BB13 ATCase (Allosteric Enzyme)

Question 0

ATCase is inhibited by

Answer 0

CTP

• the end product

Question 1

Aspartate transcarbamolase

Answer 1

• ATCase
• allosteric enzyme
• catalyzes the 1st step in the biosynthesis of pyrimidines

Question 2

CTP

Answer 2

• structurally dissimilar from reactants
• binds at site different from the substrate
(allosteric/regulatory site)
• ALLOSTERIC INHIBITOR

Question 3

Rate of CTP formation

Answer 3

• fast at low concentration
• slows as concentration increases
• make new CTP until enough pyrimidines accumulate
= feedback inhibition

Question 4

Allosteric enzyme distinguished by

Answer 4

• response to change in substrate concentration

* susceptibility to regulation by other molecules

Question 5

ATCase consists of

Answer 5

• separate catalytic and regulatory subunits
• 2 catalytic trimers
• 3 regulatory dimers (zinc)

Question 6

ATCase can be separated into regulatory and catalytic subunits by

Answer 6

p-hydroxymercuribenzoate
• reacts with sulfhydryl groups
• cysteine binds zinc
• PCMBS displaces zinc and destabilizes the domain

Question 7

Oligomeric states of ATCase

Answer 7

• tense

* relaxed

Question 8

PALA

Answer 8

potent competitive inhibitor
• mimic transition state = inhibit enzymes
• bisubstrate analog
• binds at boundary of 2 catalytic subunits

Question 9

Adding more substrate to ATCase has 2 effects

Answer 9

1. increases probability that each enzyme molecule will bind at least 1 substrate molecule
2. increases average number of substrate molecules bound to each enzyme
   * equilibrium (R-T) depends on the number of active sites that have substrate

Question 10

Relaxed state

Answer 10

• more catalytically compotent
• active
• PALA mimics intermediate, stabilizes relaxed state

Question 11

CTP favors

Answer 11

the T-state
• inhibits the enzyme
• binds to regulatory site, less active

Question 12

Catalytic subunits

Answer 12

• unresponsive to CTP

Question 13

Regulatory subunits

Answer 13

• binds CTP but not catalytic

Question 14

The interaction of the subunits in the enzyme

Answer 14

produces its regulatory and catalytic properties

Question 15

PALA stabilizes

Answer 15

the relaxed state
more active
favoured by substrate binding

Question 16

CTP stabilizes

Answer 16

the tense state
less active
favoured by CTP binding

Question 17

ATP

Answer 17

competes with CTP for binding to C and R sites

high ATP prevents CTP inhibition

Question 18

ATCase displays

Answer 18

sigmoidal kinetics
• importance of quaternary structure
(allostery)
• from cooperation in subunits

Question 19

Cooperation of subunits (ATCase)

Answer 19

• binding of substrate to 1 active site increases the likelihood that substrate will bind to other active sites

Question 20

The catalytic subunit shows

Answer 20

Michaelis-Menton kinetics

Question 21

Tense

Answer 21

regulatory dimers hold the 2 catalytic trimers close

key loops collide and interfere with the conformational adjustments necessary for high affinity binding and catalysis

Question 22

Sigmoid curve

Answer 22

t state below r state

Question 23

Reason for increased ATCase activity in response to increased ATP concentration

Answer 23

1. high ATP concentration = high purine concentration, increased ATCase activity to balance purines and pyrimidines
2. high ATP concentration = energy available for mRNA synthesis and DNA replication, synthesis of pyrimidine for these processes

Question 24

Allosteric inhibitor vs activator

Answer 24

CTP – allosteric inhibitor – moves curve right (lower)

ATP – allosteric activator – moves curve left (higher)