Mia40 catalyses the oxidative folding of disulphide-containing proteins in the mitochondria. The folding pathway is directed by the formation of the first mixed disulphide between Mia40 and its substrate. Here, we employ Cox17 to elucidate the molecular determinants of this reaction. Mia40 engages initially in a dynamic non-covalent enzyme-substrate complex that forms and dissociates within milliseconds. Cys36 of Cox17 forms the mixed disulphide in an extremely rapid reaction that is limited by the preceding complex formation with Mia40. Cys36 reacts much faster than the three other cysteines of Cox17, because it neighbours three hydrophobic residues. Mia40 binds preferentially to hydrophobic regions and the dynamic nature of the non-covalent complex allows rapid reorientation for an optimal positioning of the reactive cysteine. Mia40 thus uses the unique proximity between its substrate-binding site and the catalytic disulphide to select a particular cysteine for forming the critical initial mixed disulphide.

Background: PrsA is a foldase for secreted proteins and pathogenicity factors in Gram-positive bacteria. Results: Crystallographic, enzymatic, and NMR spectroscopic analysis provide insights to PrsA function. Conclusion: Substrate peptides interact around a unique crevice generated by PrsA dimerization via its chaperone-like domain. Significance: The comprehensive characterization of PrsA promotes its utilization as foldase and drug target.

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Mia40 catalyzes oxidative protein folding in mitochondria. It contains a unique catalytic CPC dithiol flanked by a hydrophobic groove, and unlike other oxidoreductases, it forms long-lived mixed disulfides with substrates. We show that this distinctive property originates neither from particular properties of mitochondrial substrates nor from the CPC motif of Mia40. The catalytic cysteines of Mia40 display unusually low chemical reactivity, as expressed in conventional pK values and reduction potentials. The stability of the mixed disulfide intermediate is coupled energetically with hydrophobic interactions between Mia40 and the substrate. Based on these properties, we suggest a mechanism for Mia40, where the hydrophobic binding site is employed to select a substrate thiol for forming the initial mixed disulfide. Its long lifetime is used to retain partially folded proteins in the mitochondria and to direct folding toward forming the native disulfide bonds.