Figure 5. MsrB3 can act as a stereospecific oxidase.

Msr3B was initially incubated with methionine sulfoxide in the absence of an electron donor to sulfenylate the active site thiol. After removal of methionine sulfoxide, the enzyme was incubated with dabsyl-methionine for 12 h at 37°C before RP-HPLC. The elution profiles of dabsyl-methionine (grey) and a mixture of dabsyl-methionine S- and R- sulfoxide (red) are shown to illustrate the retention times for these molecules. The elution profiles of reactants of dabsyl-methionine with WT MsrB3 (blue), MsrB3 DM (green) and MsrB3 C126A (cyan) are shown. No reaction occurs with MsrB3 C126A, but stoichiometric conversion of methionine into methionine R-sulfoxide occurs with both MsrB3 WT and DM.