Overview of Purine Ring Structure and Atom Numbering

The purine ring is a fused bicyclic structure with nine atoms, numbered as follows:

• Nitrogen atoms at positions N1, N3, N7, and N9.

• Carbon atoms at positions C2, C4, C5, C6, and C8.

Each atom is derived from specific precursors during biosynthesis.

Sources of Nitrogen Atoms in Purine Biosynthesis

• N1: Derived from the amino group of aspartic acid.

• N3 and N9: Both nitrogen atoms come from the amide nitrogen of glutamine.

• N7: Provided by the α-amino nitrogen of glycine.

Sources of Carbon Atoms in Purine Biosynthesis

• C4 and C5: Supplied by the carbon atoms of glycine.

• C2 and C8: Provided by formyl groups donated by 10-formyltetrahydrofolate (10-formyl-THF), which are derived from one-carbon metabolism involving serine, formate, and histidine.

• C6: Comes from carbon dioxide (CO₂).

Clarifying the Nitrogen at N9 Position

• The nitrogen at N9 is specifically from the amide nitrogen of glutamine’s side chain, not the α-amino nitrogen attached to the α-carbon (Cα).

• This distinction is important because glutamine contains two nitrogen atoms: the α-amino nitrogen and the side-chain amide nitrogen. Only the amide nitrogen contributes to purine biosynthesis.

Evaluating the Given Options

Summary Table of Atom Sources

Conclusion

The most accurate description of nitrogen and carbon atom sources in purine biosynthesis is:

(2) N1 is from aspartic acid; N3 and N9 are from glutamine side-chain; N7, C4, and C5 are from glycine.

This reflects the well-established biochemical pathway and isotope labeling studies confirming the origins of purine atoms.