Central component of a redox-sensitive mitochondrial intermembrane space import machinery which is required for the biogenesis of respiratory chain complexes (By similarity). Functions as chaperone and catalyzes the formation of disulfide bonds in substrate proteins, such as COX17 or MICU1. Required for the import and folding of small cysteine-containing proteins (small Tim) in the mitochondrial intermembrane space (IMS). Precursor proteins to be imported into the IMS are translocated in their reduced form into the mitochondria. Monomer. Can form homooligomers. The CHCH domain contains a conserved twin Cys-X(9)-Cys motif which is required for import and stability of MIA40 in mitochondria.