Prebiotic formation of 'energy-rich' thioesters from glyceraldehyde and N-acetylcysteine

The 'energy-rich' thioester, N-acetyl-S-lactoylcysteine, is formed from low concentrations of glyceraldehyde and N-acetylcysteine under anaerobic conditions at ambient temperature in aqueous solutions of sodium phosphate (pH 7.0). Reactions with 2mM glyceraldehyde, 2mM N-acetylcysteine, and 500 mM sodium phosphate (pH 7.0) convert about 0.3 percent/day of the glyceraldehyde to lactoyl thioester. The formation of lactoyl thioester in similar reactions with 500 mM imidazole hydrochloride (pH 7.0) is supported by the thiol-dependence of lactate formation, which is 3-fold greater in the presence of thiol (0.11 percent/day) than in the absence of thiol (0.04 percent/day). The formation of lactoly thioester is thought to proceed by the phosphate (or imidazole)-catalyzed dehydration of glyceraldehyde, which adds to the thiol to form a hemithioacetal that rearranges to the thioester. A limited amount of a second thioester, N-acetyl-S-glyceroyl-cysteine, is also formed at the beginning of these reactions. The significance of these reactions to the origin of life is discussed.