Which of the following is the impoant reactive group of glutathione in its role as an antioxidant?

Glutathione : lt is a tripeptide composed of 3 amino acids. Chemically, glutathione is y- glutamyl-cysteinyl-glycine. lt is widely distributed in nature and exists in reduced or oxidized states. 2G-SH =+G=S-S-G Reduced Oxidized Functions: In a steady state, the cells generally maintain a ratio of about 100/1 of CSH to G-S-S-C. The reversible oxidation- reduction of glutathione is impoant for many of its biological functions. Ref: Biochemistry by U. Satyanarayana 3rd edition Pgno : 65