Enzymes

Question 1

What is meant by enzyme specificity?

Answer 1

Due to the 3D arrangement of the active site of an enzyme. Each enzyme only has the ability to interact with a single substrate.

Question 2

What is the induced fit model for enzymes?

Answer 2

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.

Question 3

What is the transition state complex?

Answer 3

When substrates are undergoing reaction they first need to be activated. This high energy state is called the transition state.

Question 4

How do enzymes increase reaction rate?

Answer 4

They lower activation energy, by providing an alternative mechanism by which the reaction can take place.

Question 5

Some enzymes work with functional groups. What is the difference between a cofactors and a coenzyme?

Answer 5

A cofactors is a metal ion, whilst a coenzyme is an organic molecule which provides it’s functional group.

Question 6

What is the Michaelis-menlen equation? What enzymes is this not true for?

Answer 6

V0 = (Vmax * [S]) / (Km + [S])

This will not work for enzymes which show cooperativity.

Question 7

What is the difference between a competitive and non-competitive inhibitor? What is the effect on Vmax and Km?

Answer 7

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.

Question 8

What can be said about the affinity of an enzyme for the substrate if Km is low?

Answer 8

It has high affinity.

Question 9

In a line weaver-burk plot, what value is indicated by the y intersection?

Answer 9

1/ Vmax

Question 10

How can you find the value for Km on a line weaver-burk plot?

Answer 10

The X axis intercept is -1/Km so from this value it can be calculated.

Question 11

Give two examples of short term enzyme regulation

Answer 11

Substrate/ product concentration

Change in enzyme conformation

Question 12

What are isoenzymes?

Answer 12

These are enzymes which both catalyse the same reaction but have different kinetic properties such as Vmax and Km.

Question 13

How does accumulation of product affect the reaction rate of an enzyme?

Answer 13

Product fits the active site and so acts as a competitive inhibitor and slows reaction rate.

Question 14

Haemoglobin shows a sigmoidal binding curve. What type of enzyme regulation usually produces a curve with this shape?

Answer 14

Allosteric regulation.

Question 15

What is the most common example of covalent modification of an enzyme and what is the enzyme which allows this to happen?

Answer 15

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.

Question 16

What is the action of protein phosphotases?

Answer 16

These remove phosphates from R groups on amino acids and alter the shape of enzymes so that they are inhibited.

Question 17

What is a kinase cascade?

Answer 17

When enzymes activate enzymes this means that a signal can become multiplied in size very very quickly.

Question 18

Explain how glycogen synthesis and breakdown are reciprocally regulated.

Answer 18

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.

Question 19

What does proteolytic cleavage involve?

Answer 19

The breaking of an amino acid chain, followed by removal of some amino acids and then formation of a glycosidic bond.

Question 20

What is a problem concerning enzymes which are activated by proteolytic cleavage?

Answer 20

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.

Question 21

What is a zymogen?

Answer 21

An inactive precursor of an enzyme.

Question 22

What are the two ways in which an enzyme can be controlled in the long term?

Answer 22

Change in the rate of protein synthesis.

Change in the rate of protein degradation.

Question 23

Why can’t we live with out enzymes?

Answer 23

Without enzymes all the reactions which take place in our body would occur too slowly and so we wouldn’t be viable for life.

Question 24

Describe the structure of prothrombin.

Answer 24

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.

Question 25

How are clotting factors attracted to the damaged phospholipid bilayer?

Answer 25

Calcium is attracted to damage in the phospholipid bilayer and this has a positive charge and so attracts the negatively charged clotting factors.

Question 26

What is the significance of Prothrombin only being activated when it binds to calcium?

Answer 26

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.

Question 27

Explain how a fibrinogen molecule is activated.

Answer 27

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.

Question 28

What type of feedback occurs in the clotting cascade?

Answer 28

Positive feedback.

Question 29

State two ways in which the clotting cascade can be stopped.

Answer 29

Dilution of clotting factors

Digestion by proteases.

Question 30

Factors Va and VIIIa are degraded by protein C. How is protein C activated?

Answer 30

Protein C is activated when thrombin binds to thrombomodulin (endothelial receptor).

Question 31

What is Haemophilia?

Answer 31

Haemophilia is a defect in factor VIII and so the activity of IXa cannot be stimulated and so there is limited proteolysis by thrombin.

Question 32

What is fibrinolysis? How is this activated?

Answer 32

The process by which clots are broken down. This process is controlled by proteolytic activation, where plasminogen –> plasm in which then breaks fibrin down into fibrin fragments.