Enzymes

Question 1

Define Co-factor

Answer 1

Non-protein component needed for activity

Question 2

Define Coenzyme

Answer 2

Complex organic molecule, usually formed from vitamins

Question 3

Define Prosthetic Group

Answer 3

Cofactor covalently bound to the enzyme or very tightly associated with the enzyme

Question 4

Define Apoenzyme

Answer 4

The protein component of an enzyme-containing a prosthetic group

Question 5

Discuss the ways in which enzymes catalyze reactions

Answer 5

Enzymes

• Increase rates of spontaneous reactions
• Lower the activation energy of biochemical reactions
• Accelerate movement toward reaction equilibria

Question 6

Describe how the rate of an enzyme-catalyzed reaction may vary with changes in the concentration of the enzyme and of the substrate.

Answer 6

The y axis is the initial velocity V0 (microM/min)

The X-axis is the substrate concentration, (S) (mM)

1/2 Vmax = Km

Vmax is where substrate concentration is used up. Levels off.

Question 7

Define the ‘Michaelis Constant’, describe how it may be experimentally estimated, and describe what knowledge of the constant may allow being deduced about the nature of the catalyzed reaction.

Answer 7

Km=1/2 Vmax= k1 +k2/k1

Larger Km values indicate a less stable ES complex

Lower Km values indicate a more stable ES complex

Therefore, it tells us about the affinity of the enzyme for the substrate.

Question 8

Describe Competitive inhibition

Answer 8

These inhibitors bind to enzymes in the same way that the intended substrate does. They bind non-covalently and tend to resemble the proper substrate.

This leads to an increase in Km as the affinity for the proper substrate decreases. However, increasing the concentration of the substrate can displace the competitor and Vmax can still be achieved.

Question 9

Describe Non-competitive inhibition

Answer 9

These inhibitors also bind non-covalently, but instead, bind to a site other than the active site of the enzyme. The enzyme can still bind to the active site and so Km is unchanged, however, the inhibitor cannot be displaced by increasing the concentration of the substrate, so Vmax will decrease.

Question 10

Describe the operation of allosterically-regulated and covalently modified enzymes.

Answer 10

Metabolites can bind to allosteric sites on some enzymes to act as inhibitors or activators. This is an example of non-competitive inhibition.

2 models explain almost all allosteric enzyme kinetics.

Concerted Model

• Each subunit can exist in 2 different conformations.
• One will have a low Km, the other will have a high Km
• With no substrate, the enzyme flips between conformations
• All units must be in the same conformation (i.e in concert)
• When 1 substrate binds, it holds the enzyme in the open conformation
• This increases affinity and explains the sigmoid curve

Sequential Model

• No flipping between different conformation states
• The binding causes a conformational change in 1 subunit
• This causes a change in another subunit, making further binding easier
• Like the first model, binding sensitizes the enzyme to more substrate.