Discuss the role of tripartite G proteins in neurotransmitte

Discuss the role of tripartite G proteins in neurotransmitter signaling. What different signaling pathways are activated by the different G proteins?

Solution

The heterotrimeric GTP – binding proteins are known as G proteins. These act as molecular switches to the transducing ligands binding to their receptors basing on intracellular signals. G proteins are composed of and subunits. The subunit binds to the GTP and GDP (guanosinetriphosphate, guanosinediphosphate).

The second messenger cAMP is involved in the transduction of G – protein coupled receptors (GPCRs) signaling mechanisms. In this mechanism phosphatidyl inositol cycle and calcium ions are also involved. cGMP is synthesized from guanylate cyclase like cAMP from adenylate cyclase with a similar mechanism.

The phosphatidyl inositol cycle mediates the actions of hormones like vasopressin and neurotransmitter like epinephrine. Diacyl glycerol is a second messenger molecule formed during phosphatidylinositol – 4, 5 – bisphosphate (PIP₂) cleavage by phospholipase C enzyme. The DAG activates the protein kinase C (PKC). The reaction of phosphatidylinositol – 4, 5 – bisphosphate (PIP₂) cleavage by phospholipase C enzyme results in formation of inositol – 1,4,5 – triphosphate and diacylglycerol (DAG). After formation the IP₃ diffuses into calcisome of smooth endoplasmic reticulum and binds to its receptor (a calcium channel).

Calcium ions mediate the large number of cellular processes including the activation of plasma membrane – associated protein kinase C (PKC). Calcium binding sites of calcium regulated proteins exhibit the high affinity for calcium ions. Calmodulin a type of calcium binding protein involved in the mediation of many calcium regulated reactions.

The tripartite G proteins are also involved in cell cycle regulation and in Rho family GTPase signaling.