Guanines

Guanine is one of the four main nucleobases in DNA and RNA. It is a purine with a 2-amino group and a 6-one (keto) group, commonly described as 2-amino-6-oxopurine. In cellular nucleotides it appears as GMP, GDP, and GTP. Guanine is derived from or reclaimed through purine metabolism, with both de novo synthesis and salvage pathways contributing to its pool.

In base pairing, guanine forms three hydrogen bonds with cytosine, producing the G-C base pair in DNA and RNA. This pairing contributes to the overall stability of the double helix, and GC content can influence the thermal stability of DNA. In RNA, guanine participates in canonical G-C pairing and can engage in noncanonical interactions, such as wobble pairings in certain structures.

Guanine-rich sequences can form higher-order structures called G-quadruplexes, where four guanine bases assemble into a square plan, stabilized by monovalent cations like potassium. G-quadruplexes are implicated in the regulation of transcription, replication, and telomere maintenance, though their exact biological roles vary by context.

In metabolism, guanine nucleotides are essential building blocks for nucleic acids and also function in energy transfer and signaling. GTP serves as an energy source and a substrate for G-proteins and RNA synthesis. Salvage pathways recycle guanine to GMP, via the enzyme guanine phosphoribosyltransferase, helping to maintain nucleotide balance within the cell.