CTP

STRUCTURE: Cytosine (pyrimidine base) + ribose + three phosphates. Its deoxy form is dCTP (DNA synthesis). Cytidine = cytosine + ribose (nucleoside); CMP/CDP/CTP = mono-/di-/triphosphate nucleotides.

SYNTHESIS (de novo pyrimidine pathway): Glutamine + CO2 + 2 ATP -> carbamoyl phosphate (CPS II, cytosolic; rate-limiting, regulated step) -> + aspartate (aspartate transcarbamoylase, ATCase) -> carbamoyl aspartate -> (dihydroorotase) -> dihydroorotate -> (dihydroorotate dehydrogenase, mitochondrial inner membrane) -> orotate -> + PRPP (orotate phosphoribosyltransferase) -> OMP -> (OMP decarboxylase) -> UMP. Together, CAD is the multifunctional enzyme for CPS II, ATCase, and dihydroorotase; UMP synthase has orotate phosphoribosyltransferase and OMP decarboxylase activity. UMP -> UDP -> UTP (kinases). CTP synthetase then aminates UTP to CTP using glutamine as the nitrogen donor and ATP as the energy source.

REGULATION: CPS II is activated by PRPP and ATP and inhibited by UTP (end-product feedback). CTP synthetase is inhibited by CTP (feedback). In bacteria, ATCase is the regulated step (CTP inhibits, ATP activates).

SALVAGE: Cytidine -> CMP via uridine-cytidine kinase. Pyrimidine salvage conserves nucleotides.

CONVERSION TO dCTP: CDP -> dCDP by ribonucleotide reductase (inhibited by hydroxyurea; regulated by dATP, ATP), then phosphorylated to dCTP for DNA synthesis.

FUNCTIONS: 1. RNA synthesis (substrate for RNA polymerase). 2. Phospholipid synthesis: CTP activates intermediates. CTP + phosphocholine -> CDP-choline (rate-limiting step is CTP:phosphocholine cytidylyltransferase) -> phosphatidylcholine (lecithin). CTP + phosphoethanolamine -> CDP-ethanolamine -> phosphatidylethanolamine. CTP + phosphatidic acid -> CDP-diacylglycerol -> phosphatidylinositol, phosphatidylserine (in bacteria/ mitochondria), cardiolipin. 3. Glycoprotein/glycolipid synthesis: CMP-sialic acid (CMP-NANA) is the donor for sialylation.

CLINICAL CORRELATES: - Hereditary orotic aciduria: Autosomal recessive defect in UMP synthase (orotate phosphoribosyltransferase and OMP decarboxylase) -> decreased UMP, UTP, CTP. Presents with megaloblastic anemia unresponsive to B12/folate, failure to thrive, developmental delay, orotic acid in urine, no hyperammonemia. Treatment: uridine monophosphate (or uridine triacetate), which bypasses the block and provides negative feedback on CPS II to reduce orotic acid. - OTC deficiency: Carbamoyl phosphate accumulates in mitochondria, spills into the cytosol, and enters the pyrimidine pathway -> increased orotic acid WITH hyperammonemia and low BUN. Distinguish from orotic aciduria (no hyperammonemia). - Antimetabolite drugs: Cytarabine (ara-C) is a cytidine analog, which is converted to ara-CTP that inhibits DNA polymerase (S-phase specific; used in AML). Gemcitabine is a cytidine analog (dFdCTP) which inhibits DNA synthesis and ribonucleotide reductase (pancreatic cancer). Azacitidine/decitabine are cytidine analogs that inhibit DNA methyltransferase (MDS). Lamivudine (3TC), emtricitabine, and zalcitabine are cytosine analog NRTIs (HIV, HBV). - Leflunomide/teriflunomide inhibit dihydroorotate dehydrogenase (RA, MS), and so deplete pyrimidines including CTP in proliferating lymphocytes. - Hydroxyurea inhibits ribonucleotide reductase -> decreased dCTP/dNTPs. - Methotrexate, 5-FU: Inhibit thymidylate synthesis (dTMP) rather than CTP directly. - Mycophenolate inhibits IMP dehydrogenase (purines, not pyrimidines). - CTP synthetase activity is increased in some malignancies and lymphocytes; it is a drug target (e.g., cyclopentenyl cytosine).

HIGH-YIELD PEARLS: Pyrimidines are synthesized as the ring first, then attached to PRPP (purines are built on PRPP). CTP is a pyrimidine with C4 amino group (cytosine deamination -> uracil; repaired by uracil DNA glycosylase). Cytosine/thymine are pyrimidines (CUT the PY). Pyrimidine catabolism end products (beta-alanine, beta-aminoisobutyrate) are water soluble and not associated with gout (unlike purine -> uric acid).