Structural and Thermodynamic Consequences of Burying a Charged Residue Within the Hydrophobic Core of T4 Lysozyme

To determine the energetic and structural consequences of placing a charged group within the core of a protein, two "buried charge" mutants, Met 102----Lys (M102K) and Leu 133----Asp (L133D) were constructed in phage T4 lysozyme. Both proteins fold at neutral pH, although they are substantially less stable than wild type. The activity of M102K is about 35% that of wild type, while that of L133D is about 4%. M102K could be crystallized, and its structure was determined at high resolution. The crystal structure (at pH 6.8) of the mutant is very similar to that of wild type except for the alpha-helix that includes residues 108-113. In wild-type lysozyme, one side of this helix is exposed to solvent and the other contacts Met 102. In the M102K structure this alpha-helix becomes much more mobile, possibly allowing partial access of Lys 102 to solvent. The stability of M102K, determined by monitoring the unfolding of the protein with CD, is pH-dependent, consistent with the charged form of the substituted amino acid being more destabilizing than the uncharged form. The pKa of Lys 102 was estimated to be 6.5 both by differential titration and also by NMR analysis of isotopically labeled protein with 13C incorporated at the C epsilon position of all lysines. As the pH is lowered below pH 6.5, the overall three-dimensional structure of M102K at room temperature appears to be maintained to pH 3 or so, although there is evidence for some structural adjustment possibly allowing solvent accessibility to the protonated form of Lys 102.