Sulfide Oxidation from Cyanobacteria to Humans: Sulfide–Quinone Oxidoreductase (SQR)

Abstract

After the discovery that anoxygenic, sulfidotrophic photosynthesis can be induced in cyanobacteria, sulfide- quinone reductase (SQR) was identified and characterized in Oscillatoria limnetica. This was closely followed by the study of SQR in the purple bacterium Rhodobacter capsulatus. Subsequently the genes of the purple bacterium and of two cyanobacteria, as well as of the hyperthermophilic hydrogen bacterium Aquifex aeolicus were cloned, sequenced and expressed in Escherichia coli, and the enzymes were characterized. Sequence analysis showed that SQR belongs to the disulfide oxidoreductase flavoprotein family, together with flavocytochrome c (FCC), another sulfide oxidizing enzyme. All the members of this family are characterized by two redox active cysteines which cooperate with the flavin in the redox cycle. A redox mechanism for SQR is proposed on the basis of site directed mutations of the cysteins and of other amino acid residues. Furthermore, a 3d-structural model is derived from the crystal structure of FCC. The search into the genomes accessible in the internet documents a widely spread occurrence of SQRgenes in bacteria. From the 19 completed canobacterial genomes, five contain the gene. Phylogenetic analysis classifies these genes into at least two clades – SQR-type I and SQR-type II. However, SQRlike enzymes are not confined to prokaryotes. They occur in the mitochondria of some fungi, as well as of all animals for which the genomes have been sequenced. From these eukarytotic SQR-like proteins (SQRDL) only the one of fission yeast was isolated and was enzymatically characterized. It is involved in heavy metal tolerance, and has therefore been denoted HMT2. Since sulfide has been indentified as a gaseous transmitter substance in animals, a possible role for SQRDL signalling is considered. Finally, phylogenetic scenarious for the descent of SQR from a common ancestor are discussed. Two observations are of special interest: (i) The mitochondrial SQRDL is of type II, although the endosymbiontic ancestor of mitochondria is considered to be a proteobacterium, which should have had a type I-SQR. (ii) The two essential cysteines among the flavoprotein family must have changed positions in the primary structure during evolution, thus constituting an example of functional plasticity within phylogenies.