Publications

• Reduction of benzoquinones to hydroquinones via spontaneous reaction with glutathione and enzymatic reaction by S-glutathionyl-hydroquinone reductases.

  Lam L.K., Zhang Z., Board P.G., Xun L.

  S-Glutathionyl-hydroquinone reductases (GS-HQRs) are a new class of glutathione transferases, widely present in bacteria, halobacteria, fungi, and plants. They catalyze glutathione (GSH)-dependent reduction of GS-trichloro-p-hydroquinone to trichloro-p-hydroquinone. Since GS-trichloro-p-hydroquino ... >> More

  S-Glutathionyl-hydroquinone reductases (GS-HQRs) are a new class of glutathione transferases, widely present in bacteria, halobacteria, fungi, and plants. They catalyze glutathione (GSH)-dependent reduction of GS-trichloro-p-hydroquinone to trichloro-p-hydroquinone. Since GS-trichloro-p-hydroquinone is uncommon in nature, the extensive presence of GS-HQRs suggests they use common GS-hydroquinones. Here we demonstrate that several benzoquinones spontaneously reacted with GSH to form GS-hydroquinones via Michael addition, and four GS-HQRs from yeast and bacteria reduced the GS-hydroquinones to the corresponding hydroquinones. The spontaneous and enzymatic reactions led to the reduction of benzoquinones to hydroquinones with the concomitant oxidation of GSH to oxidized glutathione (GS-SG). The enzymes did not use GS-benzoquinones or other thiol-hydroquinones, for example, S-cysteinyl-hydroquinone, as substrates. Apparent kinetic parameters showed the enzymes preferred hydrophobic, bulky substrates, such as GS-menadiol. The broad substrate range and their wide distribution suggest two potential physiological roles: channeling GS-hydroquinones back to hydroquinones and reducing benzoquinones via spontaneous formation of GS-hydroquinones and then enzymatic reduction to hydroquinones. The functions are likely important in metabolic pathways with quinone intermediates. << Less

  Biochemistry 51:5014-5021(2012) [PubMed] [EuropePMC]

• Structural understanding of GSH-dependent reduction mechanism of glutathionyl-hydroquinone reductases.

  Green A.R., Hayes R.P., Xun L., Kang C.

  Glutathionyl-hydroquinone reductases (GS-HQRs) are a newly identified group of glutathione transferases, and they are widely distributed in bacteria, halobacteria, fungi, and plants. GS-HQRs catalyze glutathione (GSH)-dependent reduction of glutathionyl-hydroquinones (GS-hydroquinones) to hydroqui ... >> More

  Glutathionyl-hydroquinone reductases (GS-HQRs) are a newly identified group of glutathione transferases, and they are widely distributed in bacteria, halobacteria, fungi, and plants. GS-HQRs catalyze glutathione (GSH)-dependent reduction of glutathionyl-hydroquinones (GS-hydroquinones) to hydroquinones. GS-hydroquinones can be spontaneously formed from benzoquinones reacting with reduced GSH via Michael addition, and GS-HQRs convert the conjugates to hydroquinones. In this report we have determined the structures of two bacterial GS-HQRs, PcpF of Sphingobium chlorophenolicum and YqjG of Escherichia coli. The two structures and the previously reported structure of a fungal GS-HQR shared many features and displayed complete conservation for all the critical residues. Furthermore, we obtained the binary complex structures with GS-menadione, which in its reduced form, GS-menadiol, is a substrate. The structure revealed a large H-site that could accommodate various substituted hydroquinones and a hydrogen network of three Tyr residues that could provide the proton for reductive deglutathionylation. Mutation of the Tyr residues and the position of two GSH molecules confirmed the proposed mechanism of GS-HQRs. The conservation of GS-HQRs across bacteria, halobacteria, fungi, and plants potentiates the physiological role of these enzymes in quinone metabolism. << Less

  J. Biol. Chem. 287:35838-35848(2012) [PubMed] [EuropePMC]

• S-Glutathionyl-(chloro)hydroquinone reductases: a novel class of glutathione transferases.

  Xun L., Belchik S.M., Xun R., Huang Y., Zhou H., Sanchez E., Kang C., Board P.G.

