Second Messenger

SECOND MESSENGERS:

• Molecules relaying signals from membrane receptors to target molecules inside cells.

Common second messengers:

• Cyclic Adenosine 3′ 5′- Monophosphate (cAMP)
• Cyclic Guanosine Monophosphate (cGMP)
• Diacylglycerol (DAG)
• Calcium ions (Via Calmodulin)
• Inositol Triphosphate (IP3)
• Gaseous molecules – Nitric oxide (NO), Carbon-mono-oxide (CO) & Hydrogen Sulphite (H2S)

Note: NO acts as both 1° messenger (through cGMP) & 2° messenger.

CLASSIFICATION:

Group I hormones:

• Lipid soluble
• Readily diffuse across cell membrane .
• Directly bind to intracellular receptors.
• No need for second messenger.

Group II hormones:

• Most are polypeptides
• Water-soluble.Act on cell membrane receptors.
• Hence, cannot diffuse across cell membrane.
• Membrane receptors convey effector signals to target molecule via 2nd messenger.
• Hormone/Ligand – 1st messenger exerting its effects on 2nd messenger

MECHANISM OF ACTION:

1. Intracellular receptors:

• Stimulates transcription.
• Formation of mRNA translating into effector protein.

2. Secondary messengers of membrane receptors:

• Act by post-translational modification.
• Most predominant protein modification – “Phosphorylation”.

Cellular phosphorylation:

• Under control of 2 groups of protein – 

1. Protein kinases –

• Enzymes adding/catalyzing phosphorylation tyrosine or serine or threonine residues in proteins
• 2nd messengers act on protein kinases stimulating protein phosphorylation.

2. Phosphatases I–

• Enzymes removing phosphates from protein (dephosphorylation).

CALMODULIN:

• Calcium-dependent regulatory protein is calmodulin
• Second messenger Ca²⁺ effects through calmodulin.
• Calmodulin- four Ca²⁺ binding sites & its full site occupancy leads to marked conformational change.

Functions of calmodulin:

Don’t Forget to Solve all the previous Year Question asked on SECOND MESSENGER