| CTP synthase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
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| Identifiers | |||||||||
| EC no. | 6.3.4.2 | ||||||||
| CAS no. | 9023-56-7 | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
| Gene Ontology | AmiGO / QuickGO | ||||||||
| |||||||||
CTP synthase is an enzyme (EC 6.3.4.2) involved in pyrimidine biosynthesis that interconverts UTP and CTP. [1] [2]
CTP (cytidine triphosphate) synthetase catalyzes the last committed step in pyrimidine nucleotide biosynthesis: [3]
ATP + UTP + glutamine → ADP + Pi + CTP + glutamate
It is the rate-limiting enzyme for the synthesis of cytosine nucleotides from both the de novo and uridine salvage pathways. [4]
The reaction proceeds by the ATP-dependent phosphorylation of UTP on the 4-oxygen atom, making the 4-carbon electrophilic and vulnerable to reaction with ammonia. [5] The source of the amino group in CTP is glutamine, which is hydrolysed in a glutamine amidotransferase domain to produce ammonia. This is then channeled through the interior of the enzyme to the synthetase domain. [6] [7] Here, ammonia reacts with the intermediate 4-phosphoryl UTP. [8]
Two isozymes with CTP synthase activity exist in humans, encoded by the following genes:
Active CTP synthase exists as a homotetrameric enzyme. At low enzyme concentrations and in the absence of ATP and UTP, CTP synthase exists as inactive monomer. As enzyme concentration increases, it polymerizes first to a dimer (such as the form shown to the left) and, in the presence of ATP and UTP, forms a tetramer. [5] [9]
The enzyme contains two major domains, responsible for the aminotransferase and synthase activity, respectively. The amidotransferase domains are located away from the tetramer interfaces and are not affected by the oligomeric state. The ATP-binding site and CTP-binding site in the synthase domain are located at the tetramer interface. It is for this reason that ATP and UTP are required for tetramerization. [10]
CTP synthase is precisely regulated by the intracellular concentrations of CTP and UTP, and both hCTPS1 and hCTPS2 have been seen to be maximally active at physiological concentrations of ATP, GTP, and glutamine. [11]
The activity of human CTPS1 isozyme has been demonstrated to be inhibited by phosphorylation. [12] One major example of this is phosphorylation of the Ser-571 residue by glycogen synthase kinase 3 (GSK3) in response to low serum conditions. [13] Additionally, Ser568 has been seen to be phosphorylated by casein kinase 1, inhibiting CTP synthase activity. [11]
CTP is also subject to various forms of allosteric regulation. GTP acts as an allosteric activator that strongly promotes the hydrolysis of glutamine, but is also inhibiting to glutamine-dependent CTP formation at high concentrations. [14] This acts to balance the relative amounts of purine and pyrimidine nucleotides. The reaction product CTP also serves as an allosteric inhibitor. The triphosphate binding site overlaps with that of UTP, but the nucleoside moiety of CTP binds in an alternative pocket opposite the binding site for UTP. [15]
CTP synthase levels have been shown to be dependent on levels of the transcription factor Myc. In turn, CTP synthase activity is required for Myc related phenotypes. [16]
The glutamine analog DON has also been seen to act as an irreversible inhibitor, and has been used as an anti-cancer agent. [17]
CTP synthase has been reported to form filaments in several different organisms. These include bacteria ( C. crescentus ), [18] yeast ( S. cerevisiae ), [19] fruit flies ( D. melanogaster ) [20] and human cells. [21] These filamentous structures have been referred to as cytoplasmic rods and rings, [22] cytoophidia (from the Greek "cyto" meaning cell and "ophidium" meaning serpent, due to the structures morphology) or simply CTP synthase filaments. It has been shown that filamentation downregulates or upregulates CTP synthase activity depending on the species. [23] [24] [25] [26] [27] In Drosophila, only one of the CTP synthase isoform forms the filament. [28] Since the discovery of this novel mode of enzyme regulation in CTP synthase, multiple other enzymes have been shown to exhibit similar characteristics, suggesting that this is an important and well conserved strategy for enzymatic regulation. [29] CTP synthase remains a model enzyme for the study of filament formation.
Upregulated CTP synthase activity has been widely seen in human and rodent tumors. [30] Evidence from fly models [16] and human cancer cells [31] suggests that Myc-dependent cell growth may be more susceptible to suppression of CTP-synthase activity.
Mutations in the CTP synthase have been seen to confer resistance to cytotoxic drugs such as cytosine arabinoside (ara-C) in a Chinese hamster ovary (CHO) cell model of leukemia though such mutations were not found in human patients with ara-C resistance. [32]
Nucleotides are organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules within all life-forms on Earth. Nucleotides are obtained in the diet and are also synthesized from common nutrients by the liver.
Aspartate carbamoyltransferase catalyzes the first step in the pyrimidine biosynthetic pathway.
Uridine-5′-triphosphate (UTP) is a pyrimidine nucleoside triphosphate, consisting of the organic base uracil linked to the 1′ carbon of the ribose sugar, and esterified with tri-phosphoric acid at the 5′ position. Its main role is as substrate for the synthesis of RNA during transcription. UTP is the precursor for the production of CTP via CTP synthetase. UTP can be biosynthesized from UDP by Nucleoside Diphosphate Kinase after using the phosphate group from ATP. UDP + ATP ⇌ UTP + ADP; both UTP and ATP are energetically equal.
