Proteolytic activation(proteases involved in digestion,blood clotting). Regulation at protein level often allows a faster response to changes in cellular conditions (substrate concentrations,hormone action,etc.) than regulation of gene expression. Changes of 3D-structures of proteins are pivotal in many mechanisms of regulation at protein level. Changes in structure regulate binding of substrates(e.g.formation of active sites in enzymes)or interactions with other proteins (e.g. covalent modifications like phosphorylation in signal transduction).• Proteolytic activation (proteases involved in digestion, blood clotting). • Regulation at protein level often allows a faster response to changes in cellular conditions (substrate concentrations, hormone action, etc.) than regulation of gene expression. • Changes of 3D-structures of proteins are pivotal in many mechanisms of regulation at protein level. Changes in structure regulate binding of substrates (e.g. formation of active sites in enzymes) or interactions with other proteins (e.g. covalent modifications like phosphorylation in signal transduction)