Guanosine 5′-diphosphate (GDP) is used as a substrate of pyruvate kinase to produce GTP in support of RNA biosyntheis. GDP is used to study the kinetics and characteristics of GTPases such as those associated with G-protein coupled receptors (GPCR). GDP is also used to study cell signaling processes mediate by guanine nucleotide exchange factors. Guanosine 5′-diphosphate (GDP) is used as a substrate of pyruvate kinase to produce GTP in support of RNA biosyntheis. GDP is used to study the kinetics and characteristics of GTPases such as those associated with G-protein coupled receptors (GPCR). GDP is also used to study cell signaling processes mediate by guanine nucleotide exchange factors.