نيوكليوتايد

(تم التحويل من Nucleotide)

هي الوحدة اساسية لبناء الدنا و الرنا. في الأحماض النووية الوراثية (دنا و رنا) تتكون وحدة النيوكليوتيد من:

  • سكر خماسي (Ribose Sugar) وفي حمض الدنا يكون هذا السكر منقوص الأكسجين.
  • مجموعة فوسفات.
  • قاعدة نيتروجينية، في حمض الدنا القواعد النيتروجينية هي أدينين (Adenine) و كوانين (Guanine) و ثايمين (Thymine) و سايتوسين (Cytosine). أم في حمض الرنا فيكون اليوراسيل (Uracil) بدل من الثايمين.

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بنية النيوكليوتيد

The structure elements of the most common nucleotides

A nucleotide is composed of a ring of nitrogen, carbon and oxygen atoms, a five carbon sugar (together referred to as a nucleoside) and one or more phosphate groups. The sugar involved in the synthesis and structure of a nucleotide may be either ribose or deoxyribose; in the latter case, the prefix 'deoxy' may be added before the name of the nucleoside in all cases except Uracil. The nucleoside may bond to one, two or three functional group(s) of phosphate(s), forming respectively a monophosphate, diphosphate or triphosphate nucleotide. There are a wide variety of nucleotide and deoxynucleotide structures, as illustrated in the diagrams below.


Nucleotide structural diagrams

Chemical structure of adenosine monophosphate
Adenosine monophosphate
AMP
Chemical structure of adenosine diphosphate
Adenosine diphosphate
ADP
Chemical structure of adenosine triphosphate
Adenosine triphosphate
ATP
Chemical structure of guanosine monophosphate
Guanosine monophosphate
GMP
Chemical structure of guanosine diphosphate
Guanosine diphosphate
GDP
Chemical structure of guanosine triphosphate
Guanosine triphosphate
GTP
Chemical structure of uridine monophosphate
Uridine monophosphate
UMP
Chemical structure of uridine diphosphate
Uridine diphosphate
UDP
Chemical structure of uridine triphosphate
Uridine triphosphate
UTP
Chemical structure of cytidine monophosphate
Cytidine monophosphate
CMP
Chemical structure of cytidine diphosphate
Cytidine diphosphate
CDP
Chemical structure of cytidine triphosphate
Cytidine triphosphate
CTP

Deoxynucleotide structural diagrams

Chemical structure of deoxyadenosine monophosphate
Deoxyadenosine monophosphate
dAMP
Chemical structure of deoxyadenosine diphosphate
Deoxyadenosine diphosphate
dADP
Chemical structure of deoxyadenosine triphosphate
Deoxyadenosine triphosphate
dATP
Chemical structure of deoxyguanosine monophosphate
Deoxyguanosine monophosphate
dGMP
Chemical structure of deoxyguanosine diphosphate
Deoxyguanosine diphosphate
dGDP
Chemical structure of deoxyguanosine triphosphate
Deoxyguanosine triphosphate
dGTP
Chemical structure of thymidine monophosphate
Thymidine monophosphate
TMP
Chemical structure of thymidine diphosphate
Thymidine diphosphate
TDP
Chemical structure of thymidine triphosphate
Thymidine triphosphate
TTP
Chemical structure of deoxycytidine monophosphate
Deoxycytidine monophosphate
dCMP
Chemical structure of deoxycytidine diphosphate
Deoxycytidine diphosphate
dCDP
Chemical structure of deoxycytidine triphosphate
Deoxycytidine triphosphate
dCTP


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التخليق

Nucleotides can be synthesized through a variety of methods both in vitro and in vivo. This can involve salvage synthesis (the re-use of parts of nucleotides in resynthesizing new nucleotides through breakdown and synthesis reactions in order to exchange useful parts), or the use of protecting groups in a laboratory. In the latter case, a purified nucleoside or nucleobase is protected to create a phosphoramidite, and can be used to obtain analogues not present in nature and/or to create an oligonucleotide.

أنواع القواعد

Nucleotides can be synthesized with both purine and pyrimidine as bases. In DNA, the purine bases are adenine and guanine, while the pyrimidines are thymine and cytosine. RNA uses uracil rather than thymine (thymine is produced by adding a methyl to uracil). The nucleotide passes through numerous biochemical steps while being processed, adding and removing atoms through the use of numerous enzymes.

Pyrimidine ribonucleotides

The synthesis of UMP.
The color scheme is as follows: enzymes, coenzymes, substrate names, inorganic molecules

The synthesis of a single pyrimidine is complex; the diagram to the left demonstrates the synthesis of a single pyrimidine.


Purine ribonucleotides

The atoms which are used to build the purine nucleotides come from a variety of sources:

Nucleotides syn3.png The biosynthetic origins of purine ring atoms

N1 arises from the amine group of Asp
C2 and C8 originate from formate
N3 and N9 are contributed by the amide group of Gln
C4, C5 and N7 are derived from Gly
C6 comes from HCO3- (CO2)
The synthesis of IMP.
The color scheme is as follows: enzymes, coenzymes, substrate names, metal ions, inorganic molecules

The de novo synthesis of purine nucleotides by which these precursors are incorporated into the purine ring, proceeds by a 10 step pathway to the branch point intermediate IMP, the nucleotide of the base hypoxanthine. AMP and GMP are subsequently synthesized from this intermediate via separate, two step each, pathways. Thus purine moieties are initially formed as part of the ribonucleotides rather than as free bases.

Six enzymes take part in IMP synthesis. Three of them are multifunctional:

  • GART (reactions 2, 3, and 5)
  • PAICS (reactions 6, and 7)
  • ATIC (reactions 9, and 10)

Reaction 1. The pathway starts with the formation of PRPP. PRPS1 is the enzyme that activates R5P, which is primarily formed by the pentose phosphate pathway, to PRPP by reacting it with ATP. The reaction is unusual in that a pyrophosphoryl group is directly transferred from ATP to C1 of R5P and that the product has the α configuration about C1. This reaction is also shared with the pathways for the synthesis of the pyrimidine nucleotides, Trp, and His. As a result of being on (a) such (a) major metabolic crossroad and the use of energy, this reaction is highly regulated.

Reaction 2. In the first reaction unique to purine nucleotide biosynthesis, PPAT catalyzes the displacement of PRPP's pyrophosphate group (PPi) by Gln's amide nitrogen. The reaction occurs with the inversion of configuration about ribose C1, thereby forming β-5-phosphorybosylamine (5-PRA) and establishing the anomeric form of the future nucleotide. This reaction which is driven to completion by the subsequent hydrolysis of the released PPi, is the pathway's flux generating step and is therefore regulated too.


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