1. Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, TN 37240, USA.
2. Center for Structural Biology and Institute of Chemical Biology, Vanderbilt University, Nashville, TN 37235, USA.
3. Department of Chemistry, Vanderbilt University, Nashville, TN 37235, USA.
4. Department of Chemistry, University of Alabama in Huntsville, Huntsville, AL 35899, USA.
5. Department of Medicine, Vanderbilt University Medical Center, Nashville, TN 37232, USA.