Selectively guiding the membrane-perturbing potential of peptides towards a distinct cellular phenotype has been an elusive, but powerful property that allows the targeting of distinct populations of cells. Some naturally occurring peptides can potentiate membrane disruption through changes in their conformation, a concept that can be exploited in the design of functional biomolecules. We report the de novo design of a new class of peptide whose potential to perturb cellular membranes is coupled to an enzyme-mediated shift in their folding potential1. Cells rich in negatively charged surface components that also highly express alkaline phosphatase, for example many cancers, are susceptible to the action of the peptide. Here, unfolded, bio-inactive peptide is dephosphorylated, shifting its conformational bias towards cell-surface induced folding into a facially amphiphilic membrane-active conformer. The exact mechanistic fate of the peptide can be further tuned by peptide concentration to affect either lytic or cell penetrating properties, which are useful for selective drug delivery. This design platform, that couples enzymatic processing to control the folding potential of a peptide at a cell’s surface, represents a new design strategy to afford peptides which are selective in their membrane-perturbing activity.