BIOLOGY 2F03 Lecture Notes - Lecture 7: Serine Protease, Gla Domain, Integral Membrane Protein
19 May 2018
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1.1 – INITIATION OF COAGULATION
1. Tissue factor is the primary initiator of coagulation
Structure of TF
• 47kDA integral membrane protein
• Is the only pro-coagulant factor that does not require proteolytic cleavage for activation
Physiology of TF
• normally located on extravascular sites, namely on CSMs of VSMCs & Fibroblasts
• Is at higher concentrations on cell surfaces in certain organs to provide extra haemostatic
protection (lungs, brain, gonads, placenta)
TF has 2 functions
• Acts as a cellular receptor
• is the cofactor for factor 7/7a
2. Factor 7/7a has the typical Gla-egf-Ser domain structure
Structure of F7
• 48kDa zymogen
• Gla domain – binds (Vitamin K dependent factors) to activated phospholipid membranes (i.e.
PPDserine on outer surface)
• Serine protease domain – contains catalytic triad of his/asp/ser which cleaves after arg or lys
Physiology of F7
• secreted by the liver into plasma at concentration of 10nM
• ~10% of this is (for reasons unknown) circulating as F7a
o the circulating F7a is largely inactive unless bound to TF
3. the TF-7a complex is key for the initiation of coagulation
Structure/Function of TF-7a
• TF binds to F7a on subendothelial CSM and allosterically activates serine protease domain
o the reaction has a very high affinity (Kd < 1nM)
• all 4 F7 domains interact with TF
• due to low F7a concentrations, only a small amount of TF can form the TF-7a complex at first
o TF-7a can auto-activate (via proteolysis) other TF-F7 complexes
▪ 10a can also do this
Initiation of coagulation involves generating small amounts of thrombin by TF-7a
• TF-7a activates (via proteolysis) F9 by cleaving after Arg 145 and Arg 180
• TF-7a activates F10 by cleaving after Arg 191
• F10, without its cofactor (F5) generates trace amounts of thrombin by cleaving prothrombin
o Thrombin cleaves 8 → 8a
o Thrombin cleaves 5→ 5a
• F9a-8a activate F10; F10a-5a cleaves prothrombin much more efficiently
o hence propaogation of coagulation becomes independent of TF-7a
4. there are different forms of TFPI, each with their own roles in the regulation of coagulation
Structure of TFPI
• 43kDa molecules acidic (-ve) N terminus and alkaline (+ve) C terminus
• Has 3 kunitz domains
o Kunitz 2 inhibits 10a
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Document Summary
1. 1 initiation of coagulation: tissue factor is the primary initiator of coagulation. Structure of tf: 47kda integral membrane protein. Is the only pro-coagulant factor that does not require proteolytic cleavage for activation. Physiology of tf: normally located on extravascular sites, namely on csms of vsmcs & fibroblasts. Is at higher concentrations on cell surfaces in certain organs to provide extra haemostatic protection (lungs, brain, gonads, placenta) Tf has 2 functions: acts as a cellular receptor is the cofactor for factor 7/7a, factor 7/7a has the typical gla-egf-ser domain structure. Structure of f7: 48kda zymogen, gla domain binds (vitamin k dependent factors) to activated phospholipid membranes (i. e. ppdserine on outer surface, serine protease domain contains catalytic triad of his/asp/ser which cleaves after arg or lys. Structure of tfpi: 43kda molecules acidic (-ve) n terminus and alkaline (+ve) c terminus, has 3 kunitz domains, kunitz 2 inhibits 10a, kunitz 1 inhibits tf-7a.
