Ferredoxins Containing One [4Fe–4S] Center

Abstract

Ferredoxins (Fds) are iron–sulfur proteins that function as electron carriers in diverse metabolic pathways. Generally, Fds are acidic proteins with a low molecular weight and have very low redox potentials. Although historically the name of Fd was given to electron carrier proteins containing iron–sulfur clusters in bacteria, plants and algae, and animals, they differ not only in the foldings of the polypeptide chains but also in the structures of the active centers; the canonical bacterial Fds have one or two [4Fe–4S] (or [3Fe–4S]) centers whereas the plant‐type and animal‐type Fds have one [2Fe–2S] cluster. Bacterial Fds are evolutionally related whether they have one or two [4Fe–4S] (or [3Fe–4S]) clusters, but are distinct from high potential iron–sulfur proteins that also have one [4Fe–4S] cluster. Fds containing one [4Fe–4S] cluster, on which this article is focussed, are one of several distinct classes of bacterial Fds with low redox potentials.

Ultra–high‐resolution structure and charge‐density analysis of high‐potential iron–sulfur protein

“…A nonspherical distribution of electron density was also clearly observed around the atoms of Fe 4 S 4 (Cys-S c ) 4 in the deformation map ( Fig. 2A).…”

“…The contributions of valence electrons in the interactions between Fe 4 S 4 (Cys-S c ) 4 and H atoms of the protein environment are elucidated from the chargedensity map. Valence electrons of S atoms of Fe 4 S 4 (Cys-S c ) 4 interact with H atoms (Fig.…”

“…The atomic charges are derived for Fe 4 S 4 (Cys-S c ) 4 with consideration of the atomic boundaries using the atoms in molecules (AIM) theory [16,17]. The atomic charge of FE1 is lower than those of other iron atoms (Fig.…”

“…The redox potential of the Fe 4 S 4 cluster covers a wide range, from~À500 mV to~+500 mV [2]. The low potentials are achieved by the redox transition between [Fe 4 + subclusters. In each subcluster, two Fe atoms are ferromagnetically coupled with a d 9 configuration (S = 9/2).…”

“…The cuboidal Fe 4 S 4 cluster is one of the most common among them [2]. This cluster provides electron-conducting and -storing components in electron transfer proteins such as highpotential iron-sulfur protein (HiPIP) [3], ferredoxin [4], and photosystem I [5]. It also serves as an active site in enzymes such as aconitase [6] and quinolinate synthase [7].…”