Iron–sulfur protein

The prevalence of these proteins on the metabolic pathways of most organisms leads to theories that iron–sulfur compounds had a significant role in the origin of life in the iron–sulfur world theory.

In some instances Fe–S clusters are redox-inactive, but are proposed to have structural roles. Examples include endonuclease III and MutY.

Structural motifs

In almost all Fe–S proteins, the Fe centers are tetrahedral and the terminal ligands are thiolato sulfur centers from cysteinyl residues. The sulfide groups are either two- or three-coordinated. Three distinct kinds of Fe–S clusters with these features are most common.

Structure-function principles

Iron–sulfur proteins are involved in various biological electron transport processes, such as photosynthesis and cellular respiration, which require rapid electron transfer to sustain the energy or biochemical needs of the organism. To serve their various biological roles, iron-sulfur proteins effect rapid electron transfers and span the whole range of physiological redox potentials from -600 mV to +460 mV.

Fe³⁺-SR bonds have unusually high covalency which is expected. When comparing the covalency of Fe³⁺ with the covalency of Fe²⁺, Fe³⁺ has almost double the covalency of Fe²⁺ (20% to 38.4%). Fe³⁺ is also much more stabilized than Fe²⁺. Hard ions like Fe³⁺ normally have low covalency because of the energy mismatch of the metal lowest unoccupied molecular orbital with the ligand highest occupied molecular orbital.

External water molecules positioned close to the iron-sulfur active site reduces covalency; this can be shown by lyophilization experiments where water is removed from the protein. This reduction is because external water hydrogen bonds with cysteine S, decreasing the latter's lone pair electron donation to the Fe^3+/2+ by pulling away S electrons. Since covalency stabilizes Fe³⁺ more than Fe²⁺, Fe³⁺ is more destabilized by the HOH-S hydrogen-bonding.

The Fe³⁺ 3d orbital energies follow the "inverted" bonding scheme which fortuitously has the Fe³⁺ d-orbitals closely matched in energy with the sulfur 3p orbitals, giving high covalency in the resulting bonding molecular orbital. This high covalency lowers the inner sphere reorganization energy and ultimately contributes to a rapid electron transfer.

2Fe–2S clusters

The simplest polymetallic system, the [Fe₂S₂] cluster, is constituted by two iron ions bridged by two sulfide ions and coordinated by four cysteinyl ligands (in Fe₂S₂ ferredoxins) or by two cysteines and two histidines (in Rieske proteins). The oxidized proteins contain two Fe³⁺ ions, whereas the reduced proteins contain one Fe³⁺ and one Fe²⁺ ion. These species exist in two oxidation states, (Fe^III)₂ and Fe^IIIFe^II. CDGSH iron sulfur domain is also associated with 2Fe-2S clusters.

The Rieske proteins contain Fe–S clusters that coordinate as a 2Fe–2S structure and can be found in the membrane bound cytochrome bc1 complex III in the mitochondria of eukaryotes and bacteria. They are also a part of the proteins of the chloroplast such as the cytochrome b₆f complex in photosynthetic organisms. These photosynthetic organisms include plants, green algae, and cyanobacteria, the bacterial precursor to chloroplasts. Both are part of the electron transport chain of their respective organisms which is a crucial step in the energy harvesting for many organisms.

4Fe–4S clusters

A common motif features a four iron ions and four sulfide ions placed at the vertices of a cubane-type cluster. The Fe centers are typically further coordinated by cysteinyl ligands. The [Fe₄S₄] electron-transfer proteins ([Fe₄S₄] ferredoxins) may be further subdivided into low-potential (bacterial-type) and high-potential (HiPIP) ferredoxins. Low- and high-potential ferredoxins are related by the following redox scheme:

In HiPIP, the cluster shuttles between [2Fe³⁺, 2Fe²⁺] (Fe₄S₄²⁺) and [3Fe³⁺, Fe²⁺] (Fe₄S₄³⁺). The potentials for this redox couple range from 0.4 to 0.1 V. In the bacterial ferredoxins, the pair of oxidation states are [Fe³⁺, 3Fe²⁺] (Fe₄S₄⁺) and [2Fe³⁺, 2Fe²⁺] (Fe₄S₄²⁺). The potentials for this redox couple range from −0.3 to −0.7 V. The two families of 4Fe–4S clusters share the Fe₄S₄²⁺ oxidation state. The difference in the redox couples is attributed to the degree of hydrogen bonding, which strongly modifies the basicity of the cysteinyl thiolate ligands. A further redox couple, which is still more reducing than the bacterial ferredoxins is implicated in the nitrogenase.

Some 4Fe–4S clusters bind substrates and are thus classified as enzyme cofactors. In aconitase, the Fe–S cluster binds aconitate at the one Fe centre that lacks a thiolate ligand. The cluster does not undergo redox, but serves as a Lewis acid catalyst to convert citrate to isocitrate. In radical SAM enzymes, the cluster binds and reduces S-adenosylmethionine to generate a radical, which is involved in many biosyntheses.

The second cubane shown here with mixed valence pairs (2 Fe3+ and 2 Fe2+), has a greater stability from covalent communication and strong covalent delocalization of the “extra” electron from the reduced Fe2+ that results in full ferromagnetic coupling.

3Fe–4S clusters

Proteins are also known to contain [Fe₃S₄] centres, which feature one iron less than the more common [Fe₄S₄] cores. Three sulfide ions bridge two iron ions each, while the fourth sulfide bridges three iron ions. Their formal oxidation states may vary from [Fe₃S₄]⁺ (all-Fe³⁺ form) to [Fe₃S₄]²⁻ (all-Fe²⁺ form). In a number of iron–sulfur proteins, the [Fe₄S₄] cluster can be reversibly converted by oxidation and loss of one iron ion to a [Fe₃S₄] cluster. E.g., the inactive form of aconitase possesses an [Fe₃S₄] and is activated by addition of Fe²⁺ and reductant.

Other Fe–S clusters

Examples include the active sites of a number of enzymes:

• Nitrogenase include two P-clusters ([8Fe-7S]) and two FeMocos ([7Fe-9S-C-Mo-R homocitrate]).
• Carbon monoxide dehydrogenase and acetyl coenzyme-A synthase each features an Fe-Ni-S₄ clusters.
• [FeFe]-hydrogenase features an "H-cluster", consisting of a Fe₄S₄ bridge to Fe₂ via a cystine. The Fe₂ half features unique ligands: 3 CO, 2 CN⁻, and an azadithiolate HN(CH₂S⁻)₂.
• A special 6 cysteine-coordinated [Fe₄S₃] cluster was found in oxygen-tolerant membrane-bound [NiFe] hydrogenases.
• The "double cubane cluster" [Fe₈S₉], found in some nitrogenase-related ATPases, consists of two [Fe₄S₄] bridged by a cysteine. The functions of such proteins remain unclear.