Seeing Disulfides Via Diselenide Proxies

Replacing cysteine residues in a protein with selenocysteines—in effect, replacing sulfur with selenium—improves the capability of NMR to visualize disulfide bond networks and could turn out to be a general strategy for studying protein structure, according to a report (Angew. Chem. Int. Ed. 2009, 48, 9312). Disulfide bonds between the thiol groups of two cysteine residues stabilize many proteins, such as insulin. NMR techniques are a good option for solving the structure of these proteins, but most of the techniques are difficult to use for discerning the connectivity of multiple disulfides at once. Australian researchers led by Glenn F. King of the University of Queensland have now shown that disulfide bonding can be studied via the NMR-active ⁷⁷Se isotope. The team replaced two crucial cysteines in a spider toxin with selenocysteines, a change that didn’t affect the toxin’s structure or function. Then ⁷⁷Se NMR experiments let the researchers unambiguously assign the protein connectivity. The approach may be applicable to larger proteins by using selenocysteines enriched with ⁷⁷Se, the researchers note.