$Rhodobacter\ sphaeroides$ methionine sulfoxide reductase P reduces $R$ ‑ and $S$ ‑diastereomers of methionine sulfoxide from a broad‑spectrum of protein substrates

Methionine (Met) is prone to oxidation and can be converted to Met sulfoxide (MetO), which exists as $R$- and $S$-diastereomers. MetO can be reduced back to Met by the ubiquitous methionine sulfoxide reductase (Msr) enzymes. Canonical MsrA and MsrB were shown to be absolutely stereospecific for the reduction of S ‑ and R‑ diastereomer, respectively. Recently, a new enzymatic system, MsrQ/MsrP which is conserved in all gram‑negative bacteria, was identified as a key actor in the reduction of oxidized periplasmic proteins. The haem‑binding membrane protein MsrQ transmits reducing power from the electron transport chains to the molybdoenzyme MsrP, which acts as a protein‑MetO reductase. The MsrQ/MsrP function was well established genetically, but the identity and biochemical properties of MsrP substrates remain unknown. In this work, using the purified MsrP enzyme from the photosynthetic bacteria $Rhodobacter\ sphaeroides$ as a model, we show that it can reduce a broad spectrum of protein substrates. The most efficiently reduced MetO are found in clusters of amino acid sequences devoid of threonine and proline on the C‑terminal side. Moreover, $R.\ sphaeroides$ MsrP lacks stereospecificity as it can reduce both R‑ and S‑ diastereomers of MetO, similarly to its $Escherichia\ coli$ homolog, and preferentially acts on unfolded oxidized proteins. Overall, these results provide important insights into the function of a bacterial envelop protecting system, which should help understand how bacteria cope in harmful environments.