1. Institute of Biochemical Sciences, National Taiwan University, Taipei 106, Taiwan, Institute of Biological Chemistry, Academia Sinica, Taipei 115, Taiwan, Center for Biophysics and Computational Biology, University of Illinois at Urbana−Champaign, 607 South Mathews Avenue, Urbana, Illinois 61801, Core Facility for Protein Crystallography, Academia Sinica, Taipei 115, Taiwan, Department of Chemistry, University of Illinois at Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801