Enzymes Lec 3

Question 1

Metabolic Regulation

Answer 1

Many processes organised into pathways

Question 2

How are pathways regulated

Answer 2

Feedback and Feedforward mechanisms

Question 3

What is a committed step

Answer 3

An irreversible enzymatic reaction. After this step, the molecules are ‘committed’ to the pathway.

Question 4

Feedback activation mechanism

Answer 4

Low energy signals such as ADP or AMP

Signals produced earlier go back and inhibit

Question 5

Feedback inhibition mechanism

Answer 5

High energy signal (ATP)
Build up of intermediates (citrate, acetyl CoA)
Signals produced earlier go back and inhibit

Question 6

Feed Forward activation

Answer 6

Fructose Bisphosphate

Signals produced to go forward and activate

Question 7

Types of inhibition

Answer 7

Irreversible and Reversible

Question 8

Types of reversible inhibition

Answer 8

Competitive inhibition
Uncompetitive inhibition
Non-Competitive inhibition

Question 9

Types of irreversible inhibition

Answer 9

Group-specific covelant modifying agents
Transition stage analogues
Suicide inhibitors (mechanism-based inhibitors)

Question 10

Irreversible inhibition

Answer 10

Mimicking the substrate and reacting with the active site.
Dissociates very slowly from target enzyme because it becomes tightly bound to the enzyme covalently or non covalently.
e.g penicillin

Question 11

Group specific covalent modifying agents

Answer 11

React with specific functional group on an enzyme.
Binding deactivates the enzyme.
E.G Di-isopropyl phosphofluoridate (DIPF) is nerve gas

Question 12

Transition state analogues

Answer 12

Structurally similar to transition state, but binds more tightly to enzyme than substrate, high affinity for active site.
e.g saquinavir and ritonavir

Question 13

What are transition state analogues useful for

Answer 13

Understanding catalytic mechanisms

Very specific inhibitors of enzymes.

Question 14

Suicide substrates

Answer 14

Mechanism based inhibitors.
Structural similarity to substrate
Enzyme treats it as substrate, starting chemical catalytic process.
Chemical mechanism leads enzyme to react covalently with the inhibitor thus ‘committing suicide’

Question 15

Competitive Inhibitors

Answer 15

Binds to active site
Enzyme can bind to inhibitor or substrate but not both
Increasing [S] increases rate of reaction
Similar structure to substrate
Km increases with more inhibitor

e.g Non steroidal anti-inflammatory agents e.g ibuprofen

Question 16

Uncompetitive Inhibitors

Answer 16

Binds to enzyme substrate complex
Binding site is created on interaction of enzyme and substrate
Cannot be overcome by addition of more substrate
Km reduced, Vmax decreases

Question 17

Non Competitive Inhibition

Answer 17

Changes the shape of the enzyme and the active site.
Allosteric mechanism
Can bind free enzyme or ESC
Decreases functional concentration of enzyme
Reduces turnover rate
reduces Vmax
Km unchanged

Question 18

Multistep reaction

Answer 18

Regulated enzymes have two or more protein components (subunits)
Feedback in a multistep reaction

Question 19

Allosteroid Enzymes

Answer 19

Alteration of one subunit can have significant effect on others
Do not obey MM kinetics
Cooperative if binding of substrate to one active site facilitates the binding of substrate to the other active sites.

Question 20

Allosteric Effectors

Answer 20

Binds to site that alters the activity of the rest of the subunits - changes in protein conformation
Do not bind the active site (non competitive)
Structurally unrelated to substrate
Can activate or inhibit