Liberation of labile sulfur from ferredoxins by alkaline zinc reagent: an appraisal of the methylene blue method

Nitrogenase Fe protein-like Fe-S cluster is conserved in L-protein (BchL) of dark-operative protochlorophyllide reductase fromRhodobacter capsulatus

Dark-operative protochlorophyllide reductase (DPOR) in bacteriochlorophyll biosynthesis is a nitrogenase-like enzyme consisting of L-protein (BchL-dimer) as a reductase component and NB-protein (BchN-BchB-heterotetramer) as a catalytic component. Metallocenters of DPOR have not been identified. Here we report that L-protein has an oxygen-sensitive [4Fe-4S] cluster similar to nitrogenase Fe protein. Purified L-protein from Rhodobacter capsulatus showed absorption spectra and an electron paramagnetic resonance signal indicative of a [4Fe-4S] cluster. The activity quickly disappeared upon exposure to air with a half-life of 20s. These results suggest that the electron transfer mechanism is conserved in nitrogenase Fe protein and DPOR L-protein.

Assay methods and biological roles of labile sulfur in animal tissues

Sulfur is a chemically and biologically active element. Sulfur compounds in animal tissues can be present in two forms, namely stable and labile forms. Compounds such as methionine, cysteine, taurine and sulfuric acid are stable sulfur compounds. On the other hand, acid-labile sulfur and sulfane sulfur compounds are labile sulfur compounds. The sulfur atoms of labile sulfur compounds are liberated as inorganic sulfide by acid treatment or reduction. Therefore, the determination of sulfide is the basis for the determination of labile sulfur. Determination of sulfide has been performed by various methods, including spectrophotometry after derivatization, ion chromatography, high-performance liquid chromatography after derivatization, gas chromatography, and potentiometry with a sulfide ion-specific electrode. These methods were originally developed for the determination of sulfide in air and water samples and were then applied to biological samples. The metabolic origin of labile sulfur in animal tissues is cysteine. The pathways of cysteine metabolism leading to the formation of sulfane sulfur are discussed. Finally, reports on the physiological roles and pathological considerations of labile sulfur are reviewed.

GroEL-assisted refolding of adrenodoxin during chemical cluster insertion

Chemical reconstitution of recombinant bovine adrenal mitochondrial apoadrenodoxin was carried out in the presence of the nonhomologous chaperone protein GroEL and of the cochaperone GroES, both in the presence and in the absence of ATP. The approach used here was different from the one characterizing studies on chaperone activity, as we used an adrenodoxin apoprotein, devoid of the cluster iron and sulfide, rather than a denaturant-unfolded form of the protein, and catalytic amounts of the chaperone proteins. A possible scaffolding role for two bacterial sulfur transferases, namely, rhodanese from Azotobacter vinelandii and a rhodanese-like sulfurtransferase from Escherichia coli, was also investigated in the absence of the enzyme substrates. The extent and the rate of adrenodoxin refolding following cluster insertion was measured by spectroscopy and by monitoring the activity recovery in a NADPH-cytochrome c reduction assay. These measurements were carried out on the unresolved reaction mixture and on the adrenodoxin-containing fraction obtained by HPLC fractionation of the reconstitution mixture at different reaction times. The rate and extent of cluster insertion and activity recovery were substantially improved by addition of GroEL and increased with increasing the GroEL/apoadrenodoxin ratio. GroES and ATP had no effect by themselves, and did not enhance the effect of GroEL. A. vinelandii rhodanese, the E. coli sulfurtransferase, and bovine serum albumin had no effect on the rate and yield of chemical reconstitution. The accelerated chemical reconstitution of apoadrenoxin in the presence of GroEL is therefore attributable to a scaffolding effect of this protein.

