L12 - enzyme as drug targets

Question 1

What do simple models of reversible enzyme inhibition assume?

Answer 1

• 1 inhibitor molecule binds to enzyme
• inhibitor binds is reversible
• linear kinetics observed
• inhibitors are dead-end

Question 2

What do inhibitors resemble?

Answer 2

substrate in structure

Question 3

what do inhibitors affect

Answer 3

either Km, Vmax or both

Question 4

what is potency measured by?

Answer 4

Ki value (conc of inhibitor that causes a 2 fold change in Km or Vmax

Question 5

what is Ki values related to?

Answer 5

binding energy (deltaG)

Question 6

what do smaller Ki numbers mean?

Answer 6

tighter bonding (higher potency)

Question 7

what do many potent inhibitors resemble?

Answer 7

the transition state of reaction

Question 8

what does the development of new inhibitors require?

Answer 8

understanding of enzyme mechanism

Question 9

what is an example of of an inhibitor?

Answer 9

proline racemase
- transition state is relatively flat
- inhibition by pyrrole-2-carboxylate

Question 10

what are the forms of inhibition?

Answer 10

• competitive
• non-competitive
• mixed competitive
• uncompetitive

Question 11

what is the most common form of inhibition?

Answer 11

competitive inhibition

Question 12

what are the plots like for competitive inhibition?

Answer 12

• Km increases (x int changes)
• Km (I) = Km (1 + [I]/Ki)
• no change in Vmax (y int the same)

Question 13

when does inhibition decreases in competitive inhibition?

Answer 13

with increased substrate concentration

Question 14

what is non-competitive inhibition uncommon for?

Answer 14

single substrate enzymes

Question 15

What are plots like for non-competitive inhibition?

Answer 15

• Vmax is reduced
• Vmax/Km (I) = Vmax/Km(1 + [I]/Ki)
• Km not affected

Question 16

When is inhibition reduced in non-competitive inhibition?

Answer 16

it’s not reduced by increasing substrate

Question 17

What is a common form of inhibition?

Answer 17

mixed competitive inhibition

Question 18

what are plots like for mixed competitive inhibition?

Answer 18

• Km increased (comp)
• Vmax decreased (non-comp)

Question 19

what does the alpha factor define?

Answer 19

the competitive and non-competitive components

Question 20

what is an uncommon mode of inhibition?

Answer 20

uncompetitive inhibition

Question 21

How does the uncompetitive inhibition?

Answer 21

inhibitor binds to the ES complex

Question 22

What are plots like for uncompetitive inhibition?

Answer 22

• parallel lines in lineweaver-burk plot
• both Km and Vmax reduced by same amount

Question 23

What happens when there’s an increase in substrate concentration in uncompetitive inhibition?

Answer 23

it increases inhibition

Question 24

what do dose response curves measure?

Answer 24

drug potency at a fixed substrate concentration

Question 25

what is IC50?

Answer 25

conc of drug required for 50% reduction in activity
- not the same as Ki

Question 26

what does the value for IC50 depend on?

Answer 26

assay conditions
- especially substrate conc

Question 27

when does tight binding inhibition occur? and what does this mean?

Answer 27

when the conc of active enzyme and inhibitor are similar
- conc of free inhibitor doesn’t approximate to total inhibitor

Question 28

When does the Ki value increase?

Answer 28

with increasing enzyme conc

Question 29

what does tight-binding inhibition often have? and what does this mean?

Answer 29

• very slow onset
• the enzyme inhibitor complex can have a very long half life
• infrequent dosing regiments can be used

Question 30

what are examples of irreversible inhibitors?

Answer 30

penicillins and other B-lactam antibiotics

Question 31

what are enzymes inhibited by with penicillins?

Answer 31

penicillin-binding proteins (PBPs)

Question 32

what do irreversible inhibitors act as?

Answer 32

pseudo-substrates but get “stuck”
- at acyl-enzymes intermediate)

Question 33

what is kinetic behaviour like with irreversible inhibitors?

Answer 33

more similar to chemical systems