ribityl

Ribityl refers to a ribitol-derived substituent, a five-carbon polyol fragment used in various biomolecules. It is most familiar in relation to flavin cofactors, where the ribityl chain connects the isoalloxazine ring to additional chemical groups.

Chemical structure: Ribitol is HO-CH2-CHOH-CHOH-CHOH-CH2-OH; the ribityl descriptor indicates this fragment when linked through one of its hydroxyls to another moiety, commonly a phosphate or a heterocycle.

Occurrence and role: The ribityl moiety is a defining feature of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD); the chain bears a phosphate in FMN and FAD, allowing enzyme interactions and solubility. The ribityl linker confers conformational flexibility and contributes to hydrogen-bonding networks in flavin-dependent enzymes.

Stereochemistry and biosynthesis: In nature, ribityl-containing cofactors use a defined isomer of the ribitol-derived moiety; multiple stereoisomers are possible in principle, but biological systems select specific forms during riboflavin biosynthesis.

See also: ribose, ribitol, FMN, FAD.