Thiol-disulfide oxidoreductase that possesses disulfide reductase and insulin disulfide bonds reducing activities. Heat shock causes oligomerization and formation of high molecular weight (HMW) complexes with concomitant functional switching from a disulfide reductase to chaperone. Interacts with FBA5 and FBA8 (PubMed:15352244). Interacts with FBA6 (PubMed:21782461). Interacts with MDH1 (PubMed:29194485). High sensitivity to heat shock. Belongs to the thioredoxin family. Plant H-type subfamily. The active site contains a CPPC motif wich differs from the conserved CGPC motif.