Question

Describe the intracellular mechanisms of the following GPCRs, membrane lipid-derived 2^nd messengers and RTKs.

Answer 1

G-protein-coupled receptors (GPCRs) are the largest and most diverse group of membrane receptors in eukaryotes. G proteins are specialized proteins with the ability to bind the nucleotides guanosine triphosphate (GTP) and guanosine diphosphate (GDP). The G proteins that associate with GPCRs are heterotrimeric, meaning they have three different subunits: an alpha subunit, a beta (β) subunit, and a gamma (γ) subunit. Two of these subunits - alpha (α-) and gamma (γ) - are attached to the plasma membrane by lipid anchors. Ligand binding to the GPCR causes a change in the receptor conformation that in turn binds and activates the G-protein. The active form of the G-protein is then released from the surface of the receptor, dissociating into its α- and β/γ subunits. Both subunits will then activate their specific effectors, causing the release of second messengers. These messengers are recognised by protein kinases leading to their activation and triggering the signaling cascade towards a cellular event.GPCR function is associated with cell sensing of external factors including odorants, taste ligands, light, metals, neurotransmitters, biogenic amines, fatty acids, amino acids, peptides, proteins, steroids and other lipids

Intracellular second messengers

insulin and glucagon interact with a receptor on the cell surface. The activated receptor on cell surface then generates so-called secondary messengers within the cell that transmit the information to the biochemical systems whose activities must be altered to produce a particular physiological effect. The magnitude of the end effect is generally proportional to the concentration of the second messengers.

Receptor tyrosine kinase

The receptor tyrosine kinase are second major type of cell surface receptors. The legand of RTKs may be insulin, epidermal growth factor, transforming growth factor, platelet derived growth factor, fibroblasts growth factor, colony stimulating growth factor.

All rtk comprise an extracellular domain that includes a region with protein kinases activity. Most of the rtk are monomeric and ligand binding to the extracellular domain induces formation of receptor dimers.

Ligand binding leads to activation of the internship protein tyrosine kinase activity of the receptor and autophosphorylation of tyrosine residues in its cytosolic domain. The activated receptor also can phosphorylate other protein substrate. Some of these proteins are GTPase activating protein (GAP), phospholipase c gamma (PLC gamma), and sar like non receptor kinases

The three major pathway that transduce a signal from activated, RTKs

1. Ras MAP kinase pathway

2. IP3/DAG pathway

3. PI3 kinase pathway

RAS MAP Kinase pathway

RAS is an intracellular monomeric GTPase swtich protein and function in many different RTKs. RAS is lipid link protein present present at cytoplasmic face of membrane

RAS cycle required assistance of two protein guanine nucleotide exchange factor (GEF) and GTPase activating complexes (GAP). RAS activating is accelerated by a protein called GEF, which binds to the RAS GDP complex, causing dissociation of the bound GDP. Because GTP present in the cell in higher concentration then the GDP, GTP bind spontaneously to empty RAS molecule. With the release of GEF. Avarage life time of ras-gtp complex has one minute. GAP which binds to ras gtp, accelerated it's internsic GTPase activaty binding of ligand to an RtK leads to activation of RAS.