SESSION 6 Flashcards
How do you increase the rate of a reaction?
1) temperature- increase the number of molecules with the activation nervy
2) concentration- increases the chance of molecular collisions
3) enzymes- biological catalysts that increases the rate of reaction by lowering the activation energy
What are the important features of enzymes?
- highly specific
- unchanged after the action
- do not affect the reaction equilibrium
- increase the rate of reaction
- proteins ( usually globular)
- may require associated cofactors
Define the active site
The active site of an enzyme is the place where substrates bind and where the chemical reaction occurs
1) the active site occupies a small part of the enzyme
2) the active site is formed by amino acids from different parts of the primary structure
3) active sites have a complementary shape to the substrate - lock and key theory
4) active sites are clefts/ crevices
5) binding of substrates can induce changes in the conformation- induced fit theory
6) substrates are bound to enzymes by multiple weak bonds
Describe the lock- and- key hypothesis
- the idea that the shape of the active site of enzymes is exactly complementary to the shape of the substrate
- when a substrate molecule collides with an enzyme whose active site shape is complementary, the substrate will fit into the active site and as enzyme- substrate will form
- the enzyme will catalyse the reaction, and the products together with the enzyme, will form an enzyme- product complex
Describe the induced fit hypothesis
- the idea that the shape of active sites are not exactly complementary, but change shape in the presence of a specific substrate to become complementary
- when a substrate molecule collides with an enzyme, the shape of the enzyme’s active site will change so that the substrate fits into it and an enzyme- substrate complex forms
Explain the effects of enzymes on chemical reactions
Enzymes work by lowering the activation energy needed for a reaction to occur.
Binding of substrate to a distinct part of the enzyme, the active site, increases the local concentration of reactants and also stabilises the formation of the high energy transition state
Describe how the reaction rates vary as a function of enzyme and substrate concentration
An enzyme (E), combines reversible with substrate (S), to form an enzyme- substrate intermediate (ES) ES can then break to form free enzymes and the reaction product (P)
Michaelis- Menten equation: describes how the reaction velocity (V0) varies with substrate concentration:
V0= (Vmax [S]) / Km + [S]
Define the terms Vmax and Km
Vmax- maximal rate when all enzyme active sites are saturated with substrate
Km- substrate concentration that gives half maximal velocity
What is the significance of Vmax?
Vmax- the rate at which the enzyme catalysed a reaction
This takes place when all enzyme active sites are saturated
The rate of an enzyme catalysed reaction is proportional o the concentration of enzyme - double the amount of enzyme, double the rate
What is the significance of Km?
Km values give a measure of the affinity of an enzyme for its substrate
Low Km= high affinity for substrate
High Km= low affinity for substrate
Define enzyme inhibitor and the different types
Molecules that slow down or prevent an enzyme reaction- including many drugs
1) irreversible: bind very tightly, generally form covalent bonds
2) reversible: non- covalent - can freely dissociate
- competitive: binds at the active site
- non- competitive: binds another site on the enzyme
Describe the effects of enzyme inhibitors on enzyme kinetics and be able to distinguish between the two from simple graphs
1) enzyme with inhibition- reaches Vmax
2) non- competitive inhibition: doesn’t reach Vmax, Km is unaffected
3) competitive inhibitions: adding enough substrate will overcome the effect of the inhibitor
Vmax unaffected, Km increases
List the major regulatory mechanisms that control enzyme activity
- allosteric control (enzymes with more than one subunit)- the binding of substrate to one active site enhances substrate binding to the other active sites. These allosteric effectors may either inhibit or activate an enzyme
- substrate and product concentration
- covalent modification - many different types of groups can be attached covalently to proteins via amino acids.
Phosphorylation- phosphate groups can be added. The attachment of a phosphate group is carried out by kinase enzymes and their removal by phosphatase enzymes - proteolytic activation- enzyme secreted as an inactive protein precursor and cleaved by proteases to the active enzyme
- changes in the amount of enzyme- regulation of enzyme synthesis and protein degradation
Explain how activation of the clotting cascade leads ti the formation of a fibrillin clot
The intrinsic pathway- damaged membrane of blood cells promotes the binding of factor XI
The extrinsic pathway- trauma releases tissue factor (factor III)
Positive feedback- the activation of thrombin then promotes further activation in the cascade. Positive feedback allows formation of clot from the activation of a very small amount of initial factor
Discuss the mechanisms that are involved in the regulation of clot formation and breaks down
Concentration of zymogens- dilution of clotting factors by blood flow and removal by the liver
Digestion of proteases- e.g. FVa and VIIIa are degraded by protein C
Specific inhibitors- antithrombin III (AT3) enhanced by heparin binding. AT3- heparin doesn’t act on thrombomodulin- bound thrombin
