Polysulfide reductase

Polysulfide reductase Bacterial aPy (MPTpG)Mo( )h Unknown Unknown 228,294... 

Tetrathionate reductase converts tetrathionate to two equivalents of thiosulfate, with no change in the number of oxygen atoms. Polysulfide reductase catalyzes the conversion of sulfur (in the form of a polysufide) to sulfide (HS ), a two-electron transfer process, through which a sulfur atom is reduc-... 

Most of the substrate reactions catalyzed by the molybdenum and tungsten enzymes involve either incorporation or removal of an oxygen atom. For a CEPT process to apply to these reactions, transfer of protons and electrons must occur concomitantly with either the addition or elimination of water (see Section VLD). However, the substrate reactions of polysulfide reductase and formate dehydrogenase [122,228] (Eq. 14 and 15)... 

Mechanisms of action for the metal centers in acetylene hydratase, polysulfide reductase, and formate dehydrogenase have been briefly described in Sections VI.A and VLB. The discussion, in each case, was relatively straightforward insofar as the natures of these reactions lend themselves to simple mechanistic proposals. The mechanism by which the metal centers function in most of the other Mo and W enzymes is not as obvious. We elect to discuss mechanistic roles for the molybdenum centers in xanthine oxidase, sulfite oxidase, and dmso reductase. These enzymes are representative members of each large class of molybdenum enzymes, and the large body of literature on each enzyme makes detailed discussion possible. 

Included in Table I are molybdenum enzymes that are as yet unclassified due to their partial characterization (46—49, 58). These enzymes includes polysulfide reductase that accomplish sulfur reduction to sulfide (46), underlining its role in the global sulfur cycling. Chlorate and selenate reductase are examples of relatively rare enzymes using simple oxyanions of third-row elements as substrates (47 19, 58). 

As for the reactions catalyzed by members of this third family of molybdenum enzymes, there are several variations from the principal theme of oxygen atom transfer. Formate dehydrogenase from E. coli catalyzes the oxidation of formate to CO2, a reaction that isotope studies have shown does not pass through a bicarbonate intermediate (Khangulov et al., 1998). Instead, it appears likely that C02is formed by direct hydride transfer from substrate to the molybdenum center. Polysulfide reductase is another molybdenum enzyme that catalyzes a non-canonical reaction the... 

Jankielewicz, A., Klimmeck, O., and Kroger, A., 1995, The electron transfer from hydrogenase and formate dehydrogenase to polysulfide reductase in the membrane of Wolinella suc-cinogenes, Biochim. Biophys. Acta 1231 1579162. 

Reductive cleavage of sulfur-sulfur bonds in inorganic sulfur species is exploited in terminal electron transfer in some bacteria. The tetrathionate and thiosulfate reductases of the enteric bacterium Salmonella enterica LT2 and the polysulfide reductase of the rumen bacterium Wolinella succinogenes effect such reactions. ... 

The physiological fimction of 3HCc3 is not completely established. The cytochrome from D. acetoxidans, has a high polysulfide reductase activity and was therefore proposed to act as the terminal reductase when this bacterium is growing with elemental sullur as electron acceptor. It also displays high activity in the reduction of several metal oxides... 

Figure 3 Sequence alignment of the N-termini of ArrA and molybdenum-containing proteins. The sequences belong to the arsenate reductase of Chrysiogenes arsenatis C.a ArrA) (14), the polysulfide reductase of WoUnella succinogenes (Wii. PsrA), and the periplasmic nitrate reductases of Escherichia coli (E.c. NapA) and Pseudomonas sp. G-179 P.s. sp. NapA). Boxed amino acids show identity to C. arsenatis ArrA.

Polysulfide reductase The isolated polysulfide reductase consists of the three subunits predicted by the psrABC operon, and contains a molybdenum ion coordinated by two molecules of molybdopterin guanine dinucleotide (MGD) [26,28] (Fig. 2). These cofactors are likely to be bound to the catalytic subunit PsrA together with a [4Fe-4S] iron-sulfur center which is predicted by the sequence of PsrA. Four additional [4Fe-4S] iron-sulfur centers are predicted by the sequence of PsrB. The isolated enzyme contains 20 mol of free iron and... 

