a protein-L-methionine + an oxidized thioredoxin + a [protein]-L-methionine = a protein-L-methionine-(S)-S-oxide + a reduced thioredoxin + a protein-L-methionine-(S)-S-oxide
a protein with reduced sulfide groups + a protein with reduced sulfide groups = a protein with oxidized disulfide bonds + a protein with oxidized disulfide bonds