An Unusual Blue Protein Chromophore

Ranasmurfin, a blue protein isolated from the nests of a type of Malaysian tree frog, has an unusual chromophore, as well as uncommon amino acid cross-links. An international team led by Alan Cooper of the University of Glasgow and James H. Naismith of the University of St. Andrews, both in Scotland, has solved a 1.16-Å crystal structure showing that ranasmurfin, a 26-kilodalton homodimer, is stabilized by unusual lysine tyrosyl quinone (LTQ) linkages (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200802901). This is the first time such modifications have been identified in a native protein. The crystal structure also reveals a four-residue bis(LTQ) linkage between the protein's subunits. The bis(LTQ) structure, together with two histidine residues, serves as the binding site for a Zn²⁺ ion and is a likely candidate for the protein's blue chromophore. The bridging atom in bis(LTQ) can't be unequivocally identified by just electron density, but the researchers suspect that it's nitrogen because a nitrogen moiety would make a good ligand for zinc, and similar N-linked compounds are often highly colored.