AIR synthetase is the fifth enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 in the de novo synthesis of purine

Purine

A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycle in nature....

 nucleotides. It catalyzes the reaction to form 5-aminoimidizole ribonucleotide (AIR) from formylglycinamidine-ribonucleotide FGAM. This reaction closes the ring and produces a 5-membered imidazole ring of the purine nucleus (AIR). 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 a nitrogen. In humans and many other organisms, this enzyme is contained within the trifunctional purine biosynthetic protein adenosine-3 polypeptide.

Nomenclature

The systematic name 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 phosphoribose pyrophosphate (PRPP) and the end product is inosine monophospate (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

Plastid

Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...

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.