Ranasmurfin, a Smurf-blue protein

© W. A. Djatmiko, 2005.

Biofoams are soft biological materials that have unique chemical and physical properties, and they contain a rich library of undiscovered proteins and other large molecules. Certain species of tree frog use biofoams to create nests for their eggs, protecting them from dehydration, predation, microbial infections, and other dangers. In a recent Angewandte article, British, French, Brazilian, and Malaysian scientists solved the structure of ranasmurfin, a novel blue protein that has been isolated from atreefrog found in South-East Asia. HangZhou Night Net

Blue proteins are uncommon in biology. Normally, their color comes from chemicals such as carotenoids, porphyrins, or polyenes, but ranasmurfin's coloration has a different source. X-ray diffraction of ranasmurfin's deep blue crystals reveals that it is composed of two monomers: A (blue) and B (green). Each monomer contains 113 amino acids that fold into distinctive α-helical forms. The most informative part of the structure is related to how the two monomers are connected together.

A zinc atom (most likely Zn2+) is at the center of the dimer, joining the two parts through an unusual arrangement of two histidine residues and a linkage containing four amino acid residues with intra- and inter-subunit cross-links. The blue color comes from the amino acid arrangement where the proteins bind the zinc, which creates a structure that is somewhat similar to known blue dyes like indophenol.

At this stage, the scientists can only guess at ranasmurfin's biological role in the biofoam. They speculate that it has at least some mechanical function, such as stabilization or adhesion. As for the significance of the blue color, they hypothesize that it could act as sunscreen or camouflage for the eggs, or it could just be an inconsequential side-effect. Nonetheless, ranasmurfin's crystal structure adds to our list of known protein structures and the continuing effort to catalog the contents of biofoams.

Angewandte Chemie Int. Ed., 2008. DOI: 10.1002/anie.200802901

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