Lecture 1 - Enzyme Kinetics

Question 1

What is the steady state assumption in the Michaelis-Menten equation?

Answer 1

The rate of ES production is equal to the overall rate of all the reactions that decrease [ES].

Question 2

What is the Km?

Answer 2

The substrate concentration at which the reaction rate is half the maximal rate (Vmax).

Question 3

What does it mean if an enzyme has a low Km?

Answer 3

It achieves maximal catalytic efficiency at low [S]

Question 4

What is Kcat?

Answer 4

The turnover number- the number of reaction processes that each active site catalyses per unit time.

Question 5

What are the disadvantages of the Lineweaver-Burk plot?

Answer 5

Most measurements of [S] are at high values, and the 1/[S] values end up crowded on the left side of the graph- makes drawing the straight line difficult and inaccurate.
For small [S], there can be large errors in Km and Vmax.

Question 6

What are the disadvantages of the Hofstee-Eadie plot?

Answer 6

Can have large errors- both coordinates contain the dependent variable.

Question 7

What are the advantages of the Hanes-Woolf plot?

Answer 7

There is equal weighting of the data, so a better fit of the straight line.

Question 8

Describe a competitive inhibitor.

Answer 8

• resembles substrate structure
• competes directly with substrate for active site
• binds to the active site but is unreactive

Question 9

What parameters are affected by competitive inhibition?

Answer 9

Km - affecting the affinity for substrate.Vmax is not affected by competitive inhibition.

Question 10

Describe an uncompetitive inhibitor.

Answer 10

• binds to an allosteric site on the ES complex

- can cause distortion of the active site

Question 11

What parameters are affected by uncompetitive inhibition?

Answer 11

Both Vmax and Km

Question 12

Describe a mixed inhibitor.

Answer 12

• bind to an allosteric site
• can bind to free enzyme or ES complex
• effective inhibition at both low and high substrate concentrations

Question 13

Which parameters are affected by mixed inhibition?

Answer 13

Both Vmax and Km.

Question 14

Describe a non-competitive inhibitor.

Answer 14

• binds at an allosteric site

- effectiveness depends on inhibitor concentration

Question 15

Which parameters are affected by non-competitive inhibition?

Answer 15

Vmax lowers as inhibitor concentration increases.Km is not affected as EI can still bind substrate.

Question 16

How can competitive inhibition be overcome?

Answer 16

The inhibitor is displaced at high concentrations of substrate.

Question 17

How can uncompetitive inhibition be overcome?

Answer 17

Lowering the concentration of the substrate makes the effect of the inhibitor negligible.

Question 18

What are the features of a perfectly evolved enzyme?

Answer 18

Diffusion controlled. Km is much higher than physiological [S] value.