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.

[8][9] The furanose (5-carbon) sugar in AIR comes from the pentose phosphate pathway, which converts glucose (as its 6-phosphate derivative) into ribose 5-phosphate (R5P).

[1][12] The first enzyme, amidophosphoribosyltransferase, attaches ammonia from glutamine to the ribotide at its anomeric carbon, forming phosphoribosylamine (PRA): Next, PRA is converted to glycineamide ribonucleotide (GAR) by the action of phosphoribosylamine—glycine ligase, forming an amide bond with glycine in a process driven by ATP: A third enzyme, phosphoribosylglycinamide formyltransferase, adds a formyl group from 10-formyltetrahydrofolate to GAR, giving phosphoribosyl-N-formylglycineamide (FGAR): The penultimate step converts FGAR to an amidine by the action of phosphoribosylformylglycinamidine synthase, transferring an amino group from glutamine and giving 5′-phosphoribosylformylglycinamidine (FGAM) in a reaction that also requires ATP: FGAM is finally converted to AIR by the action of AIR synthetase which uses ATP to activate the terminal carbonyl group to attack by the nitrogen atom at the anomeric centre: The purine ring system of the nucleotide inosine monophosphate is formed in a pathway from AIR[13] that begins when phosphoribosylaminoimidazole carboxylase converts it to the carboxylated derivative in the imidazole ring, 5′-phosphoribosyl-4-carboxy-5-aminoimidazole (CAIR).

[15][5] The vitamin thiamine contains a pyrimidine ring system which is formed from AIR in a reaction catalysed by phosphomethylpyrimidine synthase.

[5][18] The initial reaction is catalysed by 5-hydroxybenzimidazole synthase, EC 4.1.99.23, and forms 5-hydroxybenzimidazole: All the carbon atoms of the product are transferred from AIR, as shown.