The influence of bound GDP on the kinetics of guanine nucleotide binding to G proteins

Purified guanine nucleotide-binding regulatory proteins, as either the oligomers or the isolated nucleotide-binding α subunits, display anomalous kinetics of nucleotide binding. This is due to the presence of tightly bound GDP in these preparations. The dissociation of bound GDP is the rate-limiting step for nucleotide binding. GDP can be removed by chromatography in the presence of 1 M (NH₄)₂SO₄ and 20% glycerol, which yields preparations of G proteins that contain less than 0.1 mol of GDP/mol of guanosine 5'-(γ-thio)triphosphate (GTPγS)-binding site. When the GDP is removed, the binding of GTPγS displays kinetics consistent with a bimolecular reaction.