Pseudokinases are non-canonical protein-kinase-like proteins deficient in kinase activity, few of which have enzymatic activity that differs from phosphorylation. Now a pseudokinase-enabled cyclization activity for the biosynthesis of ribosomally synthesized and post-translationally modified peptides has been observed. Here pseudokinases can catalyse a Michael addition for (ene)thioether crosslinking through a sandwich-like substrate-assisted process.