Pyrimidine
pyrimidine (Greek: πῦρ [pyr] 'fire' + amidine) It is an organic compound, similar to benzene and pyridine but with two nitrogen atoms that replace carbon in positions 1 and 3. It degrades into highly soluble substances such as beta alanine and beta aminoisobutyrate, precursors of acetyl-CoA and succinyl- CoA.
Derivatives
Pyrimidine has three very important derivatives for life, since they are part of the nitrogenous bases: thymine, cytosine and uracil; all three have a carbonyl group (C=O) on carbon 2; the first two form part of DNA where they pair with their complementary purines, while the latter is present only in RNA.
| Timina | Citosina | Uracilo |
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In pyrimidines, a carbamoylphosphate molecule in which glutamine is the amino group donor is attached to an aspartic acid molecule. The orotic acid, product of these reactions, is transferred to a molecule of phospho-ribosyl pyrophosphate to give rise to uridylic acid.
Nucleosides and nucleotides
Pyrimidines are mostly associated with five-carbon monosaccharides (pentoses) joined in N1 to form nucleosides which, in turn, join a phosphate group (phosphoric acid) to form form the nucleotides.
In DNA and RNA, these bases form hydrogen bonds with their complementary purines. Thus, in DNA, the purines adenine (A) and guanine (G) pair with the pyrimidines thymine (T) and cytosine (C), respectively.
In RNA, the complement of adenine (A) is uracil (U) instead of thymine (T), so the pairs that are formed are adenine: uracil and guanine: cytosine.
Very rarely, thymine can occur in RNA, or uracil in DNA, but when the other three major pyrimidine bases are represented, some minor pyrimidine bases can also occur in nucleic acids. These minor pyrimidines are usually methylated versions of the major ones and are postulated to have regulatory functions.
These modes of hydrogen bonding are for the classical Watson-Crick base pairing. Other modes of hydrogen bonding ('wobble pairing') 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
Nitrogen functions
Nitrogenous compounds, such as amines and nitriles, are characterized because, in addition to carbon and hydrogen, they contain nitrogen.