We investigated the substrate specificity of papain dynamic structures for peptides of 8–10 glycine residues (8–10GLY) via molecular dynamics and docking simulations. The substrate specificity of papain for 8–10GLY fluctuated considerably with time. There were several residues that were different among those that had a significant impact on binding (RESIDUES_IMPACT) with 10GLY, 9GLY, and 8GLY. Modification of these different residues should allow for control of substrate specificity, providing a framework for modifying substrate specificity in papain and other enzymes.