modified L-serine residue L-selenocysteine (Ser) 150.0500030517578125 150.9536285400390625 62.970001220703125 63.92160797119140625 A protein modification that effectively converts an L-serine residue to L-selenocysteine (not known as a natural post-translational modification process). C 0 H 0 N 0 O -1 Se 1 C 3 H 5 N 1 O 1 Se 1 MOD:00982 PSI-MOD S Sec(Ser) Selenocysteine (from Serine) [From DeltaMass: Average Mass: 64.] Although selenocysteine-charged tRNA(Sec) is biosynthesized from serine-charged tRNA(Sec), in peptide work selenocysteine is usually considered as either a natural residue or as a modified cysteine residue. This entry is for the artifactual formation of L-selenocysteine from serine. For encoded L-selenocysteine, use MOD:00031 [JSG]. artifact natural, standard, encoded residue substitution