Phosphoribosylformylglycinamidine cyclo-ligase (AIR synthetase) is the fifth

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

() in the ''de novo'' synthesis of

purine Purine is a heterocyclic aromatic organic compound that consists of two rings ( pyrimidine and imidazole) fused together. It is water-soluble. Purine also gives its name to the wider class of molecules, purines, which include substituted purin ...

nucleotides Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules wi ...

. It catalyzes the reaction to form

5-aminoimidazole ribotide 5′-Phosphoribosyl-5-aminoimidazole (or aminoimidazole ribotide, AIR) is a biochemical intermediate in the formation of purine nucleotides via inosine-5-monophosphate, and hence is a building block for DNA and RNA. The vitamins thiamine and coba ...

(AIR) from formylglycinamidine-ribonucleotide FGAM. This reaction closes the ring and produces a 5-membered imidazole ring of the purine nucleus (AIR):

ATP + 2-(formamido)-N1-(5-phospho-β-D-ribosyl)acetamidine

\rightleftharpoons

ADP + 5-amino-1-(5-phospho-β-D-ribosyl)imidazole + phosphate +

AIR synthetase catalyzes the transfer of the oxygen of the formyl group to phosphate. It is a sequential mechanism in which ATP binds first to the enzyme and ADP is released last. This enzyme hydrolyzes ATP to activate the oxygen of the amide in order to carry out a nucleophilic attack by nitrogen. In humans and many other animals, this enzyme is contained within the

trifunctional purine biosynthetic protein adenosine-3 Trifunctional purine biosynthetic protein adenosine-3 is an enzyme that in humans is encoded by the ''GART'' gene. This protein is a trifunctional polypeptide. It has Phosphoribosylamine—glycine ligase (EC 6.3.4.13), Phosphoribosylglycinamide ...

polypeptide.

Nomenclature

The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivial ...

of this enzyme class is 2-(formamido)-N1-(5-phosphoribosyl)acetamidine cyclo-ligase (ADP-forming). Other names in common use include: * AIR synthetase, * 5'-aminoimidazole ribonucleotide synthetase, * 2-(formamido)-1-N-(5-phosphoribosyl)acetamidine cyclo-ligase (ADP-forming), * phosphoribosylaminoimidazole synthetase, and * phosphoribosylformylglycinamidine cyclo-ligase.

Purine Synthesis

Purines are one of the two types of nitrogenous heterocyclic bases, which are one of the three components of the nucleotides that make up nucleic acids. Synthesis can be de novo or salvage — AIR synthetase is a component of the ''de novo'' pathway. The first committed step of the de novo pathway begins with phosphoribosyl pyrophosphate (PRPP) and the end product is inosine monophosphate (IMP). IMP is eventually converted to either AMP or GMP purines. The purine ring structure is composed by the attachment of 1 or 2 atoms at a time to the ribose sugar. The ''de novo'' pathway tends to be conserved across most organisms.

Cowpea AIR synthetase

AIR synthetase is found in both mitochondria and

plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosymbiotic cyanobac ...

s; the mitochondrial form has 5 more amino acids than the plastid form. The enzyme is encoded by a single gene in cowpeas despite the fact that it exists in different forms in plastids and mitochondria. This suggests that the different versions may be derived from a single transcript. One study proposes that there is tight transcriptional control of pur5, the gene encoding AIR synthetase.

References

External links

* {{Portal bar, Biology, border=no EC 6.3.3

Nomenclature

Purine Synthesis

Cowpea AIR synthetase

References

Further reading

External links