It is well established that the dynamic motion of proteins plays an important functional role, and that the adaptation of a protein molecule to its environment requires optimization of internal non-covalent interactions and protein-solvent interactions. Serine proteinases in general, and trypsin in particular has been used as a model system in exploring possible structural features for cold adaptation. In this study, a 500 p.s. and a 1200 p.s. molecular dynamics (MD) simulation at 300 K of both anionic salmon trypsin and cationic bovine trypsin are analyzed in terms of molecular flexibility, internal non-covalent interactions and protein-solvent interactions. The present MD simulations do not indicate any increased flexibility of the cold adapted enzyme on an overall basis. However, the apparent higher flexibility and deformability of the active site of anionic salmon trypsin may lower the activation energy for ligand binding and for catalysis, and might be a reason for the increased binding affinity and catalytic efficiency compared to cationic bovine trypsin.