 This usually occurs rapidly, and the GTP binding protein can then only be activated by exchanging the GDP for a new GTP molecule.The substitution of sulfur for one of the oxygens of the γ-phosphate of GTP creates a nucleotide that either cannot be hydrolyzed or is only slowly hydrolyzed.This prevents the GTP binding proteins from being inactivated, and allows the cellular processes that they carry out when active to be more easily studied.