Flashcards in Enzymes Deck (32):
What is meant by enzyme specificity?
Due to the 3D arrangement of the active site of an enzyme. Each enzyme only has the ability to interact with a single substrate.
What is the induced fit model for enzymes?
This model suggests that when a substrate interacts with an active site, the shape of the active site alters slightly to accommodate the substrate. It appreciates that active sites have a dynamic 3D structure.
What is the transition state complex?
When substrates are undergoing reaction they first need to be activated. This high energy state is called the transition state.
How do enzymes increase reaction rate?
They lower activation energy, by providing an alternative mechanism by which the reaction can take place.
Some enzymes work with functional groups. What is the difference between a cofactors and a coenzyme?
A cofactors is a metal ion, whilst a coenzyme is an organic molecule which provides it's functional group.
What is the Michaelis-menlen equation? What enzymes is this not true for?
V0 = (Vmax * [S]) / (Km + [S])
This will not work for enzymes which show cooperativity.
What is the difference between a competitive and non-competitive inhibitor? What is the effect on Vmax and Km?
Competitive bind to the active site. They have no impact on Vmax but they increase Km.
Non competitive bind to a site which is not the active site. They do not affect Km but they reduce Vmax.
What can be said about the affinity of an enzyme for the substrate if Km is low?
It has high affinity.
In a line weaver-burk plot, what value is indicated by the y intersection?
How can you find the value for Km on a line weaver-burk plot?
The X axis intercept is -1/Km so from this value it can be calculated.
Give two examples of short term enzyme regulation
Substrate/ product concentration
Change in enzyme conformation
What are isoenzymes?
These are enzymes which both catalyse the same reaction but have different kinetic properties such as Vmax and Km.
How does accumulation of product affect the reaction rate of an enzyme?
Product fits the active site and so acts as a competitive inhibitor and slows reaction rate.
Haemoglobin shows a sigmoidal binding curve. What type of enzyme regulation usually produces a curve with this shape?
What is the most common example of covalent modification of an enzyme and what is the enzyme which allows this to happen?
Phosphorylation. This is the regulation which occurs with Pyruvate kinase.
Protein kinases transfer the phosphate of ATP onto the R group of an amino acid which activates the enzyme.
What is the action of protein phosphotases?
These remove phosphates from R groups on amino acids and alter the shape of enzymes so that they are inhibited.
What is a kinase cascade?
When enzymes activate enzymes this means that a signal can become multiplied in size very very quickly.
Explain how glycogen synthesis and breakdown are reciprocally regulated.
When there are high levels of epinephrine, this leads to activation of protein kinase A. This activates phosphorylase but also inhibits synthase so it has an impact on both pathways.
What does proteolytic cleavage involve?
The breaking of an amino acid chain, followed by removal of some amino acids and then formation of a glycosidic bond.
What is a problem concerning enzymes which are activated by proteolytic cleavage?
Once they have been activated they cannot be switched off again and so they will continue to perform their function until they have been degraded.
What is a zymogen?
An inactive precursor of an enzyme.
What are the two ways in which an enzyme can be controlled in the long term?
Change in the rate of protein synthesis.
Change in the rate of protein degradation.
Why can't we live with out enzymes?
Without enzymes all the reactions which take place in our body would occur too slowly and so we wouldn't be viable for life.
Describe the structure of prothrombin.
Prothrombin is an inactive protease. It has a Gla residue and 2 Kringle units. After activation it consists of 2 chains joined by a disulphide bond.
How are clotting factors attracted to the damaged phospholipid bilayer?
Calcium is attracted to damage in the phospholipid bilayer and this has a positive charge and so attracts the negatively charged clotting factors.
What is the significance of Prothrombin only being activated when it binds to calcium?
Calcium is bound to the damaged site, and so if prothrombin is only activated on binding then this means that clots will remain localised to the damaged area which is very important to prevent blockage of blood vessels.
Explain how a fibrinogen molecule is activated.
Fibrinogen molecules consist of three chains joined by disulphide bonds. They have a and B termini which prevent aggregation. These are cleaved off (by fibrinopeptide) and then the molecules can come together and the globular ends form a mesh.
What type of feedback occurs in the clotting cascade?
State two ways in which the clotting cascade can be stopped.
Dilution of clotting factors
Digestion by proteases.
Factors Va and VIIIa are degraded by protein C. How is protein C activated?
Protein C is activated when thrombin binds to thrombomodulin (endothelial receptor).
What is Haemophilia?
Haemophilia is a defect in factor VIII and so the activity of IXa cannot be stimulated and so there is limited proteolysis by thrombin.