Single site-directed mutants were constructed following the
standard protocol using WT construct pET32a-VP as the template.
Easily oxidizable methionines Met152, Met 247, Met262, and
Met265 were replaced with high-redox-potential residues Leu
or Phe respectively. Ala77, Ala79, and Ile 81were replaced with
Ser/Glu, Ser/Leu