 Cysteine has the same structure as serine, but with one of its oxygen atoms replaced by sulfur; replacing it with selenium gives selenocysteine.Like other natural proteinogenic amino acids, cysteine has chirality in the older / notation based on homology to - and -glyceraldehyde.In the newer "R"/"S" system of designating chirality, based on the atomic numbers of atoms near the asymmetric carbon, cysteine (and selenocysteine) have "R" chirality, because of the presence of sulfur (or selenium) as a second neighbour to the asymmetric carbon.