Allosteric Enzymes

Question 1

Sequential rxns are characterized by formation of a

Answer 1

ternary complex, consisting of the two substrates and the enzyme
- May be ordered or random in terms of how substrates bind

Question 2

Double displacement (ping pong rxn) are characterized by the formation of a

Answer 2

substituted enzyme intermediate

- One or more of the products are released before all substrates bind the enzyme

Question 3

What are allosteric enzymes?

Answer 3

Enzymes that regulate the flux of biochemicals through metabolic pathways
- Catalyze the committed steps of metabolic pathways

Question 4

Feedback inhibition

Answer 4

The inhibitor binds the regulatory site on the enzyme, which inhibits activity
- Example: excess F can bind to regulatory site of e1 away from active site and inhibit its activity

Question 5

How do metabolic pathways communicate?

Answer 5

Allosteric enzymes can recognize both stimulatory and inhibitory molecules

Question 6

How are metabolic pathways coordinated to make the right amount?

Answer 6

F and I can act as stimulators or inhibitors to ensure enough F or I to make K
Allosteric enzymes are regulated by the endproducts of their pathways via stim or inhib

Question 7

Concerted Model for Allosteric Enzymes

Answer 7

• Multiple active sites on different polypeptide chains
• Relaxed form (R) catalyzes rxns
• Tense form (T) is less active but more stable and more common
• R and T are in equil
• The allosteric constant = T/R = L0 is typically in the hundreds
• Symmetry rule says all active sites must be in same state
• Substrate will bind more readily to R form

Question 8

How does the concerted model actually work?

Answer 8

1. Thermal jostling will randomly convert T to R form and that will push equil to R side = more R form
2. As [S] increases, it binds an active site on R, trapping all other active sites in R state (sym rule)
3. As more active sites are in R form, it is easier for the substrate to bind to R state
4. The binding of S to R becomes even easier as more active sites are in R form

Question 9

In the concerted model, the binding of the substrate

Answer 9

disrupts the T/R equil in favour of R

This behaviour is called cooperativity