Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
Correctly match the following Monosaccharides with their respective Epimers.
Monosaccharide Epimer
P. D-mannose 1. C-3 epimer of D-glucose
Q. D-allose 2. C-4 epimer of D-glucose
R. D-galactose 3. C-4 epimer of D-mannose
S. D-talose 4. C-2 epimer of D-glucose
5. C-5 epimer of D-glucose
Options:
(A) P-4; Q-1; R-2; S-3
(B) P-5; Q-1; R-2; S-3
(C) P-4; Q-3; R-5; S-1
(D) P-1; Q-5; R-3; S-2
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2025 Q.18)**
**1. Governing Concept**:
- **Topic**: Cell Signaling & G-Protein Coupled Receptor (GPCR) Downstream Pathways
- Activation of $G_{\alpha s}$-coupled receptors stimulates **adenylyl cyclase**, which catalyzes the conversion of ATP into **cyclic AMP (cAMP)**:
- Intracellular cAMP binds to the regulatory ($R$) subunits of **Protein Kinase A (PKA)**.
- Binding releases the active catalytic ($C$) subunits of PKA.
- Free catalytic subunits phosphorylate target serine/threonine residues on cellular enzymes and transcription factors (e.g., CREB).
**2. Option Analysis**:
- **Option (A) is INCORRECT**: Protein Kinase C (PKC) is activated by diacylglycerol (DAG) and $\text{Ca}^{2+}$ via the $G_q$-phospholipase C pathway.
- **Option (B) is CORRECT**: cAMP binds regulatory subunits of PKA, allosterically releasing active catalytic subunits.
- **Option (C) is INCORRECT**: Receptor Tyrosine Kinases (RTKs) are activated by direct ligand-induced dimerization and autophosphorylation.
- **Option (D) is INCORRECT**: Calmodulin is an intracellular calcium sensor, not a cAMP receptor.
**3. Final Answer**:
- **Correct Choice**: **B**
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