Question

Which of the following statements is correct?
(A). Allosteric enzymes don't obey Michaelis mentioned kinetics
(B). Some regulatory enzymes are modulated by reversible covalent modification
(C). Allosteric enzymes undergo reversible covalent modification
(D). Reversible covalent modification caused by phosphorylation
Choose the correct answer from the options given below:

• A. (A) and (B) only.
• B. (A), (B) and (C) only.
• C. (A), (B) and (D).
• D. (A), (B), (C) and (D).

Hint:

Classic enzyme kinetics check:
Allosteric enzymes $\rightarrow$ Sigmoidal curve (do not obey Michaelis-Menten).
Covalent modification $\rightarrow$ Phosphorylation Dephosphorylation (kinases/phosphatases).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Enzymes are highly specialized biological catalysts that regulate biochemical reactions.
To maintain metabolic homeostasis, the activity of key regulatory enzymes must be modulated in response to cellular signals.
The two primary mechanisms of enzyme regulation are:
1. Allosteric Regulation: Non-covalent, reversible binding of effector molecules at a distinct site (allosteric site) other than the active site.
2. Covalent Modification: Reversible attachment or removal of a chemical group (such as a phosphate group) via covalent bonds.

Step 2: Detailed Explanation:
Let us evaluate each of the given statements:
Statement (A) is correct:
Allosteric enzymes do not obey classical Michaelis-Menten kinetics (referred to as “Michaelis mentioned kinetics” in the question's typo).
Instead of the typical hyperbolic curve of reaction velocity (\( v \)) versus substrate concentration (\([S]\)), allosteric enzymes display a sigmoidal (S-shaped) curve.
This sigmoidal behavior is due to cooperative binding, where the binding of a substrate molecule to one subunit increases the affinity of the other subunits for the substrate.
Statement (B) is correct:
Many regulatory enzymes are modulated through reversible covalent modification.
This is a major mechanism where the enzyme's active state is turned on or off by enzymes that attach or remove specific groups.
Statement (C) is incorrect:
Allosteric regulation is defined by the *non-covalent*, reversible binding of allosteric modulators.
Although some enzymes can be regulated by both mechanisms, allosteric regulation itself does not involve covalent modification.
Therefore, stating that allosteric enzymes undergo covalent modification as a rule is incorrect.
Statement (D) is correct:
Reversible covalent modification is most commonly caused by phosphorylation and dephosphorylation.
Enzymes called protein kinases transfer a phosphate group from ATP to specific serine, threonine, or tyrosine residues of the target enzyme, while protein phosphatases remove it, altering its activity.
Therefore, statements (A), (B), and (D) are correct, which corresponds to option (C).

Step 3: Final Answer:
Thus, the correct statements are represented by (A), (B) and (D).