Plasmin

Key takeaways

Plasmin belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Plasmin to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Plasmin from memory before moving on to harder problems.

Reference excerpt

Plasmin is an important enzyme (EC 3.4.21.7) present in blood that degrades many blood plasma proteins, including fibrin clots. The degradation of fibrin is termed fibrinolysis. In humans, the plasmin protein (in the zymogen form of plasminogen) is encoded by the PLG gene.

Function

Plasmin is a serine protease that acts to dissolve fibrin blood clots. Apart from fibrinolysis, plasmin proteolyses proteins in various other systems: It activates collagenases, some mediators of the complement system, and weakens the wall of the Graafian follicle, leading to ovulation. Plasmin is also integrally involved in inflammation. It cleaves fibrin, fibronectin, thrombospondin, laminin, and von Willebrand factor. Plasmin, like trypsin, belongs to the family of serine proteases. Plasmin is released as a zymogen called plasminogen (PLG) from the liver into the systemic circulation. 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. Plasmin cleavage produces angiostatin.

Mechanism of plasminogen activation 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.

Mechanism of plasmin inactivation

Plasmin is inactivated by proteins such as α2-macroglobulin and α2-antiplasmin. The primary protein responsible for plasmin inhibition is the α2-antiplasmin which is a serpin protein. The C-terminal of the α2-antiplasmin binds plasminogen Kringle domains via lysine residues allowing for the inhibition of plasmin. Another method 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. Plasmin can also be inhibited by inhibiting its activators, inactivating PAI-1 and PAI-2 blocks the production of tPA and uPA which subsequently stop the conversion of plasminogen into plasmin. Defects in the SERPINE1 gene cause deficiencies in PA1-2. PAI-2 is only detectable during pregnancy and lacks a signal sequence it is not secreted by the cell. Because PAI-2 is not properly secreted it accumulates in the cell where it remains unglycosylated. The exposed RCL loop on the PAI-2 contains the sequence Arg380-Thr381 which is identical to the cleavage site uPA/tPA identity. When uPA/tPA bind and cleave the bond between Arg-380 and Thr381 the RCL loop inserts itself into a β-sheet A slot in the enzyme(s) and conformationally alters the active site of the enzyme allowing it to no longer function.

Pathology Plasmin deficiency may lead to thrombosis, as the clots are not adequately degraded. Plasminogen deficiency in mice leads to defective liver repair, defective wound healing, reproductive abnormalities.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Plasmin

Start with the simplest possible case. Write down what Plasmin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Plasmin before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Plasmin ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Plasmin

In research: Plasmin appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Plasmin in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Plasmin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Acute-phase proteins, EC 3.4.21, Extracellular matrix remodeling enzymes, so understanding it makes those chapters shorter.
In everyday life: Look for Plasmin outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Plasmin in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Plasmin means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Plasmin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Plasmin in simple terms?

Plasmin is an important enzyme (EC 3.4.21.7) present in blood that degrades many blood plasma proteins, including fibrin clots. The degradation of fibrin is termed fibrinolysis.

Why does Plasmin matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Plasmin?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Plasmin.