A novel enzyme of type VI sulfide:quinone oxidoreductases in purple sulfur photosynthetic bacteria

Abstract

Sulfide detoxification can be catalyzed by ancient membrane-bound flavoproteins, sulfide:quinone oxidoreductases (Sqr), which have important roles in sulfide homeostasis and sulfide-dependent energy conservation processes by transferring electrons from sulfide to respiratory or photosynthetic membrane electron flow. Sqr enzymes have been categorized into six groups. Several members of the groups I, II, III, and V are well-known, but type IV and VI Sqrs are, as yet, uncharacterized or hardly characterized at all. Here, we report detailed characterization of a type VI sulfide:quinone oxidoreductase (TrSqrF) from a purple sulfur bacterium, Thiocapsa roseopersicina. Phylogenetic analysis classified this enzyme in a special group composed of SqrFs of endosymbionts, while a weaker relationship could be observed with SqrF of Chlorobaculum tepidum which is the only type VI enzyme characterized so far. Directed mutagenesis experiments showed that TrSqrF contributed substantially to the sulfide:quinone oxidoreductase activity of the membranes. Expression of the sqrF gene could be induced by sulfide. Homologous recombinant TrSqrF protein was expressed and purified from the membranes of a SqrF-deleted T. roseopersicina strain. The purified protein contains redox-active covalently bound FAD cofactor. The recombinant TrSqrF enzyme catalyzes sulfur-dependent quinone reduction and prefers ubiquinone-type quinone compounds. Kinetic parameters of TrSqrF show that the affinity of the enzyme is similar to duroquinone and decylubiquinone, but the reaction has substantially lower activation energy with decylubiquinone, indicating that the quinone structure has an effect on the catalytic process. TrSqrF enzyme affinity for sulfide is low, therefore, in agreement with the gene expressional analyis, SqrF could play a role in energy-conserving sulfide oxidation at high sulfide concentrations. TrSqrF is a good model enzyme for the subgroup of type VI Sqrs of endosymbionts and its characterization might provide deeper insight into the molecular details of the ancient, anoxic, energy-gaining processes using sulfide as an electron donor.

H2S and reactive sulfur signaling at the host-bacterial pathogen interface

“…An important enzymatic route to the biogenesis of RSS in bacteria is the sulfide:quinone oxidoreductase (SQR) (76,(123)(124)(125). SQR catalyzes the two-electron oxidation of H 2 S to sulfane sulfur fixed as organic and inorganic per-and polysulfides ( Fig.…”

Insights into the catalytic mechanism of type VI sulfide:quinone oxidoreductases

“…The biochemical, enzymatic, and structural characterization of SQR enzymes has been limited by the difficulties in their isolation in a stable, active form from their natural host [15,16] or via heterologous protein expression [17][18][19][20][21][22][23] in, for example, E. coli. The SQR enzymes catalyze a complex redox reaction composed of reductive and oxidative halfreaction.…”

“…The bacterium has complex sulfur metabolism for the oxidizing and/or reducing sulfurs of various oxidation states [31,32]. In the genomic sequence of T. roseopersicina [33], genes of two SQR enzymes of types IV and VI SQRs, SqrD and SqrF, respectively, were discovered [16]. The genes coding for another sulfide oxidase enzyme complex, flavocytochrome c sulfide dehydrogenase (Fcc), were also identified which interestingly has a sulfide-oxidizing subunit homologous to SQR enzymes (Balogh et al, personal communication) [16].…”

A deep-sea sulfate reducing bacterium directs the formation of zero-valent sulfur via sulfide oxidation

“…The production of ZVS has been regarded as a bio-signature of sulfur-oxidizing microorganisms 9,10 . The process of ZVS production begins with the formation of polysulfide through the oxidation of thiosulfate or sulfide 3,[11][12][13] . For the formation process of ZVS mediated by thiosulfate oxidation, there are at least four pathways identified in the sulfur oxidizing bacteria (SOB) 11,13 , including Sox pathway 14 , tetrathionate (S 4 I) intermediate pathway 15 , Sox-S 4 I interaction system 16 and a novel pathway mediated by thiosulfate dehydrogenase (TsdA) and thiosulfohydrolase (SoxB) 17 .…”

“…For the formation process of ZVS mediated by thiosulfate oxidation, there are at least four pathways identified in the sulfur oxidizing bacteria (SOB) 11, 13 , including Sox pathway 14 , tetrathionate (S 4 I) intermediate pathway 15 , Sox-S 4 I interaction system 16 and a novel pathway mediated by thiosulfate dehydrogenase (TsdA) and thiosulfohydrolase (SoxB) 17 . For the formation process of ZVS mediated by sulfide oxidation, sulfide: quinone oxidoreductase (SQR) has been proposed to be the key enzyme to catalyze the formation of ZVS in various sulfur-oxidizing Alphaproteobacteria and Gammaproteobacteria 11, 12, 18 . SQR is a membrane associated protein that oxidizes sulfide to ZVS and transfers electrons to the membrane quinone pool with flavin adenine dinucleotide 19 .…”