# DBT JRF 2020 — Question 147

Exam: DBT JRF 2020 · Section: B · Topic: Biochemistry · Difficulty: Medium

## Question

Which post-translational modification is observed most commonly in signal transduction?

## Options

- **A.** carbonylation
- **B.** acetylation
- **C.** phosphorylation
- **D.** nitrosylation

## Answer

Option C

## Explanation

Protein phosphorylation is the most ubiquitous and rapidly reversible post-translational modification in signal transduction. Key features: (1) Ubiquity — $>$ 500 protein kinases in the human kinome; phosphorylation regulates essentially all signal transduction pathways; (2) Reversibility — kinases add phosphate (from ATP) to Ser, Thr, or Tyr; phosphatases (Ser/Thr phosphatases PP1, PP2A; Tyr phosphatases) rapidly remove it — creating an 'on/off switch'; (3) Electrostatic effects — the $-2$ charge of phosphate at pH 7 induces conformational changes and creates docking sites for SH2, PTB, BRCT domains; (4) Signal amplification — one activated kinase phosphorylates many substrates. Examples: EGFR → RAS → RAF → MEK → ERK cascade (all phosphorylation-based). Acetylation (histones, metabolic enzymes) and nitrosylation occur in signalling but are far less prevalent than phosphorylation.

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