In biochemistry, a ribonucleotide is a nucleotide containing ribose as its pentose component. It is considered a molecular precursor of nucleic acids. Nucleotides are the basic building blocks of DNA and RNA. Ribonucleotides themselves are basic monomeric building blocks for RNA. Deoxyribonucleotides, formed by reducing ribonucleotides with the enzyme ribonucleotide reductase (RNR), are essential building blocks for DNA. There are several differences between DNA deoxyribonucleotides and RNA ribonucleotides. Successive nucleotides are linked together via phosphodiester bonds.
A nucleoside triphosphate is a nucleoside containing a nitrogenous base bound to a 5-carbon sugar, with three phosphate groups bound to the sugar. They are the molecular precursors of both DNA and RNA, which are chains of nucleotides made through the processes of DNA replication and transcription. Nucleoside triphosphates also serve as a source of energy for cellular reactions and are involved in signalling pathways.
In molecular biology, biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules. This process often consists of metabolic pathways. Some of these biosynthetic pathways are located within a single cellular organelle, while others involve enzymes that are located within multiple cellular organelles. Examples of these biosynthetic pathways include the production of lipid membrane components and nucleotides. Biosynthesis is usually synonymous with anabolism.
Nucleoside-diphosphate kinases are enzymes that catalyze the exchange of terminal phosphate between different nucleoside diphosphates (NDP) and triphosphates (NTP) in a reversible manner to produce nucleotide triphosphates. Many NDP serve as acceptor while NTP are donors of phosphate group. The general reaction via ping-pong mechanism is as follows: XDP + YTP ←→ XTP + YDP. NDPK activities maintain an equilibrium between the concentrations of different nucleoside triphosphates such as, for example, when guanosine triphosphate (GTP) produced in the citric acid (Krebs) cycle is converted to adenosine triphosphate (ATP). Other activities include cell proliferation, differentiation and development, signal transduction, G protein-coupled receptor, endocytosis, and gene expression.
Glutamine synthetase (GS) is an enzyme that plays an essential role in the metabolism of nitrogen by catalyzing the condensation of glutamate and ammonia to form glutamine:
Argininosuccinate synthase or synthetase is an enzyme that catalyzes the synthesis of argininosuccinate from citrulline and aspartate. In humans, argininosuccinate synthase is encoded by the ASS gene located on chromosome 9.
Cytidine triphosphate (CTP) is a pyrimidine nucleoside triphosphate. CTP, much like ATP, consists of a ribose sugar, and three phosphate groups. The major difference between the two molecules is the base used, which in CTP is cytosine.
Nucleic acid metabolism is a collective term that refers to the variety of chemical reactions by which nucleic acids are either synthesized or degraded. Nucleic acids are polymers made up of a variety of monomers called nucleotides. Nucleotide synthesis is an anabolic mechanism generally involving the chemical reaction of phosphate, pentose sugar, and a nitrogenous base. Degradation of nucleic acids is a catabolic reaction and the resulting parts of the nucleotides or nucleobases can be salvaged to recreate new nucleotides. Both synthesis and degradation reactions require multiple enzymes to facilitate the event. Defects or deficiencies in these enzymes can lead to a variety of diseases.
CAD protein is a trifunctional multi-domain enzyme involved in the first three steps of pyrimidine biosynthesis. De-novo synthesis starts with cytosolic carbamoylphosphate synthetase II which uses glutamine, carbon dioxide and ATP. This enzyme is inhibited by uridine triphosphate.
Carbamoyl phosphate synthetase catalyzes the ATP-dependent synthesis of carbamoyl phosphate from glutamine or ammonia and bicarbonate. This enzyme catalyzes the reaction of ATP and bicarbonate to produce carboxy phosphate and ADP. Carboxy phosphate reacts with ammonia to give carbamic acid. In turn, carbamic acid reacts with a second ATP to give carbamoyl phosphate plus ADP.
Carbamoyl phosphate synthetase (glutamine-hydrolysing) is an enzyme that catalyzes the reactions that produce carbamoyl phosphate in the cytosol. Its systemic name is hydrogen-carbonate:L-glutamine amido-ligase .
Orotate phosphoribosyltransferase (OPRTase) or orotic acid phosphoribosyltransferase is an enzyme involved in pyrimidine biosynthesis. It catalyzes the formation of orotidine 5'-monophosphate (OMP) from orotate and phosphoribosyl pyrophosphate. In yeast and bacteria, orotate phosphoribosyltransferase is an independent enzyme with a unique gene coding for the protein, whereas in mammals and other multicellular organisms, the catalytic function is carried out by a domain of the bifunctional enzyme UMP synthase (UMPS).
Ribose-phosphate diphosphokinase is an enzyme that converts ribose 5-phosphate into phosphoribosyl pyrophosphate (PRPP). It is classified under EC 2.7.6.1.
Phosphatidate cytidylyltransferase (CDS) is the enzyme that catalyzes the synthesis of CDP-diacylglycerol from cytidine triphosphate and phosphatidate.
Uridine-cytidine kinase 2 (UCK2) is an enzyme that in humans is encoded by the UCK2 gene.
CTP synthase 2 is an enzyme that in humans is encoded by the CTPS2 gene.
CTP synthase 1 is an enzyme that is encoded by the CTPS1 gene in humans. CTP synthase 1 is an enzyme in the de novo pyrimidine synthesis pathway that catalyses the conversion of uridine triphosphate (UTP) to cytidine triphosphate (CTP). CTP is a key building block for the production of DNA, RNA and some phospholipids.
