Enzymes regulation, blood clotting, O2 transporters & protein secretion Flashcards
Name 5 short term regulatory mechanisms of enzymes
1) different enzyme forms (isoenzymes)
2) change in enzyme conformation (allosteric reg.)
3) reversible covalent modification (phosphorylation)
4) proteolytic activation
5) controlling amount of enzyme present (gene expression)
What are isoenzymes?
- enzymes that catalyse the same reaction but have different amino acid sequences
- different activity & diff regulatory properties
- synthesised from diff genes or differential spliced from same gene
Give properties of an allosterically regulated protein.
- usually multi subunit
- can exist in 2 forms: T state (low affinity), R state (high affinity)-hint higheR
- do NOT obey Michaelis-Menten kinetics
What do allosteric a) activators AND b) inhibitors do to enzymes in terms of T & R states?
- Activators increase proportion of enzymes in the R state (high affinity)
- Inhibitors increase proportion of enzymes in the T state (lower affinity)
How do the following work?
- protein kinases
- protein phosphotases
- protein kinases add a phosphate group (taken from ATP)
- protein phosphotases remove phosphoryl groups from proteins via hydrolysis (reverse effects of kinases)
Why is protein phosphorylation so effective?
- free energy of Pi bond in ATP is very high
- adds two negative charges (PO4^2-)
- phosphoryl group can make H bonds
- rate of (de)phosphorylation can be adjusted
- allows for amplification effects
What is proteolytic action as a regulatory mechanism for enzymes?
-inactive precursor molecules (zymogens or proenzymes)
-breaking of peptide bond
-irreversible therefore can be regulated
-important when processes need to be tightly controlled
eg digestive enzymes: trypsinogen (inactive)-> trypsin (active)
Blood clotting is a tightly regulated process. What is meant by amplification?
- blood clotting is a series of reactions catalysed by enzymes
- each step leads to amplification of previous signal via cascade mechanism
- very small amounts of initial signal needed to trigger formation of clot
What happens in the intrinsic and extrinsic pathways for blood clotting? At which common point do they meet?
- intrinsic= damaged endothelial lining of blood cells promotes binding of factor XII (7)
- extrinsic= trauma releases tissue factor (factor III ie 3)
- both meet in factor X (10) activation
Thrombin is then activated which forms the fibrin clot. How does this happen?
- prothrombin (inactive) activated to thrombin.
- fibrinogen is a precursor for fibrin. Has 2 heads, alpha helixes. Fibrinopeptides prevent fibrinogen molecules coming together.
- thrombin cuts off these fibrinopeptides to form fibrin. Fibrin assembles to form a ‘soft clot’-cross linking by covalent bonds catalysed by Factor XIII (7).
How is the activation of components in the pathway sustained?
- Factors VIII & V are cofactors that stimulate activity of other enzymes in pathway
- thrombin allows positive feedback on factors VIII, V and XIII, XI
What is the role of gamma-carboxyglutamate residues (Gla)?
hint: vitamin K
- post translationally modifies factors II, VIII etc
- addition of COOH to carboxyglutamate to form Gla requires Vitamin K
- allows interaction w damaged sites & brings clotting factors together
How is the clotting process stopped?
1) localisation of (pro)thrombin- dilution of clotting factor ps and removed by liver
2) digestion by proteases- eg Factors Va & Vllla degraded by protein C (protein C activated by thrombin negative feedback loop)
3) binding of specific inhibitors eg antithrombin III
State the similarities & differences between haemoglobin and myoglobin.
- haemoglobin= in blood, transports O2 around body, 4 polypeptide chains, 4 haem groups per Hb
- myoglobin= in muscle, short term storage of O2, 1 polypeptide chain, 1 haem group per Mg
- in both, O2 binding changes position of Fe ion
What is the shape of the slope in O2 binding with a) myoglobin
b) haemoglobin
Why is this so?
- myoglobin= hyperbolic ie myoglobin has a constant affinity for O2
- haemoglobin= sigmoid also (S shape), affinity for O2 increases w partial pressure for O2
- this is because of the structural differences, cooperativity enhances O2 transport in Hb; binding of first O2 is harder but causes conformational change making binding of next O2 easier. It promotes transition from low affinity T state to high affinity R state. This property means Hb has greater O2 transport than Mglbn
