ABSTRACT
Increasingly, proteins and especially enzymes come into focus as therapeutic and commercial targets, not only as isolated products but also as targets for metabolic engineering inside of a cell. However, most of the time, enzymes as isolated from nature do not fulfil the specific demands they have to meet for their industrial or medical application or in a re-engineered cell tailored for special tasks such as overproduction of natural compounds. To improve proteins/enzymes for given purposes, proteinengineering concepts have emerged over the last decades. Protein engineering describes the alteration/improvement of certain properties of a protein by changing its building blocks and, therefore, its structure and properties. This can be done by chemically modifying the amino acid residues of a protein or by altering its primary structure, which is possible since all modern DNA manipulation tools have become available. Typical chemical modifications are the cross-linking of amino acids at the protein surface or the derivatization of particular, reactive amino acid residues in the active center of enzymes such as the catalytically active serines in serine-proteases. Advantages of chemical modification include that changes of the 3-D-structure remain localized and are therefore much better predictable. However, the desired effects are generally weak and are not achievable in each case and for every protein. In addition, the protein must be extracted from the cells and sometimes even purified. It is not applicable to proteins that should be active intracellularly. Finally, chemical modification adds costs to a possible product. For these reasons, it is much more promising nowadays to directly change the properties of a protein by altering its primary structure. In order to make the most efficient use of rational protein engineering, it is still desirable to understand better the links between the primary structure of a protein and its 3-D structure on one hand, and between its 3-D structure and properties on the other hand (Table 1).
