MCGB - Enzyme Activity

Question 1

What is the name given to the high energy intermediate that lies between S and P in an enzyme reaction?

Answer 1

Transition state

Question 2

What is the activation energy?

Answer 2

The minimum energy that substrates must have to start a reaction.

Question 3

Why does increasing the temperature increase the rate of reaction?

Answer 3

Increases number of molecules with activation energy.

Question 4

Why does increasing the concentration increase the rate of reaction?

Answer 4

Increases chance of molecular collisions.

Question 5

What are enzymes?

Answer 5

Biological catalysts that increase the rate of reaction by lowering the activation energy required. This facilitates formation of the transition state.

Question 6

Give an exception to the statement that all enzymes are proteins

Answer 6

Some RNA molecules act as enzymes.

Question 7

Do enzymes affect the reaction equilibrium?

Answer 7

No

Question 8

If the active site of an enzyme is only made of a few amino acids out of hundreds, what is the role of the rest of the enzyme?

Answer 8

A scaffold to create the active site

Question 9

True or false - the active site is formed by amino acids from different parts of the primary sequence?

Answer 9

True

Question 10

What sort of reaction takes place when the substrate molecule binds to the active site?

Answer 10

Condensation reaction

Question 11

What is the difference between the lock-and-key and the induced fit model of enzyme action?

Answer 11

Lock and key states that the active site of the enzyme is already complementary to the shape of the substrate, while induced fit states that the active site only forms a complementary shape AFTER the binding of the enzyme.

Question 12

True or false - substrates bind to enzymes by covalent bonds?

Answer 12

False - covalent is too tight. They must be multiple weak bonds.

Question 13

What does V0 mean?

Answer 13

Initial rate of reaction.

Question 14

What shape would a graph of reaction rate against substrate concentration be?

Answer 14

A rectangular hyperbola.

Question 15

What does the Michaelis-Menten model suggest about enzyme catalysis?

Answer 15

A specific complex is a necessary intermediate, and that a plot of V0 vs [S] will be a rectangular hyperbola.

Question 16

What does Vmax refer to?

Answer 16

Maximal rate when all enzyme active sites are saturated with substrate (mol/min)

Question 17

What does Km refer to?

Answer 17

Substrate concentration that gives half maximal velocity

Question 18

What can Km values be used to indicate?

Answer 18

They give a measure of the affinity of an enzyme for its substrate.

Question 19

What does a low Km indicate?

Answer 19

Enzyme has high affinity for substrate.

Question 20

What does a high Km indicate?

Answer 20

Enzyme has low affinity for substrate.

Question 21

Hexokinase has a Km of 0.1mM and is always active. Glucokinase has a Km of 5mM and is only active after eating food. What is the reason for these different Km values?

Answer 21

HK has low Km and therefore high affinity. GK has high Km and low affinity. As GK is active after eating, it has to digest large amounts of glucose, so it has low affinity because so much glucose is available to it. HK serves to “clean up” the left over glucose and must have high affinity to be able to do this.

Question 22

What is one “unit” in terms of enzymes?

Answer 22

The amount of enzyme that converts 1 micromole of product per minute under standard conditions.

Often expressed per litre of serum or per gram of tissue

Question 23

Give two ways Km can be estimated from a Lineweaver-Burk plot.

Answer 23

Slope = Km/Vmax

X Intercept = -1/Km

Question 24

How can Vmax be estimated from a Lineweaver-Burk plot?

Answer 24

Y Intercept = 1/Vmax

Question 25

What sort of bond does an irreversible enzyme inhibitor generally form?

Answer 25

Covalent bonds, which are very tight.

Question 26

Give an example of an irreversible enzyme inhibitor.

Answer 26

Nerve gases such as sarin.

Question 27

What are the two types of reversible enzyme inhibitor? Do they affect Km or Vmax?

Answer 27

Competitive (binds at active site) - affects Km not Vmax

Non-competitive (binds at another site on enzyme) - affects Vmax not Km.

Question 28

Why doesn’t competitive inhibition affect the Vmax?

Answer 28

Adding enough substrate will always overcome the effect of the inhibitor, so Vmax unaffected. Inhibitor competes for active site, so Km increases.

Question 29

True or false - compounds binding outside of the active site can activate rather than inhibit enzymes?

Answer 29

True

Question 30

Why do non-competitive inhibitors affect Vmax?

Answer 30

They bind at an alternative site, decreasing turnover number of enzyme which lowers Vmax.

Question 31

How could Km be calculated from substrate concentration at Vmax?

Answer 31

Half it

Question 32

How can you tell whether an inhibitor is competitive or non-competitive from a Lineweaver-Burk plot?

Answer 32

Look at the Y intercept - if it’s the same as original, then its competitive. If it’s higher, then its non-competitive.