Department of Bioinformatics, State Key Laboratory of Proteomics, Beijing Proteome Research Center, National Center for Protein Sciences Beijing, Institute of Radiation Medicine and Department of Bioinformatics, National Engineering Research Center for Protein Drugs, Beijing 102206, Department of Automatic Control, College of Mechatronics and Automation, National University of Defense Technology, Changsha, Hunan 410073 and Department of Chemistry, Institutes of Biomedical Sciences, 130 DongAn Road, Fudan University, Shanghai 200032, P.R. China.