Primary structure of avian hepatic rhodanese

Rhodanese (thiosulfate:cyanide sulfurtransferase) from frog Rana temporaria

The molecular mass of rhodanese from the mitochondrial fraction of frog Rana temporaria liver, equaling 8.7 kDa, was determined by high-performance size exclusion chromatography (HP-SEC). The considerable difference in molecular weight and the lack of common antigenic determinants between frog liver rhodanese and bovine rhodanese suggest the occurrence of different forms of this sulfurtransferase in the liver of these animals.

The crystal structure of a sulfurtransferase from Azotobacter vinelandii highlights the evolutionary relationship between the rhodanese and phosphatase enzyme families

Rhodanese is an ubiquitous enzyme that in vitro catalyses the transfer of a sulfur atom from suitable donors to nucleophilic acceptors by way of a double displacement mechanism. During the catalytic process the enzyme cycles between a sulfur-free and a persulfide-containing form, via formation of a persulfide linkage to a catalytic Cys residue. In the nitrogen-fixing bacteria Azotobacter vinelandii the rhdA gene has been identified and the encoded protein functionally characterized as a rhodanese. The crystal structure of the A. vinelandii rhodanese has been determined and refined at 1.8 A resolution in the sulfur-free and persulfide-containing forms. Conservation of the overall three-dimensional fold of bovine rhodanese is observed, with substantial modifications of the protein structure in the proximity of the catalytic residue Cys230. Remarkably, the native enzyme is found as the Cys230-persulfide form; in the sulfur-free state the catalytic Cys residue adopts two alternate conformations, reflected by perturbation of the neighboring active-site residues, which is associated with a partly reversible loss of thiosulfate:cyanide sulfurtransferase activity. The catalytic mechanism of A. vinelandii rhodanese relies primarily on the main-chain conformation of the 230 to 235 active-site loop and on a surrounding strong positive electrostatic field. Substrate recognition is based on residues which are entirely different in the prokaryotic and eukaryotic enzymes. The active-site loop of A. vinelandii rhodanese displays striking structural similarity to the active-site loop of the similarly folded catalytic domain of dual specific phosphatase Cdc25, suggesting a common evolutionary origin of the two enzyme families.

Mutagenic analysis of Thr-232 in rhodanese from Azotobacter vinelandii highlighted the differences of this prokaryotic enzyme from the known sulfurtransferases

Azotobacter vinelandii RhdA uses thiosulfate as the only sulfur donor in vitro, and this apparent selectivity seems to be a unique property among the characterized sulfurtransferases. To investigate the basis of substrate recognition in RhdA, we replaced Thr-232 with either Ala or Lys. Thr-232 was the target of this study since the corresponding Lys-249 in bovine rhodanese has been identified as necessary for catalytic sulfur transfer, and replacement of Lys-249 with Ala fully inactivates bovine rhodanese. Both T232K and T232A mutants of RhdA showed significant increase in thiosulfate-cyanide sulfurtransferase activity, and no detectable activity in the presence of 3-mercaptopyruvate as the sulfur donor substrate. Fluorescence measurements showed that wild-type and mutant RhdAs were overexpressed in the persulfurated form, thus conferring to this enzyme the potential of a persulfide sulfur donor compound. RhdA contains a unique sequence stretch around the catalytic cysteine, and the data here presented suggest a possible divergent physiological function of A. vinelandii sulfurtransferase.

Cloning and expression of human liver rhodanese cDNA

cDNA for the human rhodanese (thiosulfate; cyanide sulfurtransferase, EC 2.8.1.1) was cloned from a human fetal liver cDNA library. Sequencing of the cDNA revealed an open reading frame that encodes a 297-residue polypeptide with a calculated mass of 33,427 daltons. When the rhodanese cDNA was transiently expressed in Escherichia coli and Cos7 cells, the rhodanese activity increased 40-fold and 150-fold, respectively. Sequence homology analysis showed that the human rhodanese is 89.6% identical to bovine, 90.2% identical to rat, 91.2% identical to mouse and Chinese hamster, and 71.4% similar to avian counterparts, respectively, and that rhodanese was highly conserved across evolution.

