Abstract
Although the x-ray crystallography is giving a relatively precise picture of spatial arrangement of the majority of protein structure, it is not able to give detailed information on the flexibility of the protein active site. The properties of active sites of cytochromes P450 (CYP) were supposed earlier to be relatively similar, reflecting the substrate specificities of the individual enzymes mainly by the amino acid residues present in the respective active site. On the other hand, the most recent experimental as well as theoretical results document that it is the flexibility of this part of protein which gives the active site the property to bind substrates correctly and to accommodate them. It is also the flexibility or plasticity of the structure which determines or limits the property of the protein to keep the conformation which guarantees the function of the enzyme.