Contribution of amino acid substitutions at two different interior positions to the conformational stability of human lysozyme

To elucidate correlative relationships between structural change and thermo dynamic stability in proteins, a series of mutant human lysozymes modified at two buried positions (Ile56 and Ile59) were examined. Their thermodynami c parameters of denaturation and crystal structures were studied by calorim etry and X-ray crystallography, The mutants at positions 56 and 59 exhibite d different responses to a series of amino acid substitutions. The changes in stability due to substitutions showed a linear correlation with changes in hydrophobicity of substituted residues, having different slopes at each mutation site. However, the stability of each mutant was found to be repres ented by a unique equation involving physical properties calculated from mu tant structures. By fitting present and previous stability data for mutant human lysozymes substituted at various positions to the equation, the magni tudes of the hydrophobicity of a carbon atom and the hydrophobicity of nitr ogen and neutral oxygen atoms were found to be 0.178 and -0.013 kJ/mol.Angs trom(2), respectively. It was also found that the contribution of a hydroge n bond with a length of 3.0 Angstrom to protein stability was 5.1 kJ/mol an d the entropy loss of newly introduction of a water molecules was 7.8 kJ/mo l.