Regulation Of Enzyme

REGULATION OF ENZYME

Q. 1 Which one of the following statements correctly describes allosteric enzymes?
 A Michaelis-Menten kinetics describes their activity
 B Effectors may enhance or inhibit substrate binding
 C Positive  cooperativity  occurs  in  all  allosteric molecules except hemoglobin
 D They  are  not  usually  controlled  by  feedback inhibition
Q. 1 Which one of the following statements correctly describes allosteric enzymes?
 A Michaelis-Menten kinetics describes their activity
 B Effectors may enhance or inhibit substrate binding
 C Positive  cooperativity  occurs  in  all  allosteric molecules except hemoglobin
 D They  are  not  usually  controlled  by  feedback inhibition
Ans. B

Explanation:

Effectors may enhance or inhibit substrate binding

The binding of an effector to the regulatory subunit of an allosteric enzyme causes a conformational change that either increases or decreases the activity of the enzyme’s separate catalytic site. Only in some allosteric molecules, such as hemoglobin, does positive cooperativity occur. A positive effector increases substrate binding. This is the case with cyclic AMP-dependent protein kinase of the glycogen phosphorylase cascade. Cyclic AMP binds the regulatory subunit that dissociates from the catalytic subunit and thereby activates it. In the absence of cyclic AMp, the regulatory subunit tightly binds the catalytic subunit and inactivates the enzymes. Many allosteric enzymes are often placed at the first, or committed, step of a metabolic pathway. The end product of the pathway then acts as a negative effector of the enzyme. This is called feedback inhibition. An allosteric enzyme does not obey Michaelis- Menten kinetics.


Q. 2 Allosteric activator of Acetyl CoA carboxylase:
 A Malonyl CoA
 B Acetyl CoA
 C Citrate
 D Biotin
Q. 2 Allosteric activator of Acetyl CoA carboxylase:
 A Malonyl CoA
 B Acetyl CoA
 C Citrate
 D Biotin
Ans. C

Explanation:

Citrate


Q. 3

Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following is the allosteric activators of phosphofructokinase-1?

 A

Fructose 2, 3 bisphosphate

 B

Fructose 2, 6 bisphosphate

 C

Glucokinase

 D

PEP

Q. 3

Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following is the allosteric activators of phosphofructokinase-1?

 A

Fructose 2, 3 bisphosphate

 B

Fructose 2, 6 bisphosphate

 C

Glucokinase

 D

PEP

Ans. B

Explanation:

The most potent positive allosteric activator of phosphofructokinase-1 and inhibitor of fructose 1,6-bisphosphatase in the liver is fructose 2,6-bisphosphate.

  • It relieves inhibition of phosphofructokinase-1 by ATP and increases the affinity for fructose 6-phosphate. 
  • It inhibits fructose 1,6-bisphosphatase by increasing the Km for fructose 1,6-bisphosphate. 
  • Its concentration is under both substrate (allosteric) and hormonal control (covalent modification).
Phosphofructokinase-1 is inhibited by citrate and by normal intracellular concentrations of ATP and is activated by 5′ AMP.
 
Ref: Bender D.A., Mayes P.A. (2011). Chapter 20. Gluconeogenesis & the Control of Blood Glucose. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper’s Illustrated Biochemistry, 29e.

Quiz In Between


Q. 4

N acetylglutamate serve as allosteric actor of which of the following enzyme in the urea cycle?

 A

Argininosuccinate lyase

 B

Argininosuccinate synthase

 C

Ornithine transcarbamoylase

 D

Carbamoyl phosphate synthetase I

Q. 4

N acetylglutamate serve as allosteric actor of which of the following enzyme in the urea cycle?

 A

Argininosuccinate lyase

 B

Argininosuccinate synthase

 C

Ornithine transcarbamoylase

 D

Carbamoyl phosphate synthetase I

Ans. D

Explanation:

N acetylglutamate serve as allosteric actor of Carbamoyl phosphate synthetase I, by enhancing the affinity of the synthase for ATP. N acetyl glutamate is synthesized from acetyl CoA and glutamate in a reaction for which arginine is an activator.
  • Some reactions of urea cycle occur in the mitochondria and some occur in the cytoplasm.
  • Synthesis of 1 mol of urea requires 3 mol of ATP, 1 mol each of ammonium ion and of aspartate, and employs five enzymes.
  • Carbamoyl phosphate synthetase I which catalyse the condensation of CO2, ammonia, and ATP to form carbamoyl phosphate is the rate limiting enzyme in urea synthesis. 
Ref: Biochemistry By Pamela C. Champe, page 253.

Q. 5

Allosteric inhibition of an enzyme is ‑

 A

Binding of inhibitor to catalytic site and inhibition of enzyme

 B

Binding of inhibitor to other site and inhibition of enzyme

 C

Inhibition of enzyme by inhibitors without binding to enzyme

 D

Inactivation by phosphorylation  or dephosphorylation

Q. 5

Allosteric inhibition of an enzyme is ‑

 A

Binding of inhibitor to catalytic site and inhibition of enzyme

 B

Binding of inhibitor to other site and inhibition of enzyme

 C

Inhibition of enzyme by inhibitors without binding to enzyme

 D

Inactivation by phosphorylation  or dephosphorylation

Ans. B

Explanation:

Ans. is ‘b’ i.e., Binding of inhibitor to other site and inhibition of enzyme

  • Some enzymes, called allosteric enzymes, posses a site, in addition to substrate binding (catalytic) site, known as the allosteric site.
  • Binding of allosteric modulator at the allosteric site affects the conformation of catalytic site.
  • Such enzymes are called allosteric enzymes.
  • The allosteric modulator (regulator) may facilitate the conformational change of catalytic site, required for substrate binding.
  • Such regulators are called allosteric activators (positive allosteric modifier); for example, fructose-2, 6-bisphosphate is an allosteric activator of Phosphofructokinase-I.
  • Some allosteric regulators prevent the conformational change required for binding of the substrate.
  • Such regulators are called allosteric inhibitors (negative allosteric modifier); for example, citrate is an allosteric inhibitor of phosphoructokinase-I.

Quiz In Between



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