Pro108 is important for folding and stabilization of adrenal ferredoxin, but does not influence the functional properties of the protein

The truncated mutant Met-adrenodoxin-(4-107)-peptide of bovine adrenal ferredoxin was expressed as apoprotein in Escherichia coli BL21 and could be reconstituted to the holoform by chemical or enzymatic methods. The reconstituted protein had spectroscopic, functional and redox properties similar to the Met-adrenodoxin-(4-108)-peptide of adrenal ferredoxin, into which the cluster was inserted upon expression in the same Escherichia coli strain. Rate of in vitro cluster insertion into the Met-adrenodoxin-(4-107) apoprotein was much lower than for the Met-adrenodoxin-(4-108) apoprotein under identical conditions. Comparative thermodynamic studies with the Met-adrenodoxin-(4-108)-peptide indicated that removal of Pro108 resulted in an extensive decrease of the overall stability of the protein in either oxidation state. The Met-adrenodoxin-(4-107)-peptide showed a higher sensitivity to urea denaturation and had a sensibly lower denaturation temperature, 44.8 degrees C, compared with 51.7 degrees C for mutant Met-adrenodoxin-(4-108). The stability of the reduced state of both mutants is slightly lower than that of the oxidized state indicating that this protein region does not undergo major structural changes upon reduction.

Reversible, non-denaturing metal substitution in bovine adrenodoxin and spinach ferredoxin and the different reactivities of [2Fe-2S]-cluster-containing proteins

The non-denaturing substitution of cluster iron by other metals was studied in spinach ferredoxin and in bovine adrenodoxin. Only some of several metal species tested (Cd2+, Zn2+, VO2+, Mn2+, Co2+, Ni2+) caused bleaching of the residual visible absorbance and of the EPR signals of the reduced ferredoxins. No formation of mixed-metal cluster was observed. The most reactive metal species were Cd2+ and Zn2+ and Cd2+ was found to react also with oxidized adrenodoxin. Metal-treated proteins were resolved into a mixture of apoprotein, metal-substituted protein and unreacted holoprotein. Their biological activity was proportional to the residual holoprotein concentration. Spinach ferredoxin and adrenodoxin were found to differ substantially with regard to their metal-substitution reactivity under oxidizing and reducing conditions, reaction time, and formation of apoprotein, which was more pronounced for spinach ferredoxin. Exchange of cluster iron with Cd2+ in adrenodoxin generated stable species containing 2 mol sulfide/mol protein and 2 or 5 mol cadmium/mol protein, respectively. The relative amount of the two substitution products depended on the experimental conditions. CD and NMR data on all the cadmium-substituted proteins suggest that iron replacement led to a significant structural rearrangement. Nevertheless, all the metal-substituted proteins could be re-converted into the native iron-containing form upon incubation with iron in the absence of reductants, of denaturing agents, and of an external source of sulfide. The different reactivity of the two proteins is discussed in terms of the cluster environment, along with the possible physiological relevance of these findings.

Reversible and non-denaturing replacement of iron by cadmium in Clostridium pasteurianum ferredoxin

Incubation of native, reduced Clostridium pasteurianum ferredoxin with different metals gave a range of modifications in the electronic and EPR spectrum of the protein, or made the signals disappear. The reduced protein, isolated after incubation with different metals under identical conditions (50 microM protein, 1 mM metal, 1 h incubation) was found to contain amounts of foreign metals increasing with their thiophylicity, i.e. Cd2+ >> Zn2+ > Co2+. Little, if any, incorporation was observed for Ni2+, Cu2+, Mn2+ or in the absence of reductant. The activity of substituted ferredoxins in a hydrogenase-coupled assay was proportional to the amount of residual iron, suggesting that the residual iron is present in a population of intact active molecules rather than in partially substituted clusters distributed among individual molecules. The cadmium-substituted ferredoxin did not contain iron, but contained eight cadmium atoms and six labile sulfide atoms/mol. Folding of the isolated, substituted proteins was investigated by CD and 1H-NMR. Both techniques showed retention of the main structural features of the protein upon metal substitution. The rate and extent of the substitution of iron by cadmium were essentially independent of pH, but were found to decrease with increasing ionic strength and to increase with the cadmium concentration. In the cadmium-substituted protein, cadmium was replaced by iron upon incubation with iron and mercaptoethanol in the absence of dithionite. In the presence of dithionite, cadmium was not replaced by iron upon incubation of the cadmium-substituted protein with excess iron and mercaptoethanol. In competition experiments, incubation of iron-containing ferredoxin with stoichiometric amounts of cadmium in the presence of dithionite and excess iron and mercaptoethanol resulted in quantifiable replacement of iron by cadmium. Therefore, substitution of iron by cadmium was only achieved under reducing conditions, and was only reversible in the absence of strong reductants.

