CTP synthase
Encyclopedia
CTP synthase is an enzyme
involved in pyrimidine biosynthesis that interconverts UTP and CTP
.
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 tertamerization.
Two isozymes with CTP synthase activity exist in humans, encoded by the following genes:
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.
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. 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 synthase domain. Here, ammonia reacts with the intermediate 4-phosphoryl UTP.
The activity of human CTPS1 isozyme has been demonstrated to be inhibited by phosphorylation. One major example of this is phosphorylation of the Ser-571 residue by glycogen synthase kinase 3 (GSK3) in response to low serum conditions. Additionally, Ser568 has been seen to be phosphorylated by casein kinase 1, inhibiting CTP synthase activity.
CTP is also subject to various froms 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. 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.
The glutamine
analog DON
has also been seen to act as an irreversible inhibitor, and has been used as an anti-cancer agent.
, D. virilis and D. peudoobscura). CTP synthase-containing filaments (i.e., cytoophidium) have also been found in bacteria including C. crescentus
and E. coli
, indicating that the cytoophidium is well conserved from prokaryotes to eukaryotes.
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.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
involved in pyrimidine biosynthesis that interconverts UTP and CTP
Cytidine triphosphate
Cytidine triphosphate is a pyrimidine nucleoside triphosphate.CTP is a substrate in the synthesis of RNA.CTP is a high-energy molecule equal to ATP, but its role in the organism is more specific than that of ATP....
.
Enzyme Structure
Active CTP synthase exists as a homeotetrameric 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.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 tertamerization.
Two isozymes with CTP synthase activity exist in humans, encoded by the following genes:
- CTPSCTP synthase 1CTP synthase 1 is an enzyme that in human s is encoded by the CTPS gene.- Function :The catalytic conversion of uridine triphosphate to cytidine triphosphate is accomplished by the enzyme cytidine-5-prime-triphosphate synthetase...
– CTP synthase 1 - CTPS2CTPS2CTP synthase 2 is an enzyme that in humans is encoded by the CTPS2 gene.-Further reading:...
– CTP synthase 2
Enzyme Mechanism
CTP synthase catalyzes the last committed step in pyrimidine nucleotide biosynthesis: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.
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. The source of the amino group in CTP is glutamine
Glutamine
Glutamine is one of the 20 amino acids encoded by the standard genetic code. It is not recognized as an essential amino acid but may become conditionally essential in certain situations, including intensive athletic training or certain gastrointestinal disorders...
, which is hydrolysed in a glutamine amidotransferase domain to produce ammonia. This is then channeled through the interior of the enzyme to the synthase domain. Here, ammonia reacts with the intermediate 4-phosphoryl UTP.
Regulation
CTP synthase is precisely regulated by the intracellular concentrations of CTP and UPT, and both hCTPS1 and hCTPS2 have been seen to be maximally active at physiological concentrations of ATP, GTP, and glutamine.The activity of human CTPS1 isozyme has been demonstrated to be inhibited by phosphorylation. One major example of this is phosphorylation of the Ser-571 residue by glycogen synthase kinase 3 (GSK3) in response to low serum conditions. Additionally, Ser568 has been seen to be phosphorylated by casein kinase 1, inhibiting CTP synthase activity.
CTP is also subject to various froms of allosteric regulation
Allosteric regulation
In biochemistry, allosteric regulation is the regulation of an enzyme or other protein by binding an effector molecule at the protein's allosteric site . Effectors that enhance the protein's activity are referred to as allosteric activators, whereas those that decrease the protein's activity are...
. 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. This acts to balance the relative amounts of purine
Purine
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....
and pyrimidine
Pyrimidine
Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring...
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.
The glutamine
Glutamine
Glutamine is one of the 20 amino acids encoded by the standard genetic code. It is not recognized as an essential amino acid but may become conditionally essential in certain situations, including intensive athletic training or certain gastrointestinal disorders...
analog DON
6-Diazo-5-oxo-L-norleucine
6-Diazo-5-oxo-L-norleucine is a glutamine antagonist, which was isolated originally from Streptomyces. It is a non-standard amino acid. The diazo compound was characterized in 1956 by Henry W Dion et al., who suggested a possible use in cancer therapy. This antitumoral efficacy was confirmed in...
has also been seen to act as an irreversible inhibitor, and has been used as an anti-cancer agent.
Cytoophidium
The cytoophidium (Greek: cyto-, meaning cell, and ophidium, meaning serpent) is a snake-like filamentary structure containing CTP synthase, first reported in the fruit flies (D. melanogasterDrosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
, D. virilis and D. peudoobscura). CTP synthase-containing filaments (i.e., cytoophidium) have also been found in bacteria including C. crescentus
Caulobacter crescentus
Caulobacter crescentus is a Gram-negative, oligotrophic bacterium widely distributed in fresh water lakes and streams.Caulobacter is an important model organism for studying the regulation of the cell cycle, asymmetric cell division, and cellular differentiation. Caulobacter daughter cells have...
and E. coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
, indicating that the cytoophidium is well conserved from prokaryotes to eukaryotes.
Disease Relevance
Upregulated CTP synthase activity has been widely seen in human and rodent tumors.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.