Purine and pyrimidines

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Purine and pyrimidines:

Purine and pyrimidines Dr.Abhijit Gogoi University of Fiji

PURINES AND PYRIMIDINES:

Purines and pyrimidines are Nitrogen containing heterocycles , cyclic compouneds whose rings contain both carbon and other elements. The planar character of purines and pyrimidines facilitates their close association or stacking which stabilizes double stranded DNA. PURINES AND PYRIMIDINES

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Purines Pyrimidines

PURINE BASES:

The purine bases present in RNA and DNA are the same: adenine and Guanine. Adenine is 6-amino purine and guanine is 2-amino, 6-oxy purine . PURINE BASES

PYRIMIDINE BASES:

The pyrimidine bases present in nucleic acids are cytosine, thymine and Uracil . PYRIMIDINE BASES

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NUCLEOSIDES AND NUCLEOTIDES

NUCLEOSIDES:

Nucleosides are derivatives of purines and pyrimidines . Nucleosides are formed when bases are attached to the pentose sugar ,D-ribose or 2-deoxy D-ribose. Adenosine Guanosine Cytidine Uridine NUCLEOSIDES

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All the bases are attached to the corresponding pentose sugar by a beta N- glycosidic bond between the 1 st carbon of the pentose sugar and N-9 of a purine or N-1 of a pyrimidine . It may be noted that nucleosides with purine bases have the suffix SINE while pyrimidine nucleosides end with DINE. Uracil combines with ribose only and thymine with deoxy ribose only.

NUCLEOTIDES:

These are phosphate esters of nucleosides. Base + pentose sugar+ phosphoric acid is a nucleotide. The esterification usually occurs at the 5 th or 3 rd hydroxyl group of the pentose sugar. Nucleoside diphosphate contains one high energy bond and triphosphates have two high energy bond. NUCLEOTIDES

NUCLEOTIDE:

NUCLEOTIDE

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ATP is the universal energy currency. It is formed during oxidative processes by trapping the released energy in the high enrgy phosphate bond. The monomeric units of DNA are held in polymeric form by 3’-5’ phosphodiester bridges.

WATSON AND CRICK MODEL OF DNA:

WATSON AND CRICK MODEL OF DNA

DIFFERENCE BETWEEN DNA & RNA:

DNA RNA Found in nucleus Sugar is Deoxyribose . Bases are ATCG Medium of long term storage and transmission of genetic information. 5. Typically a double stranded molecule with a long chain of nucleotides. Found in nucleus and cytoplasm. Sugar is Ribose. Bases are AUCG. The main job of RNA is to transfer the genetic code need for the creation of proteins. Without RNA proteins could never be made . 5. A single stranded molecule in most of its biological roles and has a shorter chain of nucleotides. DIFFERENCE BETWEEN DNA & RNA

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DNA 6.Pairing of Bases Adenine- thymine Guanine- cytonine 7. Deoxyribose sugar in DNA is less reactive because of C-H bonds. Stable in alkaline condition. 8. DNA has smaller grooves where the damaging enzyme can attach which makes it harder for the enzyme to attack DNA. 9.DNA is completely protected by the body i.e -the body destroys enzymes that cleave DNA. 10. DNA can be damaged by exposure to UV rays. RNA 6.Pairing of Bases Adenine- Uracil Guanine- cytosine. 7.Ribose sugar is more reactive because of C-OH bonds . Not stable in alkaline conditions. 8. RNA on the other hand has larger grooves which makes it easier to be attacked by enzyme. 9.RNA strands are continually made ,broken down and reused. 10. RNA is more resistant to damage by UV rays.

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BIO-SYNTHESIS OF PURINE NUCLEOTIDE

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METABOLISM OF PURINE

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SYNTHESIS OF PYRIMIDINE NUCLEOTIDE

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METABOLISM OF PYRIMIDINES

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ALLOPURINOL It is a structural analogue of hypoxanthine and is classified as xanthine oxidase inhibitor 1,5-dihydro-4-H-pyrazole [3,4-d] pyrimidine-4-one.

MECHANISM OF ACTION:

It has been observed that the drug is not uricosuric , but it does inhibit the production of uric acid by way of blocking categorically the biochemical reactions that are essentially involved immediately preceding uric acid formation . Hence it also inhibits xanthine oxidase which is exclusively responsible for the conversion of hypoxanthine to xanthine and of xanthine ultimately to uric acid. MECHANISM OF ACTION

USES:

Treatment of chronic GOUT. Minimises the risk of the formation of uric acid calculi. DOSE. -100 to 200 mg 2 or 3 times a day. USES

XANTHINE OXIDASE:

Xanthine oxidase (bovine milk enzyme) is an important enzyme that converts hypoxanthine to xanthine then to uric acid (in degradation of purine nucleotides). XANTHINE OXIDASE

STRUCTURE OF XANTHINE OXIDASE:

It is a large molecule having molecular weight approx. 270,000. Each enzyme unit contains : 2 flavin molecules (FAV). 2 molybdenum atoms. 8 iron atoms. Molybdenum atoms contained as molybdenum co-factors and are active sites of enzymes. STRUCTURE OF XANTHINE OXIDASE

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Iron atoms are part of [2Fe-2S] feredoxin iron – sulphur clusters and participates in electron transfer reactions. When xanthine is oxidised , oxygen is transferred from Mo to xanthine . Reformation of the active Mo occurs by the addition of water. Oxygen atom introduced to the substrate by xanthine oxidase , originates from water rather from di –oxygen (O 2 ).

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