Improvement of the refolding yield and solubility of hen egg-white lysozyme by altering the Met residue attached to its N-terminus to Ser

When hen egg-white lysozyme was produced in Escherichia coli, it possessed an extra methionine residue at the N-terminus (Met^-1-lysozyme). The Met^-1-lysozyme showed a decreased refolding yield and solubility compared with the native hen egg-white lysozyme, as the methionine is a hydrophobic amino acid. A Met^-2Pro^-1 or Met^-2Ser^-1 sequence was introduced at the N-terminus of hen egg-white lysozyme. The methionine residue in these hen egg-white lysozymes was completely removed by methionine aminopeptidase, as expected, since the penultimate residue was proline or serine. From the analyses of solubility, stability and refolding yield, it was found that an extra Ser residue attached to the N-terminus of hen egg-white lysozyme (Ser^-1-lysozyme) showed closer characteristics to the native hen egg-white lysozyme than did Met^-1 or an extra Pro residue attached to the N-terminus of hen egg-white lysozyme (Pro^-1-lysozyme). Moreover, the tertiary conformation of Ser^-1-lysozyme examined by NMR spectroscopy and its activity were almost identical with those of native hen egg-white lysozyme.