Crystallography of Extremophile Proteins—Structural Comparisons of Psychrophilic and Hyperthermophilic Rubredoxins

Psychrophilic organisms are able to grow at temperatures down to −15 °C, while hyperthermophiles can multiply at temperatures up to 122 °C. What structural changes in extremophile proteins are needed to maintain stable and biochemically active structures under such conditions? Understanding how such extremophiles accomplish this is relevant for human health, biotechnology, and our search for life elsewhere in the universe. The purpose of the current study is to report and compare the structures of four rubredoxins (Rds), the first ever two experimental psychrophile bacteria structures (from Gram-positive Clostridium psychrophilum and Gram-negative Polaromonas glacialis) and two hyperthermophiles from the Gram-negative Thermotoga maritima bacterium and the archaeon Pyrococcus yayanosii, also a piezophile, as part of a program to understand structural variations that support both stability and function under extreme conditions. These structures were obtained using synchrotron radiation X-ray diffraction at 100 K. All four structures had the expected overall rubredoxin fold. Rubredoxin from the only aerobic psychrophilic bacterium Polaromonas glacialis had larger variations in sequence and structure, whereas the other psychrophilic bacterium showed properties closely related to hyperthermophile rubredoxins. Multi-subunit structures showed similar RMSD variability independent from their thermal adaptation status. We propose including functional information in the analysis since temperature optimization may not be the only determinant for a specific protein adaptation.