Role of amino acid residues in the active site of rat liver mercaptopyruvate sulfurtransferase. CDNA cloning, overexpression, and site-directed mutagenesis

A complete amino acid structure of rat liver mercaptopyruvate sulfurtransferase (MST, EC 2.8.1.2) was determined by sequence analysis of cDNA and purified enzyme. The enzyme consists of 296 amino acid residues with a calculated molecular mass of 32,808 Da. Sequence identity in cDNA and the deduced amino acid sequence are 65 and 60% respectively, between rat MST and rhodanese. By their entire sequence similarity MST and rhodanese are confirmed to be evolutionarily related enzymes (Nagahara, N., Okazaki, T., and Nishino, T. (1995) J. Biol. Chem. 270, 16230-16235). The conversion of MST to rhodanese was attempted, and the role of amino acid residues was studied by site-directed mutagenesis with the isolated cDNA of rat liver MST. There is a strong possibility that Cys247 is a catalytic site of MST. Arg187 is suggested to be a binding site of both mercaptopyruvate and thiosulfate in MST. Arg196, which is missed in rhodanese, is important for catalysis in MST. On the other hand, the substitution of Arg for Gly248 or Lys for Ser249 facilitates catalysis of thiosulfate in MST. It is concluded that Arg187 and Arg196 of rat MST are critical residues in determining substrate specificity for mercaptopyruvate. On the other hand, Arg185, Arg247, and Lys248 of rat rhodanese are critical residues in determining substrate specificity for thiosulfate.

Active site structural features for chemically modified forms of rhodanese

In the course of the reaction catalyzed by rhodanese, the enzyme cycles between two catalytic intermediates, the sulfur-free and the sulfur-substituted (persulfide-containing) forms. The crystal structure of sulfur-free rhodanese, which was prepared in solution and then crystallized, is highly similar to that of sulfur-substituted enzyme. The inactivation of sulfur-free rhodanese with a small molar excess of hydrogen peroxide relies essentially on a modification limited to the active site, consisting of the oxidation of the essential sulfhydryl to sulfenyl group (-S-OH). Upon reaction of the sulfur-free enzyme with monoiodoacetate in the crystal, the Cys-247 side chain with the bound carboxymethyl group is forced into a conformation that allows favorable interactions of the carboxylate with the four peptide NH groups that participate in hydrogen bonding interactions with the transferable sulfur atom of the persulfide group in the sulfur-substituted rhodanese. It is concluded that active site-specific chemical modifications of sulfur-free rhodanese do not lead to significant changes of the protein structure, consistent with a high degree of similarity of the structures of the sulfur-free and sulfur-substituted forms of the enzyme both in solution and in the crystal.

Cloning, sequence analysis and overexpression of the rhodanese gene of Azotobacter vinelandii

A gene encoding rhodanese (rhdA) was cloned from Azotobacter vinelandii on a 2.3-kb SphI fragment. This fragment was identified by its hybridization to a PCR product obtained by amplification of genomic DNA using degenerate primers encoding the N-terminal sequence of rhodanese purified from A. vinelandii. The sequence of a 1.2-kb region revealed an 813-bp open reading frame that encoded a polypeptide of 271 amino acids, the N-terminal sequence of which was identical to that of A. vinelandii rhodanese. In a search of database entries, eukaryotic rhodaneses and rhodanese-like proteins from bacteria gave the highest scores of identity (27-30%) with the predicted product of the 813-bp open reading frame. A. vinelandii RhdA shows less sequence similarity to vertebrate rhodaneses than it does to prokaryotic rhodanese-like proteins which did not show typical rhodanese activity. Basic residues thought to be catalytically important in bovine rhodanese are not conserved in A. vinelandii rhodanese. The sequence similarity between the two structurally similar domains of rhodanese is more pronounced for the A. vinelandii enzyme than the bovine enzyme, and supports the hypothesis that the complete structure was originally generated by gene duplication. When rhdA was overexpressed in Escherichia coli, rhodanese represented 30% of total cell protein and thiosulfate:cyanide sulfurtransferase activity increased >600 fold in cell-free extracts. A. vinelandii rhdA insertion/deletion mutants had no discernible phenotype distinct from the wild-type strain with respect to growth on various sulfur sources or nitrogenase activity. Mutants retained 20% of wild-type rhodanese thiosulfate:cyanide sulfurtransferase activity suggesting the presence of redundant sulfurtransferase enzymes in A. vinelandii.

