Enzymes and Blood Clotting Cascade Flashcards
(22 cards)
How do enzymes work?
They lower activation energy of the reaction. Binding of substrate to active site increases local concentration of reactant and also stabilises formation of high energy transition state
What does Vmax signify?
Maximum velocity when enzyme saturated with substrate.
What does Km signify?
Km is substrate concentration that gives half of Vmax. The lower the Km, the higher the affinity for substrate.
Describe an example of an allosterically regulated enzyme
Phosphofructokinase. It sets the pace of glycolysis. It is activated by AMP, fructose-2-6-bisphosphate. It is inhibited by ATP, citrate, H+
What are zymogens?
Inactive enzyme precursors that become activated by cleavage with another enzyme.
Describe some ways in which enzymes are regulated
Allosteric regulation, Positive co-operativity (e.g haemoglobin), covalent modification, proteolytic activation
Describe the action of kinase enzymes
Transfer phosphate groups to -OH group of amino acid from ATP. (Serine, Threonine, Tyrosine)
Describe the action of phosphotase enzymes
Remove phosphate groups through hydrolytic activity. Phosphate groups are bulky, charged groups, so they affect enzyme conformation.
Describe the role of the intrinsic and extrinsic pathways in the blood clotting cascade. Also, what is the role of positive feedback in the cascade?
Extrinsic pathway is activated by membrane damage (factor 3 involved).
Intrinsic pathway activated by external trauma, such as blunt force (factor 12involved).
Factor X activated by both
Positive feedback in the way that activation of thrombin promotes further activation in cascade.
How is the blood clotting cascade regulated?
Zymogen concentration; digestion by proteases; specific inhibitors (e.g Heparin binding to antithrombin-3, AT3-Heparin doesn’t act on thrombulin-bound thrombin).
Describe fibrinolysis
Fibruinolysis is breaking down of clot. T-PA and streptokinase promote plasminogen becoming plasmin. Plasmin promotes fibrin breaking into fibrin fragments.
T-PA is tissue plasminogen activator
What are the differences between irreversible and reversible enzyme inhibitors?
Irreversible inhibitors bind very tightly and form covalent bonds, i.e. sarin
Reversible inhibitors can freely dissociate
How do competitive and non-competitive enzymes affect Km and Vmax?
Competitive- binds at active site, affects Km (increases it) not Vmax. adding enough substrate will always overcome substrate
Non-competitive- binds at another site on the enzyme. affects Vmax (lowers it) but not Km
Give an example of product inhibition in the body
Hexokinase is inhibited by glucose-6-phosphate
Why is phosphorylation so effective
- links energy status of cell to metabolism (ATP)
- allows for amplification effects
Why are digestive enzymes synthesised as zymogens?
So they don’t digest the cell itself
What does trypsin do?
- activates all other digestive enzymes
- trypsinogen is the inactive form
- enteropeptidase catalyses the conversion
- pancreatic trypsin inhibitor binds trypsin and stops activity
What is the clinical significance of alpha-1 antitrypsin deficiency?
emphysema
What are gla domains?
- vitamin K dependent (in the liver, modifies factors)
- calcium required
- targets factors to membrane
How is a fibrin clot formed?
1) Thrombin cleaves fibrinopeptides A and B from the central globular domain of fibrinogen.
2) Globular domains at the C-terminal ends of the b and g chains interact with exposed sequences at the N-termini of the cleaved b and a chains to form a fibrin mesh or clot.
The newly formed clot is stabilised by the formation of amide bonds between the side chains of lysine and glutamine residues in different monomers.
This cross-linking reaction is catalysed by transglutaminase (which is activated from protransglutaminase by thrombin.)
Describe the condition of haemophilia
Factor VIII (‘antihaemophilic factor’) is not a protease, but markedly stimulates the activity of factor IXa, which is a serine protease. Factor IXa catalyses the formation of factor Xa.
Haemophilia is a defect in this factor.
The activity of factor VIII is markedly increased by limited
proteolysis by thrombin and factor Xa. This positive feedback amplifies the clotting signal and accelerates clot formation.
When factor VIII isn’t present, this doesn’t occur and excessive bleeding results. Treatment with recombinant factor 8.
How is the clotting process stopped? (3 ways)
- localisation of prothrombin. Dilution of clotting factors of blood flow, and removal by liver
- digestion by proteases (e.g protein C degrades factor v and VIIa. It is activated by thrombin binding to endothelial receptor, thrombomodullin)
- specific inhibitors (e.g AT3, which is enhanced by heparin bind. AT3-heparin does not act on thrombomodulin-bound thrombin however)
