Enzyme Regulation & Inhibition

Question 1

Types of enzyme regulation

Answer 1

Allosteric enzyme control = control via enzyme shape
Phosphofructokinase

Question 2

Types of enzyme inhibition

Answer 2

Competitive
Noncompetitive
Uncompetitive

Question 3

Describe noncovalent allosteric regulation

Answer 3

• most allosteric enzymes have quaternary structure
• their activity is controlled through a change in 3D structure brought about by small molecules
• exhibit cooperativity between their subunits = conformational change in one subunit will impact the conformation of other subunits
• have a second regulatory site (allosteric site) distinct from the active site
• allosteric inhibitors or activators bind to this site and regulate enzyme activity via conformational changes

Question 4

What is cooperativity?

Answer 4

• Binding by a substrate to one active site affects subsequent binding at all other subunits
• Locks all subunits in the active form

Question 5

What are the different forms of allosteric enzymes?

Answer 5

Active form (R form) = stabilised by the binding of an activator

Inactive form (I form) = stabilised by the binding of an inhibitor

Question 6

Role of PFK-1

Answer 6

• is an allosteric enzyme
• catalyses early step in glycolysis = key regulation step
• ADP & PEP are allosteric activator and inhibitor respectively
• PEP is a product of a later reaction in the same pathway

Question 7

Effects of PEP and ADP

Answer 7

ADP is an activation so there is a lower apparent Km (curve shifts to left) whereas PEP is an inhibitor so there is a higher apparent Km (curve moves to right)

High PEP/ADP ratio (high energy in cell & lots of ATP) = decreased PFK activity and decreased glycolysis
Low PEP/ADP ratio = increased PFK activity and glycolysis

Question 8

What is competitive inhibition?

Answer 8

• Binds to the enzyme active site
• Vmax is the same as long as [S] is increased enough
• Km is increased = affinity of enzyme to substrate decreases

Question 9

What is uncompetitive inhibition?

Answer 9

• Inhibitor can only bind to the enzyme substrate complex
• Rare occurrence
• Vmax and Km decreased = formation of the product is reduced
• But Km/Vmax is constant

Question 10

What is noncompetitive inhibition?

Answer 10

• Can bind to both the free enzyme and the enzyme-substrate complex
• Binds to an allosteric site
• Km is not affected
• Vmax decreased = less product formed
• Addition of more substrate cannot restore rate

Question 11

Why is enzyme inhibition important?

Answer 11

• Crucial for control of cellular activity
• e.g. feedback inhibition
• Can provide insights into mechanisms of enzymes
• Crucial component of drug design programs