Types Of Second Messenger System

Question 1

Adenylyl cyclase-cAMP

Answer 1

Gs-protein consists of alpha, beta and gamma subunit
At rest stage, G-protein is associated with the receptor where its alpha subunit is bounded to GDP.
Binding of ligand-> conformational change->exchange of GDP for GTP->causes dissociation of alpha subunit
Alpha subunit+GTP moves along the membrane and goes to adenylyl cyclase and activates it.(membrane bound enzyme)
AC converts ATP to cAMP
[cAMP] increases in the cell—> activates PKA(protein kinase A) which is cAMP dependent
PKA phosphorylates proteins to activate them

Question 2

What happens when the ligand is no longer present at the receptor?

Answer 2

Alpha subunit hydrolysis GTP TO GDP
Alpha subunit dissociates from AC and re-associates with its beta-gamma complex
Phosphodiestases breaks cAMP to AMP
[cAMP] decreases which reduced the activity of PKA
Phosphatses causes dephosphorylation and inactivates proteins

Question 3

Calcium-calmodulin

Answer 3

All the steps are same to the point where GDP is exchanged for GTP.
(Afterwards)
Alpha subunit+GTP moves along the membrane to phospholipase C(similar to AC) (membrane bound enzyme) and activates it.
Activated phospholipase interacts with PIP2(phosphotidyinositol-4,5-bis-phosphate)
PIP2 breaks into IP3(inositol-1,4,5-triphoshate) and DAG(diacyleglyerol)
IP3 binds to IP3 receptors on ER which are ligand gated calcium channels
Channel opens-> calcium ions discharged->binds to protein calmodulin-> produces Ca-Calmodulin complex(Ca-CAM)
Ca-CAM complex activates CAM kinase which is a myosin light chain kinase in smooth muscle cells—> causes contractions
DAG activates PKC (Protein kinase C) which phosphorylates proteins to modulate their activity.