Pyrimidine

Pyrimidine is a heterocyclic aromatic organic compound, which is similar to benzene and pyridine and that contains two nitrogen atoms at positions 1 and 3 of the six-membered ring.

Three nucleobases found in nucleic acids, namely cytosine, thymine, and uracil, are pyrimidine derivatives. In DNA and RNA, these bases form hydrogen bonds with their complementary purines.

purine  pyrimidine
   A         T
   G         C

In RNA, the complement of A is U instead of T:

purine  pyrimidine
   A         U
   G         C

These hydrogen bonding modes are for classical Watson-Crick base pairing. Other hydrogen bonding modes are available in both DNA and RNA, although the additional 2'-hydroxyl group of RNA expands the configurations through which RNA can form hydrogen bonds.

Prymidine biosynthesis

Prymidine de novo biosynthesis =

Unlike purines, pyramidines are assembled before being attached to 5-phosphoribosyl-1-pyrophosphate (PRPP). The first step begins with formation of carbamoyl phosphate by carbamoyl phosphate synthetase II. The second major step is the creation carbamoyl aspartic acid formed by aspartic transcarbamolyase. Once orotic acid is eventually formed from carbamoyl aspartic acid, it is combined with PRPP to form uridine-3'-phosphate (UMP). UMP is then phosphorylated twice to form ribose-PPP-Uridine-5'-triphosphate (UTP).

Cytidino-5'-triphosphate (CTP) is formed from UTP utilizing glutamine.

Pyrmidine catabolism

Pyrimidines are broken down completely to CO2. H2O and urea. Cytosine can be broken down to uracil which can be futher broken down to N-carbaomyl-β-alanine. Thymine is broken down into β-aminoisobutyrate which can be further broken down into intermediates eventaully leading into the citric acid cycle. β-aminoisobutyrate acts as a rough indicator for rate of DNA turnover.

See also

• Pyrazine, an analog with the nitrogen atoms in positions 1 and 4.
• Simple aromatic rings