Enzyme kinetics

Question 1

What is meant by Vmax?

Answer 1

This is the maximum (theoretical) reaction rate

Question 2

What is meant by Km?

Answer 2

Km is the Michaelis constant and it is the substrate concentration which is 50% of the Vmax

Question 3

What is meant by K₁, K-₁, K₂ and Vo?

Answer 3

K₁ = the forward rate constant for enzyme association (bonding) with substrate
K-₁ = the backwards rate constant for enzyme dissociation with the substrate 
K₂ = the forward rate constant of an enzyme converting to a product
Vo = the initial reaction velocity

Question 4

What are the 2 ways that Km can be found?

Answer 4

Km can be found by using a graph or Km can be found using the equation:
Km = (K-₁ + K₂) / K₁

Question 5

How can the velocity (V) of an enzyme reaction be found?

Answer 5

By using the equation:

V = (Vmax [S]) / (Km + [S])

Question 6

How is the Vmax and Km determined using a lineweaver-burk plot?

Answer 6

Vmax is determined by intersection of the straight line with the y-axis
Km is determined by intersection of the straight line with the x-axis

Question 7

True or False: High Km means that an enzyme only need a little substrate to work at half max velocity

Answer 7

False - LOW Km means that an enzyme only need a little substrate to work at half max velocity.
HIGH Km means that an enzyme needs a lot of substrate to work at half max velocity

Question 8

What are the two types of reversible inhibition of enzyme activity?

Answer 8

Reversible inhibition can be:

1. Competitive - this means inhibitor binds to active site and blocks the substrate but substrate will still bind sometimes so it will make the rate slower but doesn’t effect the Vmax
2. Non-competitive - Inhibitor binds to a different site (allosteric site) to the substrate which changes the conformation of the active site (allosteric inhibition). This lowers the Vmax but the Km doesn’t change

Question 9

Is non-reversible inhibition of enzyme activity competitive or non-competitive?

Answer 9

It is non-competitive - this usually involves formation or breakage of covalent bonds in the enzyme complex

Question 10

What is meant by feedback inhibition?

Answer 10

This is the inhibition of the rate limiting enzyme by end products and it is a common mechanism of allosteric control

Question 11

Allosteric enzymes don’t follow michaelis-mentin kinetics, so what does this mean for a graph?

Answer 11

This means that increasing the substrate concentration results in a sigmoidal curve, instead of a hyperbola, so it shows co-operative behaviour. It can be controlled by allosteric inhibitors and allosteric activators

Question 12

What is the difference between allosteric inhibitors and allosteric activators?

Answer 12

Allosteric inhibitors modify the active site of the enzyme so that substrate binding is reduced or prevented whereas allosteric activators modify the active site of the enzyme so that the affinity for the substrate increases