Q.44 Given below are two statements : One is labelled as Assertion A and the other is labelled as Reason R.
Assertions A : Protein kinases catalyze the transfer of gamma (y) – phosphoryl group of ATP to proteins.
Serine, threonine or Wrosine are the general amino acids that are phosphorylated.
Reason R : Phosphorylation of proteins by protein kinases makes them aggregated and non-functional with
subsequent degradation.
In the light of the above statements. choose the most appropriate answer from the options given below :
1. Both A and R are correct and R is the conect explanation of A
2. Both A and R are correct but R is NOT the comect explanation of A
3. A is correct but R is not correct
4. A is not correct but R is correct

Correct Answer: 3. A is correct but R is not correct

Protein kinases accurately transfer the γ-phosphate from ATP to Ser, Thr, or Tyr residues, but phosphorylation typically regulates function—activating, inactivating, or altering localization—not causing aggregation, loss of function, and degradation as a general rule.

Assertion (A) Analysis

Assertion A is correct. Protein kinases (e.g., PKA, CDK) catalyze transfer of ATP’s γ-phosphoryl (PO₄³⁻) to hydroxyl groups on serine (Ser), threonine (Thr), or tyrosine (Tyr)—the primary phosphoacceptors (>98% of cases). This reversible PTM uses Mg-ATP; reaction: Protein-OH + ATP → Protein-O-PO₃²⁻ + ADP.

Reason (R) Analysis

Reason R is incorrect. Phosphorylation rarely causes aggregation or universal non-function/degradation. Instead:

• Activates (e.g., phosphorylase kinase).

• Inhibits (e.g., glycogen synthase).

• Creates docking sites (e.g., MAPK cascades).
  Ubiquitination/proteasome handles degradation; kinases like Aurora B prevent aggregates in mitosis. Disease-linked hyperphosphorylation (e.g., tau in Alzheimer’s) forms tangles, but this is pathological, not the norm.​

Option Breakdown

• Option 1: Both correct, R explains A—False. R wrong.

• Option 2: Both correct, R not explanation—False. R incorrect.

• Option 3: A correct, R incorrect—True. Matches analysis.

• Option 4: A incorrect, R correct—False. A precise.

Protein Kinases Phosphorylation: Assertion Reason Molecular Biology

Protein kinases gamma phosphoryl ATP serine threonine tyrosine phosphorylation drives cell signaling, correctly described in Assertion A but misrepresented in Reason R as causing aggregation—key for molecular biology exams.

Protein Kinase Mechanism

Kinases bind ATP in a conserved cleft; Asp residue deprotonates substrate OH, facilitating γ-P transfer:

Protein-OH+γ-PO3-ATP→Protein-O-PO32−+ADPProtein-OH+γ-PO3-ATP→Protein-O-PO32−+ADP

Targets: Ser/Thr (80%), Tyr (20%); atypical (His/Asp) exist.

Phosphorylation Effects

Reversible switch via phosphatases:

• Conformational change activates (e.g., PKC).

• Inhibits (e.g., IRS-1 in insulin resistance).

• Cascades (RTK → Raf → MEK → ERK).
  Not aggregation/degradation—those link to misfolds/ubiquitin.​

Exam Strategy

Tests PTM details: A recalls mechanism/amino acids right; R confuses with ubiquitination/aggregation pathologies. Prioritize kinase function for GATE Life Sciences.​