Dissimilatory Sulfite Reductase Revisited

Abstract

Assimilatory sulfite reductase contains a sirohaem that is very weakly coupled to a [4Fe-4S] cubane, i.e. five iron atoms in total. Dissimilatory sulfite reductase is a complex system with 20 Fe atoms/a,p,y, hexamer. A recent revision of the purification procedure for the Desulfovibrio vulgaris dissimilatory enzyme has afforded a preparation of only 10 Fe atoms hexamer, this has led to the conclusion that the topology of prosthetic groups parallels that of the assimilatory system [Wolfe, €3. M., Lui, S. M. & Cowan, J. A. (1994) Eur: J. Biochem. 223, 79-89]. The new purification procedure has been reproduced but the claimed molecular properties are not reproducible. The highly purified, active desulfoviridin contains 20, not 10, Fe atoms/molecule ; the sirohaem is extensively demetallated, not metallated; and the S = 9/2 iron-sulfur cluster is present, not absent.Keywords: dissimilatory sulfite reductase; desulfoviridin.S(IV) is biologically activated and reduced to S(I1) by the iron-sulfur sirohaemoenzyme sulfite reductase. Assimilatory sulfite reductase catalyses the concerted six-electron reduction of sulfite to sulfide for the biosynthesis of organic sulfur compounds. The crystallographic structure of the E.scherichia coli ylfite reductase haemoprotein has recently been solved to 1.6-A resolution [I]. The sirohaem is bridged to a [4Fe-4S] cubane via a cysteine thiolate ligand.Sulfite can also act as the final electron acceptor in the respiration of anaerobic microorganisms. This dissimilatory sulfite reductase activity is performed by a more complex enzyme system; in DesuZjbvibrio vulgaris (Hildenborough) this sulfite reductase is of the desulfoviridin type, classified according to its characteristic visible absorption spectrum [2-41. It is isolated as an a2p2y2 hcxamcr with subunit molecular masses of 50 kDa (a), 40 kDa @) and 11 kDa (1' ) [5]. The protein contains two sirohaems, one for each a subunit, believed to be the sites of substrate reduction [6]. ln the case of desulfoviridin the sirohaems are about 80% demetallated 17-91; the reason for this is not yet known. The enzyme also contains a high amount of iron and acid-labile sulfide: desulfoviridin from D. vulgaris and D. gigas has been reported to contain about 22 and 18 iron atoms/ molecule, respectively [8, 91.The arrangement of this iron-sulfur has not becn established. Initial reports have emphasized spectroscopic similarities between assimilatory and dissimilatory sulfite reductases [8, 10, 111, suggesting a similarity in active-site structure (i.e. a cubanecoupled sirohaem). We have questioned this proposal when we discovered S = 9/2 EPR signals from what appears to be a noninteracting iron-sulfur structure in desulfoviridin from D. vulgaris [9]. Subsequent studies showed that the S = 912 EPR is These authors claim that the resulting preparations contain only 10 Fe atoms a,~,y2 hexamer, and they suggest that this results from purifying away an unidentified contaminating iron protein.It is also claimed that the sirohaem in the new preparations ...

Structure of the Dissimilatory Sulfite Reductase from the Hyperthermophilic Archaeon Archaeoglobus fulgidus

“…The specific activity of 48.2 nmol sulfite min − 1 mg − 1 was in the same range as that reported for dSir from various sulfate-reducing bacteria. 19,20,37 The diffraction data were phased with the MAD/SIRAS method using intrinsic iron atoms and a mercury derivative (Table 1). Structural refinement converged to R and R free values of 18.9% and 22.4%, respectively, in the resolution range of 10.0-2.0 Å.…”

“…17 Notably, assimilatory nitrite reductases, which convert nitrite to ammonia, also belong to the Sir family. 18 dSirs have been isolated from several sulfatereducing microorganisms, such as Desulfovibrio vulgaris, 19 Desulfovibrio desulfuricans, 20 Pyrobaculum islandicum, 21 and Archaeoglobus fulgidus, 22 and have been characterized with respect to their molecular, spectroscopic, and kinetic properties. The dSirs studied so far are composed of a heterotetrameric (αβ) 2 core complex and, depending on the organism, two additional small subunits, γ and δ, resulting in an (αβ) 2 γ n δ n multisubunit complex.…”

Purification, crystallization and preliminary crystallographic analysis of a dissimilatory DsrAB sulfite reductase in complex with DsrC

“…The aSiRs are monomeric and share low sequence similarity to dSiRs, which are large oligomeric proteins with a molecular mass in the order of 200 kDa and minimally composed of two different types of subunits in a a 2 b 2 arrangement (Schiffer et al, 2008). The number of cofactors in some proteins is also not clear with studies reporting from 2 to 4 sirohemes and 10-32 non-heme irons per a 2 b 2 module (Fauque et al, 1990;Marritt and Hagen, 1996;Moura et al, 1988;Pierik and Hagen, 1991;Wolfe et al, 1994). Although some authors propose that the cubane-siroheme cofactor arrangement of dSiRs is similar to that found in aSiRs (Moura et al, 1988;Wolfe et al, 1994), other authors have proposed the presence of higher nuclearity high-spin iron-sulfur clusters (Arendsen et al, 1993;Marritt and Hagen, 1996;Pierik and Hagen, 1991).…”

The Crystal Structure of Desulfovibrio vulgaris Dissimilatory Sulfite Reductase Bound to DsrC Provides Novel Insights into the Mechanism of Sulfate Respiration

“…Cofactor quantification of dSiRs is quite disparate, with studies reporting from 2 to 4 sirohemes and from 10 to 32 non-heme irons per ␣ 2 ␤ 2 module (13,(17)(18)(19)(20). The nature of the catalytic cofactor has also been disputed, with some studies proposing a cubanesiroheme arrangement similar to that found in aSiRs (13,19) and other studies proposing the presence of higher nuclearity high spin iron-sulfur clusters (18,20,21). Finally, a most important and unresolved question is the nature of the physiological electron donor to dSiR.…”