Figure 3 Recording of the external TPP concentration in a suspension of proteoliposomes catalyzing poly sulfide sulfur reduction by formate. Proteoliposomes (3.0 g phospholipide L ) containing formate dehydrogenase (62 nmol g phospholipid), polysulfide reductase (30 nmol g phospholipid) and 8-methyl-menaquinone (10 pimolg phospholipid) were suspended in an anoxie Tris chloride buffer (pH 8.0, 37°C). Calibration of the TPP" electrode was done by four additions of 1 piM TPP" (thin arrows). Electron transport was started by the addition of polysulfide sulfur ([S]) and formate.

As seen from its amino acid sequence, the catalytic subunit PsrA of polysulfide reductase belongs to the DMSO reductase family of molybdo-oxidoreductases [26,29]. The structure of several single-subunit enzymes of this family is known. As a rule, the molybdenum ion coordinated by two MGD molecules appears to be the electron donor or acceptor to the respective substrate in these enzymes. A cavity extending from the surface of the protein to the molybdenum close to its center is seen in all structures. The substrates probably reach the active site near the molybdenum through this cavity, and the products are released at the surface through the cavity. With the assumption that PsrA... 

Figure 4 Hypothetical mechanism of polysulfide sulfur reduction at the substrate site of polysulfide reductase. A sulfur atom is cleaved from the end of the polysulfide sulfur chain (—S — SI ) and bound to the molybdenum ion (Mo) which is thereby oxidized. After the uptake of a proton and two electrons, HS is released and molybdenum ion is in the reduced state again.

The results suggest that the site of quinone reduction on HydC is located close to the cytoplasmic membrane surface as on E. coli FdhI. Furthermore, HydC and its site of quinone reduction appear to be involved in the electron transfer from hydrogenase to polysulfide reductase. The function of FdhC of W. succinogenes formate dehydrogenase in quinone reduction and in electron transfer to polysulfide reductase is thought to be equivalent to that of HydC. 

Proteoliposomes containing polysulfide reductase and either hydrogenase or formate dehydrogenase isolated from W. succinogenes do not catalyze polysulfide sulfur respiration unless 8-methyl-menaquinone is present (Table 3). Menaquinone with a side chain consisting of six or four isoprene units, or vitamin Ki served in reconstituting fumarate respiration, but did not replace 8-methyl-menaquinone in polysulfide sulfur respiration. The low activities of polysulfide sulfur respiration observed without added 8-methyl-menaquinone were probably due to the small amounts of this quinone associated with the enzyme preparations used. Maximum activity of polysulfide sulfur respiration required 10 p.mol 8-methyl-menaquinone per gram phospholipid [O. Klimmek and W. Dietrich, unpublished results]. 

Since electron transfer from the dehydrogenases to polysulfide reductase by diffusion of 8-methyl-menaquinone is unlikely, two alternative mechanism have to be considered. The dehydrogenases may either form a stable electron transport complex with polysulfide reductase, or the electron transfer is achieved by... 

Figure 6 Activity of polysulfide sulfur reduction by H2 (H2 -> [S]) or formate (HCO2 [S]) in proteoliposomes at 37°C. The proteoliposomes contained 8-methyl-menaquinone (10 p,mol phospholipid) and approximately the same molar amount of hydrogenase or formate dehydrogenase as polysulfide reductase. The activities were determined as described [6].

Sud contains a single cysteine residue which is essential for polysulfide sulfur transferase activity [Reaction (15)], for sulfur binding, and for the decrease of Am for polysulfide sulfur in polysulfide sulfiu reduction [53]. Sulfur appears to be covalently bound to the cysteine residue. When subjected to MALDI mass spectrometry, Sud incubated with polysulfide sulfur was found to carry one or two sulfur atoms per monomer. No sulfur was bound to the monomer after treatment of Sud with cyanide. A Sud variant carrying a serine instead of the cysteine residue did not carry sulfur even upon incubation with polysulfide sulfur. To explain the effect of Sud on the apparent Am for polysulfide sulfur in polysulfide sulfur reduction, it is assumed that sulfur is transferred from Sud to the active site of polysulfide reductase. From the concentration of Sud required for Am decrease and from its concentration in the bacterial periplasm it is likely that Sud is bound to polysulfide reductase during polysulfide sulfur transfer. The two enzymes occur in about equimolar amounts in cells of W. succinogenes grown with polysulfide sulfur. 

In the Cys/Sec subfamily, g-values of Mo(v) species from Fdh, Nap and Polysulfide reductase (Psr) enzymes clearly show two correlation geometries, one for the so-called high-g species and the other for very high-g species (Figure 3.3). These quite high g-values are clear evidence... 

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