Nonenzymatic formation of 'energy-rich' lactoyl and glyceroyl thioesters from glyceraldehyde and a thiol

The energy rich thioester, N-acetyl-S-lactoylcysteine, is formed under anaerobic conditions from glyceraldehyde and N-acetylcysteine at ambient temperature in aqueous solutions of sodium phosphate (pH 7.0). The conversion occurs at a rate of about 0.4 percent per day in reactions with 10 millimoles (mM) glyceraldehyde, 40 mM thiol, and 500 mM sodium phosphate (pH 7.0). Thioester formation proceeds at an estimated efficiency of 76 percent. The formation of lactoyl thioester most likely occurs by the phosphate catalyzed dehydration of glyceraldehyde to give pyruvaldehyde, which combines with thiol to form a hemithioacetal that rearranges to the thioester. A second energy rich thioester, N-acetyl-S-glyceroylcysteine, is also produced from glyceraldehyde when these reactions are carried out in the presence of oxygen and to a limited extent in the absence of oxygen. In the presence of oxygen, the formation of glyceroyl thioester continues until the thiol disappears completely by oxidation. The significance of these reactions to the energetics of the origin of life is discussed.