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Wild-type Sac7d (blue and orange) kinking DNA (lilac), from PDB 1AZP

Nanofitins (formerly called Affitins[1]) are artificial proteins with the ability to selectively bind antigens. They are structurally derived from the DNA binding protein Sac7d, found in Sulfolobus acidocaldarius, a microorganism belonging to the archaeal domain. By randomizing the amino acids on the binding surface of Sac7d and subjecting the resulting protein library to rounds of ribosome display, the affinity can be directed towards various targets, such as peptides, proteins, viruses, and bacteria.[citation needed] Nanofitins are antibody mimetics and are being developed as an alternative to antibodies as tools in biotechnology. They have also been used as specific inhibitors for various enzymes.


Nanofitins consist of 66 amino acids and have a molecular mass of about 7 kDa, small compared to antibodies with some 130–150 kDa. Being obtained from a thermophile organism, they are unusually heat resistant proteins. Unlike antibodies, nanofitins are produced in vitro, and therefore can be generated more quickly. Due to their small size and high solubility, they can be easily produced in large amounts using bacterial expression systems.