Q.1 Rajiv Gandhi Khel Ratna Award was conferred
_Mary Kom, a six-time world
champion in boxing, recently in a ceremony
_the Rashtrapati Bhawan (the President’s
official residence) in New Delhi.
(A) with, at
(B) on, in
(C) on, at
(D) to, at
Phosphomimic mutants mimic the negative charge of a phosphorylated serine residue in proteins. Replacing serine with aspartic acid creates this effect due to its acidic side chain.
Correct Answer
The correct choice is (B) aspartic acid. Phosphorylation adds a phosphate group to serine’s hydroxyl side chain, introducing a negative charge that alters protein function, often activating it. Aspartic acid (Asp, D) bears a negatively charged carboxyl group (-COO⁻) at physiological pH, closely mimicking this phosphorylated state without needing enzymatic modification.
Option Explanations
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(A) Alanine: Alanine has a small, non-polar methyl side chain (-CH₃), replacing serine with a neutral residue that prevents phosphorylation and eliminates the negative charge mimicry. This creates a non-functional or phospho-dead mutant.
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(B) Aspartic acid: Aspartic acid’s side chain (-CH₂COO⁻) provides a permanent negative charge similar to phosphoserine, enabling constitutive protein activation in studies of signaling pathways like MAPK.
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(C) Phenylalanine: Phenylalanine features a large, hydrophobic aromatic ring, which disrupts serine’s polarity and cannot replicate phosphorylation’s charge or hydrogen bonding, often used in gain-of-function studies but not as a phosphomimic.
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(D) Lysine: Lysine has a positively charged amino group (-NH₃⁺), opposing the negative charge of phosphorylation and potentially inverting regulation, sometimes used to study charge reversal effects.
Phosphomimic Applications
Phosphomimics like S-to-D mutations aid research in kinase signaling, cancer biology, and plant immunity by locking proteins in an “on” state for functional assays. Common in tools like CRISPR editing for serine/threonine sites.
