Ferredoxins of the third kind

Most low-potential ferredoxins (Fds) are of the well-known [2Fe–2S] plant or [4Fe–4S] bacterial type. Yet, an additional class of [2Fe–2S] Fds has been recognized on the basis of sequence and spectroscopic idiosyncrasies. A recent crystal structure has confirmed the uniqueness of this third kind of Fd, and shown that these proteins display an unexpected structural similarity to thioredoxin. The properties of these thioredoxin-like [2Fe–2S] Fds are summarized, and hypotheses concerning their function are discussed.