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A significant challenge in chemistry is to rationally reproduce the functional potency of a protein in a small molecule, which is cheaper to manufacture, non-immunogenic, and also both stable and bioavailable. Synthetic peptides corresponding to small bioactive protein surfaces do not form stable structures in water and do not exhibit the functional potencies of proteins. Here we describe a novel approach to growing small molecules with protein-like potencies from a functionally important amino acid of a protein. A 77-residue human inflammatory protein (complement C3a) important in innate immunity is rationally transformed to equipotent small molecules, using peptide surrogates that incorporate a turn-inducing heterocycle with correctly positioned hydrogen-bond-accepting atoms. Small molecule agonists (molecular weight <500 Da) examined for receptor affinity and cellular responses have the same high potencies, functional profile and specificity of action as C3a protein, but greater plasma stability and bioavailability. Replicating the functionality of bioactive proteins using rationally designed small molecule mimics is both economically valuable and synthetically challenging. Here the authors develop a mimic of the inflammatory protein C3a with equal biological potency but enhanced stability and bioavailability.