Why do GDP and this Quaternary Structure Go Together?
You know that a G protein is in its inactive state when it has GDP bound to the a subunit. During this time, it is coupled to the b and g subunits.
When GTP binds, the a subunit changes conformation and no longer binds to the b/g subunits and floats freely into the cytoplasm.
Therefore, the quaternary structure of 3 subunits binding to each other indicates that the a subunit has GDP bound and not GTP.
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