The most common theory about the emergence of prebiotic amino acids can’t account for cysteine, so origin-of-life researchers have proposed that it is a product of evolution.
A new study proposes a prebiotic route for making cysteine-containing compounds. In addition, Matthew W. Powner and coworkers at University College London show a plausible prebiotic pathway in which these compounds catalyze the synthesis of peptides containing other amino acids (Science 2020, DOI: 10.1126/science.abd5680).
They propose a prebiotic pathway that starts with serine nitrile. First, a reaction with thioacetic acid forms diacetylated serine nitrile. Over 4 days, it converts to acetylated dehydroalanine nitrile. Finally, reaction with hydrogen sulfide results in acetylcysteine-containing compounds.
The researchers then show that those compounds can catalyze peptide ligation. In water at neutral pH, the acylcysteines catalyzed the reaction of acylglycine nitrile with all canonical amino acids to form simple peptides. “The inherent catalytic activity of simple cysteinyl peptides makes them an excellent candidate for catalysts for protometabolic reactions in an abiotic environment,” the researchers write.
The work “highlight[s] the potential of small-molecule organocatalysis and the pivotal role that thiols might have played in the emergence of biochemistry,” Kamila B. Muchowska and Joseph Moran of the University of Strasbourg write in an accompanying commentary (Science 2020, DOI: 10.1126/science.abf1698). “If life did once use nitrile-based chemistry, it is unclear why it would have shifted to a biochemistry where nitriles are rare.”