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Staphylococcal nuclease (SNase) is a model protein that contains one domain and no disulfide bonds. Its stability in the native state may be maintained mainly by key amino acids. In this study, two point-mutated proteins each with a single base substitution [alanine for tryptophan (W140A) and alanine for lysine (K133A)] and two truncated fragment proteins {positions 1–139 [SNase(1–139) or W140O] and positions 1–141 [SNase(1–141) or E142O]} were generated. | iFEBS Journal Local stability identification and the role of a key aromatic amino acid residue in staphylococcal nuclease refolding Zhengding Su3 Jiun-Ming Wu1 Huey-Jen Fang1 Tian-Yow Tsong2 3 and Hueih-Min Chen1 1 Institute of BioAgriculturalSciences Academia Sinica Taipei Taiwan ROC 2 Institute of Physics Academia Sinica Taipei Taiwan ROC 3 Department of Biochemistry University of Minnesota College of BiologicalSciences St Paul MN USA Keywords aromatic amino acid refolding stability staphylococcal nuclease Correspondence H-M. Chen Institute of BioAgricultural Sciences Academia Sinica Taipei Taiwan 115 R.O.C. Fax 886 2 2788 8401 Tel 886 2 2785 5696 ext. 8030 E-mail robell@gate.sinica.edu.tw Received 3 May 2005 revised 3 June 2005 accepted 13 June 2005 doi 10.1111 j.1742-4658.2005.04814.x Staphylococcal nuclease SNase is a model protein that contains one domain and no disulfide bonds. Its stability in the native state may be maintained mainly by key amino acids. In this study two point-mutated proteins each with a single base substitution alanine for tryptophan W140A and alanine for lysine K133A and two truncated fragment proteins positions 1-139 SNase 1-139 or W140O and positions 1-141 SNase 1-141 or E142O were generated. Differential scanning microcalorimetry in thermal denaturation experiments showed that K133A and E142O have nearly unchanged AHcal relative to the wild-type whereas W140A and W140O display zero enthalpy change AHcal w 0 . Far-UV CD measurements indicate secondary structure in W140A but not W140O and near-UV CD measurements indicate no tertiary structure in either W140 mutant. These observations indicate an unusually large contribution of W140 to the stability and structural integrity of SNase. Staphylococcal nuclease SNase is a single-domain protein of 149 amino acids. Its 3D structure has been examined by NMR 1-3 and X-ray crystallography 4 5 . However only part of the structure positions 1141 can be confirmed. The segment 142-149 has not been .