Precursors of the atoms in the purine skeleton are
1
N1, Asp; C2 and C8, formate; N3 and N9, guanidine of Arg; C4, C5 and N7, Gly; C6, CO2
2
N1, Asp; C2 and C8, citrate; N3 and N9, amide nitrogen of Gin; C4, C5 and N7; Gly; C6, CO2
3
N1, Asp; C2 and C8, formate; N3 and N9, amide nitrogen of Gln, C4, C5 and N7, Gly; C6, CO2
4
N1, Glu; C and C8, acetate; N3 and N9, amide nitrogen of Asn; C4, C5 and N7, Gly; C6, CO2