  Sphingobium chlorophenolicum completely mineralizes PCP (pentachlorophenol). Two GSTs (glutathione transferases), PcpC and PcpF, are involved in the degradation. PcpC uses GSH to reduce TeCH (tetrachloro-p-hydroquinone) to TriCH (trichloro-p-hydroquinone) and then to DiCH (dichloro-p-hydroquinone) ... >> More

  Sphingobium chlorophenolicum completely mineralizes PCP (pentachlorophenol). Two GSTs (glutathione transferases), PcpC and PcpF, are involved in the degradation. PcpC uses GSH to reduce TeCH (tetrachloro-p-hydroquinone) to TriCH (trichloro-p-hydroquinone) and then to DiCH (dichloro-p-hydroquinone) during PCP degradation. However, oxidatively damaged PcpC produces GS-TriCH (S-glutathionyl-TriCH) and GS-DiCH (S-glutathionyl-TriCH) conjugates. PcpF converts the conjugates into TriCH and DiCH, re-entering the degradation pathway. PcpF was further characterized in the present study. It catalysed GSH-dependent reduction of GS-TriCH via a Ping Pong mechanism. First, PcpF reacted with GS-TriCH to release TriCH and formed disulfide bond between its Cys53 residue and the GS moiety. Then, a GSH came in to regenerate PcpF and release GS-SG. A TBLASTN search revealed that PcpF homologues were widely distributed in bacteria, halobacteria (archaea), fungi and plants, and they belonged to ECM4 (extracellular mutant 4) group COG0435 in the conserved domain database. Phylogenetic analysis grouped PcpF and homologues into a distinct group, separated from Omega class GSTs. The two groups shared conserved amino acid residues, for GSH binding, but had different residues for the binding of the second substrate. Several recombinant PcpF homologues and two human Omega class GSTs were produced in Escherichia coli and purified. They had zero or low activities for transferring GSH to standard substrates, but all had reasonable activities for GSH-dependent reduction of disulfide bond (thiol transfer), dehydroascorbate and dimethylarsinate. All the tested PcpF homologues reduced GS-TriCH, but the two Omega class GSTs did not. Thus PcpF homologues were tentatively named S-glutathionyl-(chloro)hydroquinone reductases for catalysing the GSH-dependent reduction of GS-TriCH. << Less

  Biochem. J. 428:419-427(2010) [PubMed] [EuropePMC]

• Maintenance role of a glutathionyl-hydroquinone lyase (PcpF) in pentachlorophenol degradation by Sphingobium chlorophenolicum ATCC 39723.

  Huang Y., Xun R., Chen G., Xun L.

  Pentachlorophenol (PCP) is a toxic pollutant. Its biodegradation has been extensively studied in Sphingobium chlorophenolicum ATCC 39723. All enzymes required to convert PCP to a common metabolic intermediate before entering the tricarboxylic acid cycle have been characterized. One of the enzymes ... >> More

  Pentachlorophenol (PCP) is a toxic pollutant. Its biodegradation has been extensively studied in Sphingobium chlorophenolicum ATCC 39723. All enzymes required to convert PCP to a common metabolic intermediate before entering the tricarboxylic acid cycle have been characterized. One of the enzymes is tetrachloro-p-hydroquinone (TeCH) reductive dehalogenase (PcpC), which is a glutathione (GSH) S-transferase (GST). PcpC catalyzes the GSH-dependent conversion of TeCH to trichloro-p-hydroquinone (TriCH) and then to dichloro-p-hydroquinone (DiCH) in the PCP degradation pathway. PcpC is susceptible to oxidative damage, and the damaged PcpC produces glutathionyl (GS) conjugates, GS-TriCH and GS-DiCH, which cannot be further metabolized by PcpC. The fate and effect of GS-hydroquinone conjugates were unknown. A putative GST gene (pcpF) is located next to pcpC on the bacterial chromosome. The pcpF gene was cloned, and the recombinant PcpF was purified. The purified PcpF was able to convert GS-TriCH and GS-DiCH conjugates to TriCH and DiCH, respectively. The GS-hydroquinone lyase reactions catalyzed by PcpF are rather unusual for a GST. The disruption of pcpF in S. chlorophenolicum made the mutant lose the GS-hydroquinone lyase activities in the cell extracts. The mutant became more sensitive to PCP toxicity and had a significantly decreased PCP degradation rate, likely due to the accumulation of the GS-hydroquinone conjugates inside the cell. Thus, PcpF played a maintenance role in PCP degradation and converted the GS-hydroquinone conjugates back to the intermediates of the PCP degradation pathway. << Less

  J. Bacteriol. 190:7595-7600(2008) [PubMed] [EuropePMC]