Circular dichroism studies on the Deinococcus radiodurans Nudix hydrolase DR_0079: an atypical thermal melt

The crystal structure for the Deinococcus radiodurans Nudix protein DR_0079 was recently determined in the metal-free form at 1.9 A resolution (2O5F). The protein adopts the fundamental fold common to the Nudix family of proteins, a large mixed b-sheet sandwiched between the a-helix of the "Nudix box" and a second a-helix. The protein's physical properties were further characterized by circular dichroism (CD) spectroscopy. A CD thermal melt at 220 nm identifies an inflection point at approximately 52 degrees C. However, unlike typical CD thermal melts, the negative ellipticity at 220 nm becomes more negative upon passing through the inflection point. Both NMR spectroscopy and size exclusion chromatography indicate that heating effects the irreversible formation of a large molecular weight complex. After cooling, the negative ellipiticity at 220 nm increases further, and overall, the CD spectrum at 25 degrees C suggests that heat-treated DR_0079 has more a-helical and b-sheet structure than non-heat treated DR_0079.