 3). Factors II, VII, IX, and X, Protein C, Protein S, and Protein Z are dependent on the presence of vitamin K [84-80-0] for their formation as biologically functionally active procoagulant glycoproteins. 

Plasmin, a serine protease (83 kDa), can degrade fibrin, and its degradation products (FDP) are soluble in the blood. Plasmin is formed from its proenzyme (zymogen, precursor), plasminogen (92 kDa), synthesized by the liver, and secreted into the blood circulation, where its concentration is 2 pM. Plasminogen is converted to plasmin by plasminogen activators (serine proteases). 

Tissue-type plasminogen activator (tPA) is a glycoprotein (68 kDa), synthesized by endothelial and tumor-cells. As a serine protease, tPA hydrolyses Arg561-Val562 peptide bond in plasminogen, resulting in plasmin formation. It needs cofactors for efficient plasminogen activation. 

Urokinase-type plasminogen activator (uPA, urokinase) is synthesized by endothelial and tumor cells as a single-chain glycoprotein (scuPA) without catalytic activity. When it is converted to a two-chain protein (tcuPA) by plasmin, an active serine protease center develops, which activates plasminogen. Thus, uPA (55 kDa) results in the amplification of fibrinolysis. 

The serine proteases are the most extensively studied class of enzymes. These enzymes are characterized by the presence of a unique serine amino acid. Two major evolutionary families are presented in this class. The bacterial protease subtilisin and the trypsin family, which includes the enzymes trypsin, chymotrypsin, elastase as well as thrombin, plasmin, and others involved in a diverse range of cellular functions including digestion, blood clotting, hormone production, and complement activation. The trypsin family catalyzes the reaction ... 

Serine proteases usually show primary specificity (occupation of subsite Si) for positively charged arginine or lysine (trypsin, plasmin, plasminogen activators, thrombin), large hydrophobic side chains of phenylalanine, tyrosine, and tryptophan (chymotrypsin, cathepsin G, chymase, and subtilisin), or small aliphatic side chains (elastases). Nevertheless, there are a large number of variations and in many cases, other subsites like S2 and S3 are more discriminating while maintaining the... 

UK is a serine protease that activates plasminogen to plasmin. Plasmin dissolves the fibrin in blood clots. The attachment of UK to the islet surface was expected to dissolve blood clots that surrounded the islets in the liver thus, IBMIR could be inhibited in the initial stages. A fibrin plate-based assay was performed to assess the... 

Urokinase is a serine protease produced by the kidney and is found in both the plasma and urine. It is capable of proteolytically converting plasminogen into plasmin. Two variants of the enzyme have been isolated a 54 kDa species and a lower molecular mass (33 kDa) variant. The lower molecular mass form appears to be derived from the higher molecular mass moiety by proteolytic processing. Both forms exhibit enzymatic activity against plasminogen. 

The carboxy-terminal region in apolipoprotein (a) closely resembles the protease domain in plasminogen [eight amino acid substitutions, nine amino acid deletions, and one insertion in apo(a) relative to plasminogen, with 94% overall nucleotide sequence identity] (G28). The most important difference is the substitution of arginine by serine in the site responsible for proteolytic activity (position 4308) (G28). As a result, Lp(a) has no protease activity towards substrates for plasmin (J3). Salonen (SI) reported a serine-protease activity of Lp(a) towards fibronectin, a glycoprotein present in connective tissue matrices. 

The effects of Lp(a) on the fibrinolytic system are based on the homology between plasminogen and Lp(a) (E3, E5, K4). Inactive Glu-plasminogen is converted to inactive glutamine-plasmin or inactive lysine-plasmin. Both can be converted to active lysine-plasmin, the activity of which is based on the serine protease part that splits fibrin and fibrinogen, but also factors V and Villa. In addition, Lp(a) is able to activate factor XII, factor VII, and the complement factors Cl and C3. 

Serine protease inhibitor that inhibits trypsin, chymotrypsin, kallikrein, and plasmin Binding is reversible, with most aprotinin-protease complexes dissociating at pH>10or <3 Peptidase inhibitor... 

Inhibitor of serine and cysteine proteases plasmin, trypsin, papain and cathepsin B Inhibitor of type 1 and type 2A protein phosphatases... 

Sheehan JJ, Tsirka SE. Fibrin-modifying serine proteases thrombin, tPA, and plasmin in ischemic stroke a review. Glia 2005 50(4) 340-50. 

Circulahng plasmin is rapidly neutralized by aj-an-tiplasmin, a physiological serine protease inhibitor that forms an inert complex with plasmin. In contrast, hbrin-bound plasmin is resistant to inactivation by 2-an-tiplasmin. Under normal circumstances plasma t-PA is inactive because it is inhibited by PAI-1, while t-PA that is bound to hbrin is unaffected by PAI-1. In addition, plasma t-PA has a very rapid turnover in blood (half-life 5 to 8 minutes). For these reasons, hbrinolysis is normally restricted to the thrombus. 

---