PYRIMIDINE SYNTHESIS

Biosynthesis of pyrimidine nucleotides can occur by a de novo pathway or by the reutilization of preformed pyrimidine bases or ribonucleosides (salvage pathway).
The pyrimidine synthesis is a similar process than that of purines. In the de novo synthesis of pyrimidines, the ring is synthesized first and then it is attached to a ribose-phosphate to for a pyrimidine nucleotide.

1. NUCLEOTIDE METABOLISM PREPARED BY; MISS RABIA KHAN BABER COURSE TITLE : BIOCHEMISTRY IV

2. LEARNING OBJECTIVES OF CLASS

3. PYRIMIDINE SYNTHESIS

4. Thymine, cytosine and uracil are pyrimidine bases. The pyrimidine ring is synthesized as free pyrimidine & it is incorporated into the nucleotide. Aspartate , glutamine (amide group) & CO2 contribute to atoms in the formation of pyrimidine ring. PYRIMIDINE BASES

5. DE NOVO PYRIMIDINE SYNTHESIS Biosynthesis of  pyrimidine   nucleotides can occur by a de novo pathway or by the reutilization of preformed pyrimidine bases or ribonucleosides (salvage pathway). The  pyrimidine synthesis is a similar process than that of purines . In the de novo synthesis of pyrimidines , the ring is synthesized first and then it is attached to a ribose-phosphate to for a  pyrimidine  nucleotide.

6. LOCATION De novo pyrimidine synthesis occurs in the cytosol of cells in all tissues. SUBSTRATES CO 2 ; glutamine; ATP; Aspartate ; H 2 O; NAD + ; Phosphoribosyl pyrophosphate (PRPP). PRODUCTS UTP; CTP; glutamate; NADH; CO 2

7. STEPS OF SYNTHESIS   Pyrimidines are synthesized from carbamoyl phosphate and aspartate . Ribose-5-phosphate is then attached to yield pyrimidine ribonucleotides . Cytosine is found in both DNA and RNA. Uracil is found only in RNA

8. Synthesis of carbomyl phosphate The reaction occurs in cytoplasm Glutamine transfers its amido nitrogen to CO2 to produce carbamoyl phosphate. This reaction is catalyzed by ATP dependent enzyme carbamoyl phosphate synthetase II (CPS II). CPS-I is involved in urea cycle. STEP#1

9. Rate limiting step Carbamoyl phosphate condenses with aspartate to form carbamoyl aspartate . This reaction is catalysed by aspartate transcarbamoylase . The atoms C2 & N3 are derived from carbamoyl phosphate & the rest are from aspartate . STEP#2

10. Formation of pyrimidine ring Dihydroorotase catalyses the pyrimidine ring closure with a loss of H2O. Oxidation NAD+ dependent dehydrogenation, leading to the formation of orotate . The enzyme is dihydro orotate dehydrogenase (DHODH). STEP#3 & STEP#4

11. Formation of OMP Ribose 5-phosphate is now added to orotate to produce orotidine monophosphate (OMP). This reaction is catalysed by orotate phosphoribosyltransferase . PRPP is the donor of ribose 5-phosphate. STEP#5

12. Decarboxylation OMP undergoes decarboxylation to uridine mono-phosphate (UMP). The enzyme is OMP decarboxylase (OMPDC). This is the first pyrimidine that is synthesized. 6-aza-uridine inhibits this step & used as an anticancer drug. STEP#6

13. Orotate phosphoribosyl transferase & OMP decarboxylase are domains of a single protein. A defect in this bifunctional enzyme causes orotic aciduria . CLINICAL CORELATION

14. STEP#7 Synthesis of triphosphates By an ATP-dependent UMP kinase reaction, UMP is converted to UDP which serves as a precursor for the synthesis of dUMP , dTMP , UTP & CTP. UDP is phosphorylated to UTP ( uridine triphosphate ) with the help of ATP. The enzyme is nucleoside diphosphate kinase .

15. STEP#8 Formation of CTP Cytidine triphosphate (CTP) is synthesized from UTP by amination . CTP synthetase is the enzyme & glutamine provides the nitrogen. CTP synthetase needs ATP.

16. REGULATION OF PYRIMIDINE SYNTHESIS In bacteria, aspartate transcarbamoylase ( ATCase ) catalyses a committed step in pyrimidine biosynthesis. ATCase is a good example of an enzyme controlled by feedback mechanism by the end product CTP. Carbamoyl phosphate synthetase II (CPS II) is the regulatory enzyme of pyrimidine synthesis in animals. It is activated by PRPP and ATP & inhibited by UDP & UTP. OMP decarboxylase , inhibited by UMP & CMP, also controls pyrimidine formation.

17. SIGNIFICANCE OF PYRIMIDINE SYNTHESIS Pyrimidine nucleotides, in common with purine nucleotides, are required for the synthesis of DNA and RNA. They also participate in intermediary metabolism. For example,  pyrimidine nucleotides are involved in the biosynthesis of glycogen and of phospholipids. Pyrimidines  have diverse biological activities such as antimicrobial, CNS depressant, anti-inflammatory, analgesic, anti- convulsant , anticancer, antihelmentic , antioxidant and herbicidal.

18. REFERENCES Textbook of Biochemistry-U Satyanarayana Textbook of Biochemistry-DM Vasudevan Text book of medical biochemistry, MN Chatterjee Fundamentals of biochemistry, J.L Jain, Sunjay Jain, Nitin Jain