A peptide catalyst can replace an essential enzyme in a eukaryotic cell

Protein enzymes are central to modern biology, yet how catalysis emerged before the evolution of large, folded proteins remains unresolved. Here we show that a short, genetically encoded peptide can replace an essential enzyme in a living eukaryotic cell. We designed minimal peptides containing a Cys-Xaa-Cys catalytic motif and an endoplasmic reticulum retention signal, and identified variants that rescue the otherwise lethal deletion of protein disulfide isomerase (PDI) in Saccharomyces cerevisiae. Cells relying on these peptides remain viable, though they grow more slowly and adapt by activating stress-response pathways, consistent with PDI being replaced by catalysts of lower intrinsic efficiency. Biochemical analyses show that peptide activity depends on local chemical environment and secondary structure rather than a globular fold. These results demonstrate that short peptides can replace an essential cellular reaction in vivo at the system level, supporting the plausibility of peptide-based catalysis as a precursor to modern protein enzymes.