C. Ley & D. Holtmann
Many redox proteins contain a cofactor or more precisely a prosthetic group as a nondiffusible organic or inorganic compound located at the enzyme’s active site. The cofactor plays an essential role for the enzyme’s catalytic activity . It is linked firmly to the protein backbone, and the linkage may be of non-covalent or covalent nature and is often accompanied by additional interactions between the cofactor and its protein surrounding (e.g., ionic or hydrophobic). The most prominent examples for cofactors of organic origin are heme and flavin adenine dinucleotide (FAD) which can be found in myoglobin and hemoglobin or in case of FAD in glucose oxidase (Fig. 1). Furthermore, pyrroloquinoline quinone (PQQ), the cofactor of, e.g., certain alcohol dehydrogenases, is of interest since it functions not only as cofactor but also as redox shuttle . Inorganic cofactors like certain metal clusters can be found, e.g., in nitrogenases and hydrogenases.