Cytosolic mercaptopyruvate sulfurtransferase is evolutionarily related to mitochondrial rhodanese. Striking similarity in active site amino acid sequence and the increase in the mercaptopyruvate sulfurtransferase activity of rhodanese by site-directed mutagenesis

Rat liver mercaptopyruvate sulfurtransferase (MST) was purified to homogeneity. MST is very similar to rhodanese in physicochemical properties. Further, rhodanese cross-reacts with anti-MST antibody. Both purified authentic MST and expressed rhodanese possess MST and rhodanese activities, although the ratio of rhodanese to MST activity is low in MST and high in rhodanese. In order to compare the active site regions of MST and rhodanese, the primary structure of a possible active site region of MST was determined. The sequence showed 66% homology with that of rat liver rhodanese. An active site cysteine residue (Cys246; site of formation of persulfide in catalysis) and an arginine residue (Arg185; substrate binding site) in rhodanese were also conserved in MST. On the other hand, two other active site residues (Arg247 and Lys248) were replaced by Gly and Ser, respectively. Conversion of rhodanese to MST was tried by site-directed mutagenesis. After cloning of rat liver rhodanese, recombinant wild type and three mutants (Arg247-->Gly and/or Lys248-->Ser) were constructed. The enzymes were expressed in Escherichia coli strain BL21 (DE3) with a T7 promoter system. The mutation of these residues decreases rhodanese activity and increases MST activity.

Cloning and sequence analysis of the human liver rhodanese: comparison with the bovine and chicken enzymes

The cDNA for the human rhodanese (thiosulfate: cyanide sulfurtransferase, EC 2.8.1.1), a nuclearly encoded protein of the mitochondrial matrix, was isolated from a human fetal liver cDNA library. Nucleotide sequence revealed an open reading frame coding for a polypeptide of 295 amino acids, which presented a 57% and 58% identity with the bovine and avian rhodanese, respectively. The analysis of the 5'-ends of the coding region gave no evidence for the presence of a cleavable signal sequence as found in other mitochondrial proteins. A comparison with two available amino acid sequences (cow and chicken) showed that sequence similarity is not restricted to the alpha-helices and beta-structures motifs which are remarkably superimposable in the two halves of bovine rhodanese, but extends to adjacent regions.

Purification of bovine liver rhodanese by low-pH column chromatography

We report a purification of bovine liver rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) using column chromatography under conditions that take advantage of recent information regarding the structure and stability of this enzyme. At low pH (e.g., pH 4-6), rhodanese is stabilized against inactivation processes. By maintaining rhodanese at low pH, column chromatography, and especially ion-exchange chromatography, becomes practical, without loss of enzymatic activity. A purification method involving the sequential use of cation-exchange, size-exclusion, and hydrophobic-interaction chromatography was developed, and rhodanese was purified with good yield to electrophoretic purity and high specific activity. Previous methods for purifying bovine liver rhodanese employ repeated ammonium sulfate fractionations and crystallization of the rhodanese. In these methods, it is difficult to separate rhodanese from yellow-brown contaminants in the final stages of the procedures. Here, yellow-brown contaminants, which copurify with rhodanese on the first two columns, are completely resolved by hydrophobic interaction chromatography. This method can be readily scaled up, requires no special equipment, eliminates the variability inherent in previous methods, and is less dependent upon experience.