Editing Nucleotide (section)

===Purine ribonucleotide synthesis===

{{main|Purine metabolism}}

The atoms that are used to build the [[purine nucleotides]] come from a variety of sources:
[[File:Nucleotide synthesis.svg|thumb|250px|class=skin-invert-image|'''The [[biosynthetic]] origins of purine ring [[atoms]]'''<br /><br />N<sub>1</sub> arises from the amine group of [[Aspartic acid|Asp]]<br />C<sub>2</sub> and C<sub>8</sub> originate from [[formate]]<br />N<sub>3</sub> and N<sub>9</sub> are contributed by the amide group of [[Glutamine|Gln]]<br />C<sub>4</sub>, C<sub>5</sub> and N<sub>7</sub> are derived from [[Glycine|Gly]] <br />C<sub>6</sub> comes from HCO<sub>3</sub><sup>−</sup> (CO<sub>2</sub>)]]
[[File:Nucleotides syn1.svg|class=skin-invert-image|thumb|600px|Diagram of the synthesis of IMP.
{{legend|blue|enzymes}}
{{legend|rgb(219,155,36)|coenzymes}}
{{legend|rgb(151,149,45)|substrate names}}
{{legend|rgb(227,13,196)|metal ions}}
{{legend|rgb(128,0,0)|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 [[Inosine monophosphate|IMP]], the nucleotide of the base [[hypoxanthine]]. [[Adenosine monophosphate|AMP]] and [[Guanosine monophosphate|GMP]] are subsequently synthesized from this intermediate via separate, two-step pathways. Thus, purine [[Moiety (chemistry)|moieties]] are initially formed as part of the [[ribonucleotides]] rather than as [[Freebase (chemistry)|free bases]].

Six enzymes take part in IMP synthesis. Three of them are multifunctional:
* [[Phosphoribosylglycinamide formyltransferase|GART]] (reactions 2, 3, and 5)
* [[Phosphoribosylaminoimidazole carboxylase|PAICS]] (reactions 6, and 7)
* [[Inosine monophosphate synthase|ATIC]] (reactions 9, and 10)

The pathway starts with the formation of [[PRPP]]. [[PRPS1]] is the [[enzyme]] that activates [[R5P]], which is formed primarily by the [[pentose phosphate pathway]], to PRPP by reacting it with [[Adenosine triphosphate|ATP]]. The reaction is unusual in that a pyrophosphoryl group is directly transferred from ATP to C<sub>1</sub> of R5P and that the product has the '''α''' configuration about C1. This reaction is also shared with the pathways for the synthesis of [[Tryptophan|Trp]], [[Histidine|His]], and the [[pyrimidine nucleotides]]. Being on a major metabolic crossroad and requiring much energy, this reaction is highly regulated.

In the first reaction unique to purine nucleotide biosynthesis, [[PPAT]] catalyzes the displacement of PRPP's [[pyrophosphate]] group (PP<sub>i</sub>) by an amide nitrogen donated from either [[glutamine]] (N), [[glycine]] (N&C), [[aspartate]] (N), [[folic acid]] (C<sub>1</sub>), or CO<sub>2</sub>. This is the committed step in purine synthesis. The reaction occurs with the inversion of configuration about ribose C<sub>1</sub>, thereby forming '''β'''-[[5-phosphorybosylamine]] (5-PRA) and establishing the anomeric form of the future nucleotide.

Next, a glycine is incorporated fueled by ATP hydrolysis, and the carboxyl group forms an amine bond to the NH<sub>2</sub> previously introduced. A one-carbon unit from folic acid coenzyme N<sub>10</sub>-formyl-THF is then added to the amino group of the substituted glycine followed by the closure of the imidazole ring. Next, a second NH<sub>2</sub> group is transferred from glutamine to the first carbon of the glycine unit. A carboxylation of the second carbon of the glycin unit is concomitantly added. This new carbon is modified by the addition of a third NH<sub>2</sub> unit, this time transferred from an aspartate residue. Finally, a second one-carbon unit from formyl-THF is added to the nitrogen group and the ring is covalently closed to form the common purine precursor inosine monophosphate (IMP).

Inosine monophosphate is converted to adenosine monophosphate in two steps. First, GTP hydrolysis fuels the addition of aspartate to IMP by adenylosuccinate synthase, substituting the carbonyl oxygen for a nitrogen and forming the intermediate adenylosuccinate. Fumarate is then cleaved off forming adenosine monophosphate. This step is catalyzed by adenylosuccinate lyase.

Inosine monophosphate is converted to guanosine monophosphate by the oxidation of IMP forming xanthylate, followed by the insertion of an amino group at C<sub>2</sub>.  NAD<sup>+</sup> is the electron acceptor in the oxidation reaction. The amide group transfer from glutamine is fueled by ATP hydrolysis.