On the origin of elementary hexoses.

A possible scenario on the origin of elementary hexoses (fructose, glucose, mannose, and galactose) is proposed: (1) Formol reaction, an autocatalytic polymerization reaction, occurred on the primitive earth under weakly alkaline conditions to generate various small molecular compounds. (2) Among them, glyceraldehyde and dihydroxyacetone undergo aldol condensation to produce stable ketohexoses, fructose and sorbose. (3) Subsequently, Lobry de Bruyn rearrangement converts fructose into stable aldohexoses, glucose, and mannose. (4) Thus, prebiotically synthesized ¿first triplet¿ hexoses would have been available for utilization by ancestral primitive microorganisms. (5) After the development of biochemical pathways, various saccharides, including galactose, were biosynthesized from glucose and mannose as ¿bricolage products¿; the utilization of galactose as a key recognition molecule, based on its distinctive axial 4-OH and its outermost location in glycoconjugates, owing to its late arrival, may have evolved concomitantly with the evolution of multicellular organisms.