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E. coli methionine sulfoxide reductase with a truncated N terminus or C terminus, or both, retains the ability to reduce methionine sulfoxide

The monomeric peptide methionine sulfoxide reductase (MsrA) catalyzes the irreversible thioredoxin‐dependent reduction of methionine sulfoxide. The crystal structure of MsrAs from Escherichia coli and Bos taurus can be described as a central core of about 140 amino acids that contains the active site. The core is wrapped by two long N‐ and C‐terminal extended chains. The catalytic mechanism of the E. coli enzyme has been recently postulated to take place through formation of a sulfenic acid intermediate, followed by reduction of the intermediate via intrathiol‐disulfide exchanges and thioredoxin oxidation. In the present work, truncated MsrAs at the N‐ or C‐terminal end or at both were produced as folded entities. All forms are able to reduce methionine sulfoxide in the presence of dithiothreitol. However, only the N‐terminal truncated form, which possesses the two cysteines located at the C‐terminus, reduces the sulfenic acid intermediate in a thioredoxin‐dependent manner. The wild type displays a ping‐pong mechanism with either thioredoxin or dithiothreitol as reductant. Kinetic saturation is only observed with thioredoxin with a low KM value of 10 μM. Thus, thioredoxin is likely the reductant in vivo. Truncations do not significantly modify the kinetic properties, except for the double truncated form, which displays a 17‐fold decrease in kcat/KMetSO. Alternative mechanisms for sulfenic acid reduction are also presented based on analysis of available MsrA sequences.