FIGURE 7.

Proposed mycothiol relay mechanism of Cd-MsrA using the MSH/Mrx1/Mtr pathway. The reaction starts with a nucleophilic attack of Cys⁵² on the sulfoxide of l-Met-SO, which results in the formation of a sulfenic acid (-SOH) on the Cys⁵², while reduced l-Met is released (step 1). MSH then attacks the sulfenic acid and forms a mixed disulfide with the sulfur of Cys⁵² (step 2). Then an MSH relay mechanism starts with a nucleophilic attack of either Cys²⁰⁶ or Cys²¹⁵ on the mixed disulfide, forming a new mixed disulfide between the sulfur of Cys²⁰⁶ or Cys²¹⁵ and MSH (steps 3 and 4). MSH needs to be transferred from Cys²¹⁵ to Cys²⁰⁶ (step 5) to become a substrate for the MSH/Mrx1/Mtr pathway (step 6).