Nitrogenase of Klebsiella pneumoniae. Rhodanese-catalysed restoration of activity of the inactive 2Fe species of the Fe protein

The inactive 2Fe species of the Fe protein of the nitrogenase of Klebsiella pneumoniae was generated by treating oxidized Fe protein (Kp2) with MgATP and chelator. Incubation of the 2Fe species of Kp2 with the sulphurtransferase rhodanese in the presence of thiosulphate, ferric citrate and reduced lipoate reproducibly restored activity. The extent of restoration of activity depended on the molar ratio of 2Fe Kp2 to rhodanese and was time-dependent. Re-activation did not occur in the reaction mixture lacking rhodanese.

Enzymic synthesis of the 4Fe-4S clusters of Clostridium pasteurianum ferredoxin

Ex novo enzymic synthesis of the two 4Fe-4S clusters of Clostridium pasteurianum ferredoxin has been achieved by incubation of the apoprotein with catalytic amounts of the sulfurtransferase rhodanese in the presence of thiosulfate, DL-dihydrolipoate and ferric ammonium citrate. This enzymic reconstitution procedure was compared to a chemical one, in which the enzyme was replaced by sodium sulfide. A further comparison was made with the results previously obtained in the enzymic synthesis of the 2Fe-2S cluster of spinach ferredoxin, allowing the following conclusions to be drawn. The nature of the cluster to be inserted into the reconstituted iron-sulfur protein is determined by the apoprotein itself. The refolding of the structure of the iron-sulfur proteins around the newly inserted cluster is the rate-limiting step in both chemical and enzymic reconstitution. Rhodanese appears to play a role in the recovery of the native architecture of the reconstituted iron-sulfur protein(s). The extension to the 4Fe-4S centers of the rhodanese-based biosynthetic system allows this enzymic route to be proposed as a general way to the in vivo synthesis of iron-sulfur structures.

Enzymic synthesis of the iron-sulfur cluster of spinach ferredoxin

A biologically active spinach ferredoxin was reconstituted from the apoprotein by incubation with catalytic amounts of the sulfurtransferase rhodanese in the presence of thiosulfate, reduced lipoate and ferric ammonium citrate. Analytical and spectroscopical features of the reconstituted ferredoxin were identical to those of the native one; yield of the reconstitution reaction was 80%. Yields and kinetic parameters of the enzymic and chemical reconstitution were also compared. The higher efficiency of the enzymic system is ascribed to a productive interaction between rhodanese and apoferredoxin favouring the process of cluster build-up and insertion. The physiological relevance of this synthetic activity is discussed.

The inhibition of rhodanese by lipoate and iron-sulfur proteins

A study was made on the effects of DL-dihydrolipoate, lipoate and iron-sulfur proteins on the activity of rhodanese (EC 2.8.1.1) with dihydrolipoate or cyanide as acceptors. DL-Dihydrolipoate inactivates rhodanese, lipoate does not, and the opposite occurs with the sulfur-free form of the transferase. The observed effects vary with the sulfane sulfur acceptor from rhodanese (i.e., dihydrolipoate or cyanide) and depend on intramolecular oxidation of the catalytic sulfhydryl or on formation of a mixed disulfide with dihydrolipoate. Thiosulfate protects against inactivation by reloading the active-site cysteine with persulfide sulfur. The inhibition of sulfur transfer by iron-sulfur proteins appears related to the amount of native iron-sulfur structure interacting with rhodanese. The implications of the results for a possible biological role of rhodanese are considered.