Date of this Version Methionine residues in proteins can be
readily oxidized to a diastereomeric mixture of methionine
sulfoxides by reactive oxygen species. In most organisms,
methionine sulfoxides are reversibly and stereospecifically
reduced back to methionine by two distinct classes of repair
enzymes, methionine-S-sulfoxide reductase (MsrA) and methionine-
R-sulfoxide reductase (MsrB). Methionine sulfoxide reduction is
thought to be an essential pathway that protects cells from
oxidative stress and regulates protein function. This pathway is
also implicated in delaying the aging process in organisms from
yeast to mammals. The first selenoprotein identified using
bioinformatics methods, SelR (also known as SelX or MsrB1), was
recently found to be a selenocysteine-containing MsrB. In
mammals, selenoprotein MsrB1 is a major MsrB, while MsrB2 and
MsrB3 contain cysteine in place of selenocysteine. It has been
found that selenocysteine- and cysteine-containing MsrBs employ
different catalytic mechanisms. Interes