lecture 12 - control of enzyme activity

Question 1

What are enzyme inhibitors?

Answer 1

Compounds that bind to enzymes and reduce their activity

Question 2

What are the 2 classes of enzyme inhibitors?

Answer 2

Reversible and irreversible

Question 3

What are irreversible inhibitors?

Answer 3

Inhibitors that permanently bind covalently to enzymes, usually in the active site

Question 4

What type of bonds are formed between irreversible inhibitors and amino acids in the active site of enzymes?

Answer 4

Covalent bonds

Question 5

What are the 3 classes of reversible inhibitor?

Answer 5

Competitive, non-competitive, mixed

Question 6

How do reversible inhibitors bind to enzymes?

Answer 6

Non-covalently

Question 7

What is a competitive inhibitor?

Answer 7

A reversible inhibitor that’ competes directly with the substrate for the active site, creating two possible mutually exclusive binding events

Question 8

How does a Vo vs [S] graph change when a competitive inhibitor is added?

Answer 8

No change in Vmax, because infinite [S] eventually outcompetes the inhibitor. However, the curve is flatter and moved to the right because a higher [S] is needed to reach Vmax - therefore higher Km

Question 9

How is the Km value affected when a competitive inhibitor is added?

Answer 9

Increased Km - because a greater [S] is needed to reach Vmax/2

Question 10

How is the Vmax value affected when a competitive inhibitor is added?

Answer 10

Unaffected

Question 11

How does a Lineweaver-Burke Plot change when a competitive inhibitor is added?

Answer 11

Steeper line, same y intercept, higher x-intercept (less negative, smaller number)

Question 12

What is a non-competitive inhibitor?

Answer 12

An inhibitor that binds a different site to the substrate (not the active site), so has no affect on the binding of S (in the case of pure non-competitive inhibition) . However, binding of I changes the structure of the active site such that transition state stabilisation of S is no longer optimal

Question 13

How does a Vo vs [S] change when a non-competitive inhibitor is added?

Answer 13

Vmax is less, so curve reaches the asymptote at a lower point. Km remains the same because the binding affinity is the same

Question 14

How is the Km value affected when a non-competitive inhibitor is added?

Answer 14

Km value is unchanged because the binding affinity of the substrate is not affected by a non-competitive inhibitor

Question 15

How is the Vmax value affected when a non-competitive inhibitor is added?

Answer 15

Decreases because the product is made more slowly as transition state stabilisation is impaired by the inhibitor

Question 16

How does a Lineweaver-Burke Plot change when a non-competitive inhibitor is added?

Answer 16

Steeper line, same x intercept, higher y-intercept

Question 17

How does mixed inhibition change Vmax?

Answer 17

Decreases Vmax

Question 18

How does mixed inhibition change Km?

Answer 18

Increases Km

Question 19

How does mixed inhibition alter a Lineweaver-Burke plot?

Answer 19

Slope, and both x- and y-intercepts change

Question 20

What is enzyme regulation?

Answer 20

Turning enzymes ‘on’ or ‘off’

Question 21

What is the key function of feedback and feed forward enzyme regulation?

Answer 21

Avoids making unnecessary metabolic intermediates

Question 22

What chemical helps to active the glycogen phosphorylase enzyme?

Answer 22

AMP

Question 23

How does AMP activate the glycogen phosphorylase enzyme?

Answer 23

Binds to an allosteric binding site (into active site) and creates a conformational change that activates it. Energy is required

Question 24

Is AMP activation of glycogen phosphorylase feedback or feed forward?

Answer 24

Feedforward

Question 25

How is glucose-6-phosphate used in a feedback mechanism in glycogen phosphorylase regulation?

Answer 25

glucose-6-p can bind to the glycogen phosphorylase enzyme in the same site as AMP, causing feedback inhibition, and preventing further ATP production so that glycogen can be stored

Question 26

How does caffeine and purines regulate glycogen phosphorylase?

Answer 26

Via feedback mechanism - they bind allosterically to inhibit the enzyme

Question 27

How do cellular signals regulate glycogen phosphorylase?

Answer 27

Phosphorylase kinase creates a conformational change that promotes enzyme activity

Question 28

What are allosteric enzymes?

Answer 28

Enzymes that contain a region where small regulatory molecules (effectors) can bind and thereby alter the enzyme’s ability to bind substrate by creating a conformational change

Question 29

What is the shape of a Vo vs [S] plot when an allosteric enzyme is added?

Answer 29

sigmoidal, rather than hyperbolic. S shape in the physiological [S] range

Question 30

What are the 5 methods of enzyme regulation?

Answer 30

Covalent modification, allosteric effects, proteolytic cleavage, gene expression, enzyme degradation

Question 31

How do non-competitive inhibitors lower “enzyme concentration”?

Answer 31

They lower the number of enzymes that are active, essentially reducing the number of enzymes that can undergo susbtrate reactions. As Vmax ultimately depends on enzyme concentration, Vmax is reduced