Unveiling the enigmatic traits of Corynebacterium glutamicum mycoredoxin-3: A tiny redox protein displaying swapped homodimer formation and DsbA-like oxidase activity

Mycoredoxins (Mrxs) are a group of small dithiol oxidoreductases that share a conserved CXXC active site sequence motif resembling glutaredoxins. They are commonly found in saprophytic microorganisms, including Actinobacteria such as Mycobacterium tuberculosis. Among the known mycoredoxins, Corynebacterium glutamicum (Cg) Mrx1, featuring a conserved CVQC active site, functions as a mycothiol-dependent monothiol oxidoreductase. On the other hand, Mrx2, also known as NrdH-redoxin and containing the same CVQC motif, exhibits dithiol oxidoreductase properties and receives electrons from thioredoxin reductase (TrxR). Recently, it has been reported that CgMrx3, featuring a CGSC motif, acts as a thioredoxin, although its structural and biophysical characteristics remain unexplored.

In this study, we successfully determined the X-ray structure of CgMrx3 at a resolution of 1.7 Å, revealing a swapped dimer arrangement. We compared the structure of CgMrx3 with those of CgMrx1 and CgMrx2 and with the AlphaFold2 predicted structure of Mrx3 from Mycobaterium tuberculosis (MtMrx3). We correlated the number of hydrogen bonds accepted by the nucleophilic cysteines with the relatively low pKa's determined for MtMrx3 and CgMrx3. Finally, we showed that CgMrx3 has DsbA-like oxidase activity. Taken together, our results provide valuable insights into the structural and functional characteristics of Mrx3, thereby enhancing our understanding of mycoredoxin-dependent redox biology.