Cytidine triphosphate (CTP), also known as 5'-CTP, is pyrimidine nucleoside triphosphate. Formally, CTP is an ester of cytidine and triphosphoric acid. It belongs to the class of organic compounds known as pentose phosphates. These are carbohydrate derivatives containing a pentose substituted by one or more phosphate groups. CTP, much like ATP, consists of a base (cytosine), a ribose sugar, and three phosphate groups. CTP is a high-energy molecule similar to ATP, but its role as an energy coupler is limited to a much smaller subset of metabolic reactions. CTP exists in all living species, ranging from bacteria to plants to humans and is used in the synthesis of RNA via RNA polymerase. Another enzyme known as cytidine triphosphate synthetase (CTPS) mediates the conversion of uridine triphosphate (UTP) into cytidine triphosphate (CTP) which is the rate-limiting step of de novo CTP biosynthesis. CTPS catalyzes a complex set of reactions that include the ATP-dependent transfer of the amide nitrogen from glutamine (i.e., glutaminase reaction) to the C-4 position of UTP to generate CTP. GTP stimulates the glutaminase reaction by accelerating the formation of a covalent glutaminyl enzyme intermediate. CTPS activity regulates the intracellular rates of RNA synthesis, DNA synthesis, and phospholipid synthesis. CTPS is an established target for a number of antiviral, antineoplastic, and antiparasitic drugs. CTP also acts as an inhibitor of the enzyme known as aspartate carbamoyltransferase, which is used in pyrimidine biosynthesis. CTP also reacts with nitrogen-containing alcohols to form coenzymes that participate in the formation of phospholipids. In particular, CTP is the direct precursor of the activated, phospholipid pathway intermediates CDP-diacylglycerol, CDP-choline, and CDP-ethanolamine ((PMID: 18439916). CDP-diacylglycerol is the source of the phosphatidyl moiety for phosphatidylserine, phosphatidylethanolamine, and phosphatidylcholine (synthesized by way of the CDP-diacylglycerol pathway) as well as phosphatidylglycerol, cardiolipin, and phosphatidylinositol (PMID: 18439916).