Topic 8 Flashcards
What are 5 short term regulatory mechanisms?
different enzyme forms- isozymes
change in enzyme regulation- allosteric regulation
reversible covalent modification- phosphorylation
proteolytic activation
controlling the amount of enzyme present- gene expression
Describe isozymes
enzymes that catalyse the same reaction but have different amino acid sequences
- different activity
- different regulatory properties
synthesised from the different genes or differentially spliced from the same gene
Describe allosteric regulation
usually multi subunit
can exist in two forms, t state (lower rate of activity) and r state (higher rate of activity)
do not obey Michaelis menten kinetics
binding at allosteric site leads to conformational change
activators increase the proportion of enzyme in r state
inhibitors increase proportion of enzyme in t state
Describe protein kinases and protein phosphatases
PK
transfer the terminal phosphate from ATP to hydroxyl group of Ser, Thy, Tyr
PP
reverse the effects of kinases by catalysing the hydrolytic removal of phosphoryl groups from proteins
Describe phosphorylation
free energy is large adds two neg charges a phosphoryl group can make H bonds rate of phosphorylation can be adjusted links energy status of cell to metabolism through ATP allow for amplification effects
What is proteolytic activation?
inactive precursor molecules, zymogens or prozymogens
involves the breaking of a peptide bond (removes pro)
irreversible
important when processes need to be tightly controlled
eg blood clotting, apoptosis
What are pancreatic zymogens?
Zymogens are a group of proteins that display no catalytic activity but can be transformed into active enzymes within an organism.
pancreatic proteases have a wide range of specificities
activation controlled by trypsin
What is the blood clotting cascade?
intrinsic pathway (damaged endothelial lining of blood cells promotes binding factor XII)
extrinsic pathway (trauma release tissue factor III)
both factor into the Factor X activation pathway
this leads to thrombin activation
this leads to the formation of the fibrin clot
it is a series of reactions each catalysed by an enzyme
each step leads to an amplification of the original signal
very small amounts of the initial signal needed to trigger the formation of a clot
What is the structure of prothrombin?
the protease function (thrombin part) is contained within the C terminal domain
the two Kringle domains help keep prothrombin in the inactive form
Gla domains target it to appropriate sites for its activation
What is fibrinogen?
precursor molecule
composed of three polypeptide chain (2 of each A𝞪2, B𝞫2 and γ2)
2 globular heads (folded units) separated by rod like triple helical alpha helices
fibrinopeptides prevent fibrinogen molecules coming together
thrombin cuts off fibrinopeptides to produce fibrin
fibrin monomers assemble by non covalent interactions (soft clot)
close linking of soft clot by covalent bonds between Lys and Gln residues- catalysed by transglutaminase (factor XIII)
How is the activation sustained?
Factors V and VIII - cofactors that stimulate activity of other enzymes in the pathway
thrombin- positive feedback on factors V, VIII and XI and XIII
once clotting has started you don’t need the damage to the blood vessel to still be there as the pathway is sustained once activated by thrombin
What is the role of γ-carboxyglutamate residues?
post transitional modification of factors II, VII, IX, X in the liver
this involves the addition of COOH groups to glutamate residues to form carboxyglutamate (requires vitamin K)
allows the interaction with sites of damage and brings together clotting factors
How is the clotting process stopped?
localisation of prothrombin
- dilution of clotting factors by blood flow and removal by liver
digestion of proteases
- eg factors Va and VIIIa are degraded by protein C
- protein C is activated by thrombin negative feedback loop
binding of specific inhibitors eg antithrombin III
How is the clot broken?
through fibrinolysis
this is where streptokinase (bacteria) and t-PA activate tissue plasminogen to plasmin which breaks down fibrin clot into fibrin fragments
How is the clotting process regulated?
- Inactive zymogens present at low concentration.
- Proteolytic activation.
- Amplification of initial signal by cascade mechanism.
- Clustering of clotting factors at site of damage.
- Feedback activation by thrombin ensures continuation of clotting.
- Termination of clotting by multiple mechanisms.
- Clot breakdown controlled by proteolytic activation
