This family consists of several mammalian selenoprotein S (SelS) sequences. SelS is a plasma membrane protein and is present in a variety of tissues and cell types [1]. Selenoprotein S (SelS) is an intrinsically disordered protein. It formsa selenosulfide bond between cys 174 and Sec 188, that has a redox potential -234 mV. In vitro, SelS is an efficient reductase that depends on the presence of selenocysteine. Due to the high reactivity, SelS also has peroxidase activity that can catalyze the reduction of hydrogen peroxide [2]. It is also resistant to inactivation by hydrogen peroxide which might provide evolutionary advantage compared to cysteine containing peroxidases [3].

1: Characterization of mammalian selenoproteomes. Kryukov GV, Castellano S, Novoselov SV, Lobanov AV, Zehtab O, Guigo R, Gladyshev VN; Science 2003;300:1439-1443. PMID:12775843

2: The intrinsically disordered membrane protein selenoprotein S is a reductase in vitro. Liu J, Li F, Rozovsky S; Biochemistry. 2013;52:3051-3061. PMID:23566202

3: Contribution of selenocysteine to the peroxidase activity of selenoprotein S. Liu J, Rozovsky S; Biochemistry. 2013;52:5514-5516. PMID:23914919