REGULATION OF ENZYME
| 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 |
| A | Malonyl CoA | |
| B | Acetyl CoA | |
| C | Citrate | |
| D | Biotin |
| A | Fructose 2, 3 bisphosphate | |
| B |
Fructose 2, 6 bisphosphate |
|
| C |
Glucokinase |
|
| D |
PEP |
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 |
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 |
| 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 |
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.
| A | Malonyl CoA | |
| B | Acetyl CoA | |
| C | Citrate | |
| D | Biotin |
Citrate
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 |
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).
| A |
Argininosuccinate lyase |
|
| B |
Argininosuccinate synthase |
|
| C |
Ornithine transcarbamoylase |
|
| D |
Carbamoyl phosphate synthetase I |
- 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.
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. 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.