The Two [4Fe-4S] Clusters in Chromatium vinosumFerredoxin Have Largely Different Reduction Potentials

We analyzed a eukaryotically encoded rubredoxin from the cryptomonad Guillardia theta and identified additional domains at the N-and C-termini in comparison to known prokaryotic paralogous molecules. The cryptophytic N-terminal extension was shown to be a transit peptide for intracellular targeting of the protein to the plastid, whereas a C-terminal domain represents a membrane anchor. Rubredoxin was identified in all tested phototrophic eukaryotes. Presumably facilitated by its C-terminal extension, nucleomorph-encoded rubredoxin (nmRub) is associated with the thylakoid membrane. Association with photosystem II (PSII) was demonstrated by co-localization of nmRub and PSII membrane particles and PSII core complexes and confirmed by comparative electron paramagnetic resonance measurements. The midpoint potential of nmRub was determined as ؉125 mV, which is the highest redox potential of all known rubredoxins. Therefore, nmRub provides a striking example of the ability of the protein environment to tune the redox potentials of metal sites, allowing for evolutionary adaption in specific electron transport systems, as for example that coupled to the PSII pathway.

Section: Rubredoxin Is Associated With Psii 30061supporting

confidence: 67%

Eukaryotically Encoded and Chloroplast-located Rubredoxin Is Associated with Photosystem II

Wastl¹,

Duin²,

Iuzzolino³

et al. 2000

“…This view was challenged in 1998 by our discovery that a ferredoxin from A. vinelandii (AvFdIII) was very similar to CvFd based on molecular weight and sequence motif and yet had very different E 0 Ј values of Ϫ486 and Ϫ644 mV for its two [4Fe-4S] 2ϩ/ϩ clusters (45). A reexamination of CvFd then led to the conclusion that it also had very different E 0 Ј of about Ϫ460 mV and about Ϫ655 mV for its two [4Fe-4S] 2ϩ/ϩ clusters (46,47). These studies further identified the lower E 0 Ј cluster of CvFd as the one with the CXXCXXC motif.…”

Section: Selection Of Mutants-eight-iron Ferredoxins Containing Two [mentioning

confidence: 85%

Azotobacter vinelandii Ferredoxin I

Chen¹,

Jung²,

Bonagura³

et al. 2002

The reduction potential (E 0 ) of the [4Fe-4S]2؉/؉ cluster of Azotobacter vinelandii ferredoxin I (AvFdI) and related ferredoxins is ϳ200 mV more negative than the corresponding clusters of Peptostreptococcus asaccharolyticus ferredoxin and related ferredoxins. Previous studies have shown that these differences in E 0 do not result from the presence or absence of negatively charged surface residues or in differences in the types of hydrophobic residues found close to the Most iron-sulfur ([Fe-S]) proteins are intimately involved in essential electron transfer reactions, including membranebound electron transport systems; their functions, however, are not restricted to electron transfer and include direct catalysis of hydration/dehydration reactions, O 2 and iron sensing, regulation of gene expression, iron storage, metal cluster assembly, and the generation and stabilization of radical intermediates (for reviews, see Refs. 1-6). To meet these diverse functions, individual proteins have increased the variety of [Fe-S] cluster structures by adding or subtracting iron and sulfide atoms to vary the cluster type, or by introducing noncysteine ligands. Even after adopting a specific cluster type (e.g. [4Fe-4S] clusters with four cysteine ligands), proteins can control reactivity further by stabilizing a particular redox couple (e.g. 3ϩ/2ϩ, 2ϩ/ϩ, or ϩ/0) or by modulation of the reduction potential (E 0 Ј) of a particular redox couple (7-19). Thus, high potential iron proteins have 3ϩ/2ϩ E 0 Ј ranging from 90 to 450 mV (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)20), while ferredoxins that contain structurally indistinguishable 2ϩ/ϩ clusters have E 0 Ј ranging from Ϫ280 to Ϫ715 mV in different native proteins (10, 21).Both experimental and theoretical research has been directed toward understanding how the polypeptide surrounding the cluster controls the reduction potential. Factors that have been proposed as being important include (a) solvent exposure of the cluster, (b) specific hydrogen bonding networks especially NH-S bonds, (c) the proximity and orientation of protein backbone and side chain dipoles, and/or (d) the proximity of charged residues to the cluster (7,(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35). This study concerns protein control of the reduction potential of a 2ϩ/ϩ cluster that is ligated via a typical CXXCXXC motif with one remote Cys ligand.Previous studies in this area have focused on comparing the environments of the clusters in structurally characterized proteins. 2ϩ/ϩ cluster of Azotobacter vinelandii ferredoxin I (AvFdI) 1 has a low E 0 Ј of about Ϫ620 mV (pH 7.0) (36), whereas the analogous clusters in Peptostreptococcus asaccharolyticus ferredoxin (PaFd) and Clostridium acidiurici ferredoxin have E 0 Ј of about Ϫ430 mV (not pH-dependent) (37). Comparison of the structures of these proteins showed that the peptide folding around the analogous clusters is highly conserved with respect to the location of the four Cys ligands, the Cys dihedral angles, and the eight amide groups hydr...

“…Similar to some bacterial ferredoxins, the [4Fe-4S] clusters of ABCE1 are stable down to redox potentials below Ϫ500 to Ϫ600 mV (32,47). Reasons for such low potentials are solventshielding of the clusters, side-chain features, cavity sizes and N-H⅐⅐⅐S hydrogen bonds (48).…”

Section: Purification and Reconstitution Of The [Fe-s] Clustermentioning

confidence: 98%

X-ray Structure of the Complete ABC Enzyme ABCE1 from Pyrococcus abyssi

Karcher

Schele

Hopfner

2008

The ATP binding cassette enzyme ABCE1 (also known as RNase-L (ribonuclease L) inhibitor, Pixie,and HP68), one of the evolutionary most sequence-conserved enzymes, functions in translation initiation, ribosome biogenesis, and human immunodeficiency virus capsid assembly. However, its structural mechanism and biochemical role in these processes have not been revealed. We determined the crystal structure of Pyrococcus abyssi ABCE1 in complex with Mg 2؉ and ADP to 2.8 Å resolution. ABCE1 consists of four structural domains. Two nucleotide binding domains are arranged in a head-to-tail orientation by a hinge domain, suggesting that these domains undergo the characteristic tweezers-like powerstroke of ABC enzymes. In contrast to all other known ABC enzymes, ABCE1 has a N-terminal iron-sulfur-cluster (FeS) domain. The FeS domain contains two [4Fe-4S] clusters and is structurally highly related to