X-ray structure determination of proteins by using the multiple-wavelength
anomalous dispersion method targeting selenomethionine is now widely employed.
Isoleucine was examined for the second choice of the substitution of methionine
next to leucine. We performed a systematic mutational study of the substitutions
of methionine for isoleucine. All mutated lysozymes were less stable than the
wild-type by about 1 kcal/mol and it is suggested that this instability was
caused by the change in residual hydrophobicity from isoleucine to methionine.
The X-ray structures of all mutant lysozymes were very similar to that of the
wild-type. In addition, both the accessible surface areas and the conformation
of the side chain of methionine in all mutant lysozymes were similar to those of
the side chain at the respective isoleucine in the wild-type. Therefore, it is
suggested that the mutation from isoleucine to methionine in a protein can be
considered as a "safe" substitution.
