Methionine sulfoxide reductase: Chemistry, substrate binding, recycling process and oxidase activity

• The mechanism of the reductase step in Msrs passes through sulfenic acid chemistry.
• The sulfenic acid intermediate is formed via a sulfonium cation.
• The activation of the recycling Cys remains a question in debate.
• The MsrA/Trx interface is typical of redox partner having short-lived interactions.
• The oxidase activity of MsrAs can be operative in the presence of thioredoxin.

Three classes of methionine sulfoxide reductases are known: MsrA and MsrB which are implicated stereo-selectively in the repair of protein oxidized on their methionine residues; and fRMsr, discovered more recently, which binds and reduces selectively free L-Met-R-O. It is now well established that the chemical mechanism of the reductase step passes through formation of a sulfenic acid intermediate. The oxidized catalytic cysteine can then be recycled by either Trx when a recycling cysteine is operative or a reductant like glutathione in the absence of recycling cysteine which is the case for 30% of the MsrBs. Recently, it was shown that a subclass of MsrAs with two recycling cysteines displays an oxidase activity. This reverse activity needs the accumulation of the sulfenic acid intermediate. The present review focuses on recent insights into the catalytic mechanism of action of the Msrs based on kinetic studies, theoretical chemistry investigations and new structural data. Major attention is placed on how the sulfenic acid intermediate can be formed and the oxidized catalytic cysteine returns back to its reduced form.

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