The role of S126 in the Staphylococcus equorum MnSOD activity and stability.
The dimeric form of manganese superoxide dismutase is instrumental for activity because each of the monomers provides amino acid residues participating in the enzymatic reaction. Hence, preventing dissociation of the dimer would maintain the enzymatic activity in detrimental conditions e.g. high temperature. To prevent dissociation of the dimer, a disulphide (S-S) bond was introduced at the dimer interface. In the wild type structure, S126 interacts with S126 of the other monomer. In the presented work, a mutant was designed with an S126C substitution. The crystal structure of the S126C mutant showed that only 50-70% of monomers formed the S-S bond. This observed imperfect S-S bonding was likely caused by photolytic S-S bond breakage mediated by the neighbouring tryptophan residue. In the wild type, S126 is located facing W163 and forms a water-mediated hydrogen bond with E164; W163 and E164 are crucial in the enzyme's activity. The replacement of S126 by a cysteine residue lowered the activity of the enzyme by ∼70%. S126 has never been considered to play a role in the enzyme's activity or stability, thus the finding showed the importance of this residue.