Step 2 : Glycine is coupled to the amino group of phosphoribosylamine

This is the only step in purine biosynthesis that contributes more than one atom to the purine skeleton.

a) The carboxylate group of Gly is activated by phosphorylation to form acylphosphate.

b) The phosphate is then displaced by the amino group of phosphoribosylamine.

Step 3: Transfer of a formyl group from N¹⁰ -formyltetrahydrofolate to the amino group of the glycine residue (catalyzed by glycinamide ribonucleotide transferase). N¹⁰ -formyl-tetrahydrofolate (THF) serves as a C1 carbon donor.