Phosphoribosylamine

Phosphoribosylamine (PRA) 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 cobalamin also contain fragments derived from PRA.
 * Phosphoribosyl pyrophosphate.svg

It is the product of the enzyme amidophosphoribosyltransferase which attaches ammonia from glutamine to phosphoribosyl pyrophosphate (PRPP) at its anomeric carbon:
 * + →  +  + PPi

The biosynthesis pathway next combines PRA with glycine in a process driven by ATP giving glycineamide ribonucleotide (GAR). The enzyme phosphoribosylamine—glycine ligase catalyses the reaction forming an amide bond:
 * + + ATP →  + ADP + Pi