Question

structure and function of G-protein coupled receptors: binding of neurotransmitter/ hormone to 7-transmembrane receptor, activation of G protein (3 subunits- what happens with each subunit), role of GTP/GDP (AH domain)

Answer 1

Solution

GPCR structure and function-

• GPCRs are integral membrane proteins that possess seven membrane-spanning domains or transmembrane helices. The extracellular parts of the receptor can be glycosylated. These extracellular loops also contain two highly conserved cysteine residues that form disulfide bonds to stabilize the receptor structure.
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G protein with activation-

• Heterotrimeric G-proteins are composed of three distinct subunits (α, β, and γ).
• There are many different α, β, and γ subunits, allowing a bewildering number of G-protein permutations.
• Regardless of the specific composition of the heterotrimeric G-protein, its α subunit binds to guanine nucleotides, either GTP or GDP.
• Binding of GDP allows the α subunit to bind to the β and γ subunits to form an inactive trimer. Binding of an extracellular signal to a G-protein-coupled receptor allows the G-protein to bind to the receptor and causes GDP to be replaced with GTP (Figure 8.5A). When GTP is bound to the G-protein, the α subunit dissociates from the βγ complex and activates the G-protein.
• Following activation, both the GTP-bound α subunit and the free βγ complex can bind to downstream effector molecules and mediate a variety of responses in the target cell
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