Structures of the Iron-Sulfur Flavoproteins fromMethanosarcina thermophilaandArchaeoglobus fulgidus

Abstract

Iron-sulfur flavoproteins (ISF) constitute a widespread family of redox-active proteins in anaerobic prokaryotes. Based on sequence homologies, their overall structure is expected to be similar to that of flavodoxins, but in addition to a flavin mononucleotide cofactor they also contain a cubane-type [4Fe:4S] cluster. In order to gain further insight into the function and properties of ISF, the three-dimensional structures of two ISF homologs, one from the thermophilic methanogen Methanosarcina thermophila and one from the hyperthermophilic sulfate-reducing archaeon Archaeoglobus fulgidus, were determined. The structures indicate that ISF assembles to form a tetramer and that electron transfer between the two types of redox cofactors requires oligomerization to juxtapose the flavin mononucleotide and [4Fe:4S] cluster bound to different subunits. This is only possible between different monomers upon oligomerization. Fundamental differences in the surface properties of the two ISF homologs underscore the diversity encountered within this protein family.The ISF (iron-sulfur flavoprotein) family is distinct from other flavin mononucleotide (FMN)-containing flavoproteins based on overall sequence identity and an unusually compact cysteine motif (24). The prototype of ISF was characterized from Methanosarcina thermophila (ISF-Mt), a strictly anaerobic methane-producing thermophile belonging to the domain Archaea (16). Since then, homologs of ISF have been identified in the genomes of numerous anaerobic prokaryotes belonging to the domains Bacteria and Archaea (24). Remarkably, many species contain multiple ISF homologs; indeed, the Methanosarcina acetivorans genome is annotated with 19 homologs (12). The only ISF homolog identified from a member of the domain Eukarya is that from the intestinal anaerobic pathogen Entamoeba histolytica (24). Thus, it appears that the ISF family occurs predominantly in anaerobic prokaryotes.ISF from M. thermophila has been reported to be a functional homodimer, with each monomer containing one FMN and one [4Fe:4S] cluster (1, 16). Ferredoxin can act as an electron donor, and redox potential measurements support the hypothesis that electron flow proceeds from ferredoxin to the low-potential (Ϫ394 mV) [4Fe:4S] cluster and then to the flavin (1). Although a downstream electron acceptor is unknown, the inability to detect flavin semiquinone during redox titrations suggests that the protein environment of ISF-Mt stabilizes the hydroquinone form and that ISF-Mt functions as a one-electron/two-electron switch. A role for ISF in electron transport coupled to methane formation has been postulated (16); however, the presence of multiple ISF homologs in metabolically diverse anaerobic prokaryotes (24) suggests broader physiological functions.The cysteine motif (Cys47, Cys50, Cys53, and Cys59) in ISF-Mt is highly conserved among the deduced sequences of all ISF homologs, consistent with a role in ligation of the [4Fe:4S] cluster. The compact nature of the motif is unusual compared to motifs known t...

Crystal Structure of the NADH:Quinone Oxidoreductase WrbA from Escherichia coli

“…At the FMN site, the N 5 atom is shielded by a phenylalanine residue, Phe 80, and a similar situation is found in ISF-3 of A. fulgidus, where Phe 84 is found at this position; in the ISF of M. thermophila a methionine, Met 86 provides a potentially more reactive environment (1). In all cases, the roof of the active site cavity is set to interact with aromatics in a stacking interaction between the FMN moiety and Trp 98 in E. coli WrbA, Arg 104 in M. thermophila ISF, and Arg 102 in A. fulgidus ISF-3.…”

“…The tetrameric quaternary structure of WrbA of E. coli, as well as of its homologs from D. radiodurans and P. aeruginosa, was first observed in a different family of flavoproteins, the ISFs from M. thermophila and A. fulgidus (1,37). These proteins show the same, typical flavodoxin fold with a fivestranded ␤-␣-␤-topology, but they are characterized by an additional loop region between strand ␤2 and helix ␣2 that contains a compact Cys-X 2 -Cys-X 2 -Cys-X 5 -Cys motif.…”

“…These proteins show the same, typical flavodoxin fold with a fivestranded ␤-␣-␤-topology, but they are characterized by an additional loop region between strand ␤2 and helix ␣2 that contains a compact Cys-X 2 -Cys-X 2 -Cys-X 5 -Cys motif. A cubane-type [4Fe:4S] cluster is bound to this motif, and the loop attains a conformation that brings this cluster in close proximity to an FMN cofactor not of the same, but of a neighboring, subunit in the tetramer (1). A structural comparison between WrbA and ISF underlines the high homology of both classes of proteins, particularly in the region of the central ␤-sheet (residues 70 to 150), where the structures of M. thermophila ISF and E. coli WrbA align with an RMSD of their C ␣ atoms of 0.74 Å (Fig.…”

The Prevalence of Excessive Mobile Phone Use and its Relation With Mental Health Status and Demographic Factors Among the Students of Gonabad University of Medical Sciences in 2011 - 2012

“…For example, studies showed that mobile phone addiction could be related or effective on impulsivity and addiction (7), low self-esteem and depression (8), control problems and the symptoms of depression, anxiety and Internet addiction and lower self-confidence (9), high impulsivity, feeling of loneliness and social anxiety (10), extroversion and self-esteem (5), depression, distractibility, lack of tolerance, withdrawal, and escape (11), impulsivity and loneliness (12) and negative consequences such as emotionally-damaged relationships and also reduction of academic achievement (13), family relationship decrease, social isolation, loneliness and depression (7,14); in a study conducted on Hong Kongese youth, psychological characteristics of so-called addiction of mobile phone were identified and four addiction symptoms to mobile phone use were shown such as: loss of control and complaining, anxiety and ravenous and excessive spending habit, withdrawal, neglecting or loss of productivity. The results showed that those who feel mostly fatigue in unemployment times are mostly dependent on mobile phone use and on the contrary the individuals with more self-esteem feel less dependency on or less willing to use a mobile phone (15).…”

WrbA from Escherichia coli and Archaeoglobus fulgidus Is an NAD(P)H:Quinone Oxidoreductase

“…In surveying the BLAST results, it was discovered that several sequences annotated as WrbA contain a motif strictly conserved in another FMN-containing protein family called iron-sulfur flavoprotein (Isf) (2,5,38,39,67). However, the Isf family contains an unusual compact cysteine motif (CX 2 CX 2 CX 5-7 C) that binds a 4Fe-4S cluster not found in the sequences of WrbA proteins or the NQO enzymes aforementioned.…”