Hemerythrins are oxygen-binding proteins found in the body fluids and tissues of certain invertebrates. Oxygen is bound at a nonheme iron centre consisting of two oxo-bridged iron atoms bound to a characteristic set of conserved histidine: aspartate and glutamate residues with the motifs H-HxxxE-HxxxH-HxxxxD. It has recently been demonstrated biochemically that two bacterial proteins bearing the same motifs do in fact possess similar iron centres and bind oxygen in the same way. The recent profusion of prokaryotic genomic sequence data has shown that proteins bearing hemerythrin motifs are present in a wide variety of bacteria, and a few archaea. Some of these are short proteins as in eukaryotes; others appear to consist of a hemerythrin domain fused to another domain, generally a putative signal transduction domain such as a methyl-accepting chemotaxis protein, a histidine kinase, or a GGDEF protein (cyclic di-GMP synthase). If, as initial evidence suggests, these are in fact hemerythrin-like oxygen-binding proteins, then their diversity in prokaryotes far exceeds that seen in eukaryotes. Here, a survey is presented of prokaryotic protein sequences bearing hemerythrin-like motifs, for which the designation 'bacteriohemerythrins' is proposed, and their functions are speculated.

Flavocytochrome c from the Gram-negative, food-spoiling bacterium Shewanella putrefaciens is a soluble, periplasmic fumarate reductase. We have isolated the gene encoding flavocytochrome c and determined the complete DNA sequence. The predicted amino acid sequence indicates that flavocytochrome c is synthesized with an N-terminal secretory signal sequence of 25 amino acid residues. The mature protein contains 571 amino acid residues and consists of an N-terminal cytochrome domain, of about 117 residues, with four heme attachment sites typical of c-type cytochromes and a C-terminal flavoprotein domain of about 454 residues that is clearly related to the flavoprotein subunits of fumarate reductases and succinate dehydrogenases from bacterial and other sources. A second reading frame that may be cotranscribed with the flavocytochrome c gene exhibits some similarity with the 13-kDa membrane anchor subunit of Escherichia coli fumarate reductase. The sequence of the flavoprotein domain demonstrates an even closer relationship with the product of the yeast OSM1 gene, mutations in which result in sensitivity to high osmolarity. These findings are discussed in relation to the function of flavocytochrome c.

Two groups have now published sequences of the six genes contained in the operon coding for the sodium-linked NADH-ubiquinone oxidoreductase of Vibrio aiginolyticus. Sequence analyses indicate that this enzyme is unrelated to other known respiratory NADH dehydrogenases. A search for cofactor motifs suggests that the enzyme contains only one FAD, a ferredoxin-type iron sulphur centre, and the NADH-binding site. These are all located on NqrF, a subunit that can be recognized as a new member of a large diverse family of NAD(P)H-oxidizing flavoenzymes. A possible model of ion-coupling is presented, based upon this new information.Key words." NADH-ubiquinone oxidoreductase; NADH dehydrogenase; Sodium ion translocation BackgroundSeveral types of enzyme (NQR) are known to catalyse the respiratory reaction of oxidation of NADH by membranebound ubiquinone. Only one, termed complex I, is found in mammalian mitochondria, where it oxidizes internally generated NADH. Bovine complex I is remarkably complicated, with seven mitochondrially encoded and at least 34 further nuclear encoded subunits [1]. Gene clusters coding for proton translocating NADH-ubiquinone oxidoreductases have also been identified in the prokaryotes Paracoccus denitrificans, Rhodobacter capsulatus and Escherichia coli [24]. These comprise homologues of only 14 of the mammalian subunits [5] and, presumably, represent a more minimal core catalytic structure. The purified enzyme from E. eoli can be subfractionated into three domains, viz. a fragment (FP) of three subunits which contains the NADH site, FMN and iron sulphur centres; a more amphipathic fragment (IP) of four subunits also containing iron sulphur centres; and a very hydrophobic fragment (HP) of seven proteins which are homologues of the mitochondrially encoded subunits of eukaryotes. In the mitochondrial enzyme, one HP subunit (ND 1) contains a ubiquinone-binding site while another (ND2) is reactive with rotenone and DCCD [1]. All homologues of complex I are presumed to be coupled to proton translocation and have been generically termed NDH-1 types [5,6]. *Corresponding author. Fax: (44) (1208) 821 575. E-mail: mbprr@seqnet.dl.ac.uk Two other types of NADH dehydrogenase can be found in mitochondria from fungal and plant sources [7]. One of these oxidizes internally generated NADH but, in contrast to complex I, it is not coupled to proton translocation. It is likely to be homologous to a non-protonmotive NQR in bacteria, termed NDH-2 [6]. The enzyme may be a single polypeptide, whose sequence has been established [8][9][10], and with FAD as the only redox cofactor [6]. A third NADH dehydrogenase in the inner membrane of plant and fungal mitochondria, which we propose might be termed NDH-3, has an externally facing NADH site and can directly oxidize cytosolic NADH [11]. This enzyme is also not coupled to proton translocation [12]. A bacterial homologue has not been identified and its detailed structure and composition remain unknown.A further type of NQR was first found in the marine bacter...

