Mirror image proteins

• Native chemical ligation enables facile synthetic access to mirror image proteins.
• Mirror image proteins facilitate protein crystallization and structure solution.
• Mirror image proteins aid drug discovery of novel classes of therapeutics.
• Mirror image proteins can be a powerful mechanistic tool for biology.

Proteins composed entirely of unnatural d-amino acids and the achiral amino acid glycine are mirror image forms of their native l-protein counterparts. Recent advances in chemical protein synthesis afford unique and facile synthetic access to domain-sized mirror image d-proteins, enabling protein research to be conducted through ‘the looking glass’ and in a way previously unattainable. d-Proteins can facilitate structure determination of their native l-forms that are difficult to crystallize (racemic X-ray crystallography); d-proteins can serve as the bait for library screening to ultimately yield pharmacologically superior d-peptide/d-protein therapeutics (mirror-image phage display); d-proteins can also be used as a powerful mechanistic tool for probing molecular events in biology. This review examines recent progress in the application of mirror image proteins to structural biology, drug discovery, and immunology.