Entry for the Table of Contents Modular, supramolecular catalysts based on the coiled coil peptide scaffold and designed to mimic nonribosomal peptide synthetases are demonstrated to catalyze the formation of diketopiperazine and linear dipeptides for several aminoacyl substrates. We further demonstrate that the nature of the active site residues in the peptide catalysts can be used to effect directed intermodular aminoacyl transfer processes and govern the relative yields of diketopiperazine, linear dipeptide, and hydrolyzed substrate.