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The protein disulfide isomerase (PDI) family comprises a wide set of enzymes mainly involved in thiol–disulfide exchange reactions in the endoplasmic reticulum. Class A PDIs (PDI-A) constitute the smallest members of the family, consisting of a single thioredoxin (TRX) module without any additional domains. To date, their catalytic activity and cellular function are still poorly understood. To gain insight into the role of higher-plant class A PDIs, the biochemical properties of rAtPDI-A, the recombinant form of Arabidopsis thaliana PDI-A, have been investigated. As expressed, rAtPDI-A has only little oxidoreductase activity, but it appears to be capable of binding an iron–sulfur (Fe–S) cluster, most likely a [2Fe-2S] center, at the interface between two protein monomers. A mutational survey of all cysteine residues of rAtPDI-A indicates that only the second and third cysteines of the CXXXCKHC stretch, containing the putative catalytic site CKHC, are primarily involved in cluster coordination. A key role is also played by the lysine residue. Its substitution with glycine, which restores the canonical PDI active site CGHC, does not influence the oxidoreductase activity of the protein, which remains marginal, but strongly affects the binding of the cluster. It is therefore proposed that the unexpected ability of rAtPDI-A to accommodate an Fe–S cluster is due to its very unique CKHC motif, which is conserved in all higher-plant class A PDIs, differentiating them from all other members of the PDI family.