Q.84 The covalent modification performed by kinases which regulate proteins in signaling pathways is
(A) glycosylation
(B) methylation
(C) ubiquitination
(D) phosphorylation
Kinases perform phosphorylation, the key covalent modification that adds a phosphate group to proteins, regulating signaling pathways like cell growth and response to stimuli. This process activates or deactivates proteins rapidly, making it essential in molecular biology.
Correct Answer
The correct answer is (D) phosphorylation. Kinases catalyze the transfer of a phosphate group from ATP to specific amino acids like serine, threonine, or tyrosine on target proteins, altering their activity, localization, or interactions in signaling cascades such as MAPK or insulin pathways.
Option Breakdown
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Glycosylation (A): Enzymes called glycosyltransferases attach sugar molecules to proteins or lipids, aiding cell recognition, stability, and folding, but not performed by kinases.
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Methylation (B): Methyltransferases add methyl groups to lysine or arginine residues, often on histones to regulate gene expression, unrelated to kinase activity.
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Ubiquitination (C): E3 ligases attach ubiquitin chains to proteins, typically marking them for degradation via the proteasome, though it can interact with phosphorylation; kinases do not perform this.
Why Phosphorylation Dominates Signaling
Phosphorylation enables reversible switches in signaling, with over 500 kinases in humans controlling pathways like cancer-related PI3K/AKT. Unlike ubiquitination’s degradative role or glycosylation’s structural function, phosphorylation’s speed suits dynamic regulation.