Exoelectrogenic bacteria are organisms that can transfer electrons to extracellular insoluble electron acceptors and have the potential to be used in devices such as microbial fuel cells (MFCs). Currently, exoelectrogens have been identified in the Alpha-, Beta-, Gamma-and Deltaproteobacteria, as well as in the Firmicutes and Acidobacteria. Here, we describe use of culture-independent methods to identify two members of the genus Arcobacter in the Epsilonproteobacteria that are selectively enriched in an acetate-fed MFC. One of these organisms, Arcobacter butzleri strain ED-1, associates with the electrode and rapidly generates a strong electronegative potential as a pure culture when it is supplied with acetate. A mixed-community MFC in which ϳ90% of the population is comprised of the two Arcobacter species generates a maximal power density of 296 mW/liter. This demonstration of exoelectrogenesis by strain ED-1 is the first time that this property has been shown for members of this genus.

The major soluble cytochrome isolated from microaerobically grown cells of Shewanella putrefaciens has been shown to be a novel type of flavocytochrome with fumarate reductase activity. This flavocytochrome, located in the periplasmic fraction of cell extracts, has been purified to homogeneity and shown to contain 4 mol of haem c and 1 mol of non-covalently bound FAD per mol of protein. An M(r) value of 63,800 is estimated from sequence analysis assuming 4 mol of haem/mol of protein. In the presence of the artificial electron donor, reduced methyl viologen, the flavocytochrome catalysed the reduction of fumarate but not that of nitrite, dimethylsulphoxide, trimethylamine-N-oxide or sulphite. The pH optimum was 7.4 with calculated pKa values of 6.8 and 8.0 for contributing catalytic groups. The Km and kcat. values for fumarate reduction were 21 microM and 250 s-1 respectively, whereas the corresponding values for succinate oxidation with 2,6-dichlorophenol-indophenol as electron carriers were 200 microM and 0.07 s-1 respectively. Mesaconic acid was a competitive inhibitor of fumarate reduction with a Ki of 2 microM. Zymogram staining of polyacrylamide gels with purified protein showed a band of fumarate reductase activity. Polyclonal antibodies, raised to the purified flavocytochrome, were shown to titrate out fumarate reductase activity. We conclude that the physiological role of this enzyme is as a fumarate reductase. Optical absorption spectra of the flavocytochrome indicated that all the haems were of the c-type and gave alpha, beta and gamma peaks at 552.3, 523 and 418 nm in the reduced spectrum with epsilon values of 30.2, 15.9 and 188.2 mM-1.cm-1 respectively. Oxidized spectra showed no 695 nm band that would be indicative of His-Met coordination. Two redox potentials were resolved at -220 mV and -320 mV. The cytochrome was reduced by formate in the presence of particulate cell fractions. The relationship of this cytochrome to other low-potential flavocytochromes c is discussed.