Thermodynamic Analysis of Protein Stability

Understanding how tertiary structures of proteins are coded for in their amino acid sequences is one of the main targets in modern life science research.However，this goal has not yet been achieved。One of the reasons is that a protein structure is prescribed not only by a part of the amino acid sequence but also by an extensive interaction。In fact，the same kinds of substitutions give different changes in conformational stability.These results show that the effect of a single amino acid substitution on protein stability depends on the location of the mutation site and its environment in the protein structure.In order to estimate the contribution of each factor to protein stability，the data of structure and stability changes must be expanded by studying many mutant proteins with systematic and comprehensive substitutions。In this project，we tried to estimate some factors that contribute to conformational stability of a protein using data base of stability/structure，which are obtained from a series of mutant human lysozymes.Then，we constructed more than100 mutant human lysozymes。Changes in stabilities and structures due to mutations were evaluated by DSC and X-ray analysis，respectively.Using database of structure and stability changes due to mutations，which are obtained in this project，the magnitude of hydrophobicity of carbon atom(C)，and the hydrophobicity of nitrogen and neutral oxygen atoms(N/O)could be estimated to be 0.178kJ/mol/埃D12 I D 1，and-0.013 kJ/mol/A D 12 D 1，respectively.The net contribution of an intramolecular hydrogen bond，an intermolecular one between protein and ordered water molecules，and an intermolecular one between ordered water molecules was estimated to be8.5，5.2，and 5.0kJ/mol，respectively，for a hydrogen bond with 3埃length。The entropic cost due to the introduction of a water molecule could be also estimated to be about8kJ/mol。

Understanding how tertiary structures of proteins are coded for in their amino acid sequences is one of the main targets in modern life science research. However, this goal has not yet been achieved. One of the reasons is that a protein structure is prescribed not only by a part of the amino acid sequence but also by an extensive interaction. In fact, the same kinds of substitutions give different changes in conformational stability. These results show that the effect of a single amino acid substitution on protein stability depends on the location of the mutation site and its environment in the protein structure. In order to estimate the contribution of each factor to protein stability, the data of structure and stability changes must be expanded by studying many mutant proteins with systematic and comprehensive substitutions. In this project, we tried to estimate some factors that contribute to conformational stability of a protein using data base of stability/structure, which are obtained from a series of mutant human lysozymes. Then, we constructed more than 100 mutant human lysozymes. Changes in stabilities and structures due to mutations were evaluated by DSC and X-ray analysis, respectively. Using database of structure and stability changes due to mutations, which are obtained in this project, the magnitude of hydrophobicity of carbon atom (C), and the hydrophobicity of nitrogen and neutral oxygen atoms (N/O) could be estimated to be 0.178 kJ/mol/ÅィイD12ィエD1, and -0.013 kJ/mol/ÅィイD12ィエD1, respectively. The net contribution of an intramolecular hydrogen bond, an intermolecular one between protein and ordered water molecules, and an intermolecular one between ordered water molecules was estimated to be 8.5, 5.2, and 5.0 kJ/mol, respectively, for a hydrogen bond with 3 Å length. The entropic cost due to the introduction of a water molecule could be also estimated to be about 8 kJ/mol.