Cell Communication & Cell Signalling

• cAMP Signaling Mechanism: Cyclic AMP (cAMP) acts as a ubiquitous second messenger generated from ATP by adenylyl cyclase upon G-protein coupled receptor (GPCR) activation.
• Activation of Protein Kinase A (PKA): In animal cells, cAMP exerts its primary biochemical effects by activating PKA (cAMP-dependent protein kinase):
• Inactive PKA exists as a tetramer containing two regulatory ($R$) and two catalytic ($C$) subunits ($R_2C_2$).
• Binding of four cAMP molecules to the regulatory subunits causes a conformational change that releases the active catalytic subunits ($C$), which then phosphorylate serine and threonine residues on target enzymes and the transcription factor CREB.

Correct option: (2) PKA

• Ras GTPase Signaling Pathway:
• Ras is a monomeric G-protein that operates as a molecular binary switch: it is active when bound to GTP and inactive when bound to GDP.
• Active Ras-GTP binds and recruits Raf (a serine/threonine protein kinase) to the plasma membrane, activating the MAPK/ERK signaling cascade.
• GTP Hydrolysis: Hydrolysis of GTP to GDP (accelerated by GTPase-activating proteins, GAPs) returns Ras to its inactive state, terminating downstream kinase activation.
• Therefore, GTP hydrolysis inactivates signaling, making the statement that "Hydrolysis of GTP by Ras can activate a protein kinase" FALSE.

Correct option: (4) Hydrolysis of GTP by Ras can activate a protein kinase.

• Nitric Oxide Synthase (NOS):
• Catalyzes the synthesis of nitric oxide ($\cdot\text{NO}$), a gaseous signaling molecule that regulates vasodilation, neurotransmission, and immune defenses.
• Reaction: NOS oxidizes the guanidino nitrogen of L-arginine in a two-step reaction requiring molecular oxygen and NADPH, producing $\cdot\text{NO}$ and L-citrulline:

$$\text{L-Arginine} + 2\text{ O}_2 + 1.5\text{ NADPH} + 1.5\text{ H}^+ \xrightarrow{\text{NOS}} \text{L-Citrulline} + \cdot\text{NO} + 1.5\text{ NADP}^+ + 2\text{ H}_2\text{O}$$

• Isoforms: Neuronal NOS (nNOS / NOS1), Inducible NOS (iNOS / NOS2), and Endothelial NOS (eNOS / NOS3).

Correct option: (1) Nitric oxide synthase (NOS)

• Nitric Oxide and Soluble Guanylyl Cyclase (sGC) Signaling:
• Signaling Mechanism:
• Correct: Nitric oxide ($\cdot\text{NO}$) diffuses across the plasma membrane and acts as an intracellular signaling molecule that binds and activates sGC.
• Correct: sGC functions as both an intracellular receptor for NO (via its prosthetic heme iron group) and an intracellular signaling enzyme.
• sGC converts GTP into cyclic GMP (cGMP), which activates protein kinase G (PKG) to mediate vascular smooth muscle relaxation.
• Incorrect: NO is synthesized by nitric oxide synthase (NOS), not guanylyl cyclase.
• Exam Context: Option (4) is designated in the official answer key.

Correct option: (4) (B), (C) and (D) only.