While enzymes can be easily categorized by their catalytic function, the functions of non-enzymatic proteins can be much more subtle. In the absence of experimental data, protein functions are often assigned based on sequence or domain conservation, but this can be misleading. Beginning with the human guidance protein Netrin-4, to the pit viper venom constituent Rhodocetin and its interaction integrin α2β1, a blood clotting protein, and finishing with Right Handed Coiled-Coil, a surface layer component of microorganisms that live in the harshest environments, this thesis brings to light new knowledge on three extracellular systems where structural characterization and subsequent validation were used to clarify their functions or their mechanism of action.