Learning from trypsin(ogen) structures: conformational flexibility and functional disorder

Our understanding of trypsin, its zymogenicity and its inhibition is intimately intertwined with the history of biochemistry. Early structural studies revealed its close relationship to chymotrypsin, with its active site triad, oxyanion hole and N-terminus involved in a buried salt bridge near the active site. Its complex with basic pancreatic trypsin inhibitor provided a model for how peptide substrates bind to and are cleaved by the proteinase. Analysis of crystals of trypsinogen by Wolfram Bode and Robert Huber revealed that a large region of the zymogen is disordered prior to proteolytic activation, with an associated disruption of the oxyanion hole and substrate binding pockets, highlighting the importance of disorder in protein function. As archetype of numerous therapeutically important serine proteinases, trypsin can also serve as a surrogate for structure-based drug design. Trypsin variants designed for ligand binding studies resulted however in an unexpected plasticity of the mutant proteins that underlines the complexity of protein stability. A trypsin variant selected for peptide ligation (reverse proteolysis) was shown to possess zymogen-like characteristics that proved central to its application in modification of therapeutic proteins. The review pays homage to the seminal works of Bode and Huber and their influence on modern structural biology.