Guanosine-5'-triphosphate (GTP) is a purine nucleoside triphosphate. It is one of the building blocks needed for the synthesis of RNA during the transcription process. Its structure is similar to that of the guanosine nucleoside, the only difference being that nucleotides like GTP have phosphates on their ribose sugar. GTP has the guanine nucleobase attached to the 1' carbon of the ribose and it has the triphosphate moiety attached to ribose's 5' carbon. GTP is essential to signal transduction, in particular with G-proteins, in second-messenger mechanisms where it is converted to guanosine diphosphate (GDP) through the action of GTPases. Guanosine triphosphate, also known as 5'-GTP or H4GTP, belongs to the class of organic compounds known as purine ribonucleoside triphosphates. These are purine ribonucleotides with a triphosphate group linked to the ribose moiety. Thus, a GTP-bound tubulin serves as a cap at the tip of microtubule to protect from depolymerization; and, once the GTP is hydrolyzed, the microtubule begins to depolymerize and shrink rapidly. Guanosine triphosphate exists in all living species, ranging from bacteria to humans. In humans, guanosine triphosphate is involved in intracellular signalling through adenosine receptor A2B and adenosine. Guanosine-5'-triphosphate (GTP) is a purine nucleoside triphosphate. Outside of the human body, guanosine triphosphate has been detected, but not quantified in several different foods, such as mandarin orange (clementine, tangerine), coconuts, new zealand spinachs, sweet marjorams, and pepper (capsicum). Cyclic guanosine triphosphate (cGTP) helps cyclic adenosine monophosphate (cAMP) activate cyclic nucleotide-gated ion channels in the olfactory system. It also has the role of a source of energy or an activator of substrates in metabolic reactions, like that of ATP, but more specific. It is used as a source of energy for protein synthesis and gluconeogenesis. For instance, a GTP molecule is generated by one of the enzymes in the citric acid cycle. GTP is also used as an energy source for the translocation of the ribosome towards the 3' end of the mRNA. During microtubule polymerization, each heterodimer formed by an alpha and a beta tubulin molecule carries two GTP molecules, and the GTP is hydrolyzed to GDP when the tubulin dimers are added to the plus end of the growing microtubule. The importing of these proteins plays an important role in several pathways regulated within the mitochondria organelle, such as converting oxaloacetate to phosphoenolpyruvate (PEP) in gluconeogenesis. GTP is involved in energy transfer within the cell.