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Hydrogen Photoproduction by Oxygenic Photosynthetic Microorganisms
Published in Farshad Darvishi Harzevili, Serge Hiligsmann, Microbial Fuels, 2017
Fabrice Franck, Bart Ghysels, Damien Godaux
[FeFe]-hydrogenases are mainly monomeric enzymes. Their catalytic center is highly conserved and consists of a binuclear iron site bound to a [4Fe-4S] by a cysteine bridge. Nonprotein ligands CN– and CO are attached to the iron atoms of the binuclear Fe center (Nicolet et al., 2000). The Fe atoms also share two bridging sulfur ligands of a small five-atom molecule (Nicolet et al., 2002). Additional domains accommodate the Fe-S center and relay the electron transfer from the external electron source to the H cluster buried inside this monomeric protein. Moreover, a hydrophobic channel from the surface to the active site provides access for protons and egress for H2. This unique active center leads to about a 100-fold higher activity than for the other types of hydrogenases (Frey, 2002; Vogt et al., 2007). Three accessory proteins, named HydE, HydF, and HydG, are required for the proper biosynthesis of [FeFe]-hydrogenases. HydE and HydG belong to the radical S-adenosylmethionine (radical SAM) superfamily, whereas the HydF maturation factor belongs to the GTPase protein family (for review, see Vignais et al., 2001).
The enigma of environmental organoarsenicals: Insights and implications
Published in Critical Reviews in Environmental Science and Technology, 2022
Xi-Mei Xue, Chan Xiong, Masafumi Yoshinaga, Barry Rosen, Yong-Guan Zhu
The biosynthetic pathways for arsenosugars have not been studied well compared with those for simple methylarsenicals, probably because arsenosugars are identified mainly in marine macroalgae or animals, more complex systems than bacteria. However, the recent discovery that arsenosugars are produced by prokaryotic cyanobacteria has stimulated research on these systems (Xue et al., 2014, Xue et al., 2017a). Dimethylated arsenosugars are composed of a pentavalent dimethylarsinoy moiety and a 5′-deoxyriboside. Cells of the cyanobacterium Synechocystis sp. PCC 6803 produce arsenosugars when exposed to As(III), MAs(V) or DMAs(V). In contrast, cells in which the arsM gene was deleted produced arsenosugars only when incubated with DMAs(V), but not with either As(III) or MAs(V), suggesting that the ArsM is involved in arsenosugar biosynthesis via production of the precursor DMAs(III) (Xue et al., 2017b). In prokaryote genomes, multiple genes with related functions often form a cluster, or an operon, under the control of a single promoter. A gene adjacent to SsarsM (the arsM gene of Synechocystis sp. PCC 6803) has been termed SsarsS and is annotated to encode a radical SAM superfamily protein characterized by four iron-four sulfur ([4Fe-4S]) clusters and SAM-binding sites (Xue et al., 2019). Neither SsarsM nor an SsarsS mutant was able to produce arsenosugars, suggesting that both genes are necessary for the synthesis of arsenosugars. Recently, purified SsArsS was demonstrated to catalyze the formation of 5′-deoxy-5′-dimethylarsinoyladenosine (Table 1.23) by forming an adenosine radical (Fig. 1C) (Cheng et al., 2021). E. coli cells expressing both the SsarsM and SsarsS genes produced DMAs(V) and dimethylarsinoyl hydroxycarboxylic acid derivatives but not arsenosugars, suggesting that E. coli cells may degrade 5′-deoxy-5′-dimethylarsinoyladenosine (Xue et al., 2019). Dimethylarsinoylalchols (DMAE and thio-DMAE), dimethylarsinoyl-carboxylic acids (DMAA and thio-DMAA), and methylarsenicals (MAs(V) and DMAs(V)) have been detected in mammalian urine and feces after ingestion of arsenosugars (Francesconi et al., 2002). Bacteria may play a role in degradation of arsenosugars in animals. In contrast, Oxo-Gly was only converted into thio-Gly in an in vitro artificial gastrointestinal digestion system, indicating the complexity of degradation of arsenosugars (Hata et al., 2019). Microorganisms and enzymes involved in the pathway of arsenosugar degradation remains to be identified.