Q.8 Human genome sequencing project involved the construction of genomic library in
(A)bacterial artificial chromosome (B)pBR322
(C) bacteriophage (D) pcDNA3.1
Protein phosphorylation regulates countless cellular processes through precise modification of specific amino acid side chains, excluding proline due to its rigid structure. This MCQ challenges biochemistry students on kinase target specificity fundamental to signal transduction pathways.
Correct Answer
(C) Pro cannot undergo standard phosphorylation. Proline’s unique imino ring structure eliminates the free hydroxyl (-OH) group required for phosphate ester formation, lacking the nucleophilic oxygen present in Ser, Thr, and Tyr.
Option Breakdown
(A) Ser
Serine represents ~76% of phosphorylation sites with its primary -CH₂OH group. Protein kinase A (PKA) and MAPK cascades frequently target Ser for rapid signal propagation.
(B) Thr
Threonine accounts for ~20% of sites via its secondary -CH(OH)CH₃ alcohol. CDKs and AKT kinases modify Thr residues critical for cell cycle progression and survival signaling.
(D) Tyr
Tyrosine (~4% sites) features phenolic -OH targeted by receptor tyrosine kinases (EGFR, insulin receptor). Tyr phosphorylation drives growth factor responses and oncogenesis.
Phosphorylation Chemistry
Kinases catalyze nucleophilic attack of substrate -OH oxygen on ATP’s γ-phosphate, forming unstable phosphoester bonds (ΔG hydrolysis ~ -9 kcal/mol). Proline’s cyclic constraint prevents this geometry, rendering it kinase-resistant while maintaining its proline kink in alpha helices.
