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.