Activated GPCRs activate G proteins by reducing the strength of binding of GDP to the ? subunit of the G protein, allowing GDP to dissociate and GTP (which is present at much higher concentrations in the cell than GDP) to bind in its place
How would the activity of a G protein be affected by a mutation that reduces the affinity of the ? subunit for GDP without significantly changing its affinity for GTP?
The mutant G protein would be constantly active. Each time the ? subunit hydrolyzed GTP to GDP, the GDP would dissociate spontaneously, allowing GTP to bind and reactivate the ? subunit, especially because the intracellular concentration of GTP is higher than that of GDP. Normally, GDP is tightly bound by the ? subunit, which keeps the G protein in its inactive state until interaction with an appropriate activated GPCR stimulates the release of GDP.
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