BIOCHEMICAL AND FUNCTIONAL ANALYSIS OF SULFIDE OXIDASE ENZYMES IN PURPLE SULFUR BACTERIA
András Tóth1,2, Ágnes Duzs2, Enikő Kiss1, Brigitta Németh1, Kornél L. Kovács1,2, Gábor Rákhely1,2
1Biological Research Centre, Hungarian Academy of Sciences, Institute of Biophysics;
2University of Szeged, Department of Biotechnology
Phototrophic purple sulfur bacteria can utilize various reduced inorganic sulfur compounds (eg. sulfide) as electron donor of the metabolism during anoxygenic photosynthetic growth. In these bacteria flavocytochrome c and sulfide quinone oxidoreductase play a role in the first step of the electron transport from sulfide into the photosynthetic electron transport chain.
These enzymes are members of disulfide oxidoreductase protein family. Flavocytochrome c (Fcc) is a periplasmic enzyme consisting of a large sulfide-binding flavoprotein (FccB) and a smaller, heme c binding cytochrome c subunit (FccA). Sulfide quinone oxidoreductases are monomeric membrane-bound flavoproteins which present in all domains of life. Sqr can transfer electrons directly from sulfide into the membrane quinone pool while Fcc indirectly via small periplasmic c-type cytochrome proteins.
Thiocapsa roseopersicina is a photosynthetic purple sulfur bacterium. Three genes presumably encoding sulfide oxidizing disulfide oxidoreductase proteins were identified in the genom sequence: fcc, sqr and sqn. The Sqr and Sqn belong to different groups of Sqr-type proteins: group IV and group VI, respectively. There have not been yet detailed biochemical and structural studies about the members of the group VI. proteins.
For biochemical and functional analysis Fcc and Sqn proteins fused to Strep II affinity tag were expressed in T. roseopersicina and successfully purified. Redox active heme c and flavin prosthetic groups were identified in the purified Fcc and Sqn proteins which seem to bind covalently to the enzymes. Recombinant Fcc catalyzes the reduction of bovine heart cytochrome c in the presence of sulfide. The purified Sqn protein has sulfide-dependent quinone reductase activity. Detailed biochemical and kinetic analysis of the Sqn was performed which revealed that the enzyme affinity for sulfide is low, accordingly Sqn could play role in the sulfide oxidation at high sulfide concentration. Our model is that Fcc, Sqr and Sqn enzymes have role in the sulfide oxidation metabolic pathways at different sulfide concentration in T. roseopersicina cells. Based on our results the model of sulfur metabolism of purple sulfur bacteria have been supplemented.
Acknowledgement:
This research was supported by the European Union and the State of Hungary, co-financed by the European Social Fund in the framework of TÁMOP 4.2.4. A/2-11-1-2012-0001
‘National Excellence Program’.
