S-adenosylhomocysteine hydrolase from the thermophilic archaeon Sulfolobus solfataricus: purification, physico-chemical and immunological properties

Functional and Pathological Roles of AHCY

Adenosylhomocysteinase (AHCY) is a unique enzyme and one of the most conserved proteins in living organisms. AHCY catalyzes the reversible break of S-adenosylhomocysteine (SAH), the by-product and a potent inhibitor of methyltransferases activity. In mammals, AHCY is the only enzyme capable of performing this reaction. Controlled subcellular localization of AHCY is believed to facilitate local transmethylation reactions, by removing excess of SAH. Accordingly, AHCY is recruited to chromatin during replication and active transcription, correlating with increasing demands for DNA, RNA, and histone methylation. AHCY deletion is embryonic lethal in many organisms (from plants to mammals). In humans, AHCY deficiency is associated with an incurable rare recessive disorder in methionine metabolism. In this review, we focus on the AHCY protein from an evolutionary, biochemical, and functional point of view, and we discuss the most recent, relevant, and controversial contributions to the study of this enzyme.

Heat-stable enzymes from extremely thermophilic and hyperthermophilic microorganisms

Only in the last decade have microorganisms been discovered which grow near or above 100°C. The enzymes that are formed by these extremely thermophilic (growth temperature 65 to 85°C) and hyperthermophilic (growth temperature 85 to 110°C) microorganisms are of great interest. This review covers the extracellular and intracellular enzymes of these exotic microorganisms that have recently been described. Polymer-hydrolysing enzymes, such as amylolytic, cellulolytic, hemicellulolytic and proteolytic enzymes, will be discussed. In addition, the properties of the intracellular enzymes involved in carbohydrate and amino-acid metabolism and DNA-binding and chaperones and chaperone-like proteins from hyperthermophiles are described. Due to the unusual properties of these heat-stable enzymes, they are expected to fill the gap between biological and chemical processes.

Recombinant S-adenosylhomocysteine hydrolase from Thermotoga maritima: cloning, overexpression, characterization, and thermal purification studies

S-Adenosylhomocysteine hydrolase (SAHase) encoded by sahase gene is a determinant when catalyzing the reversible conversion of adenosine and homocysteine to S-adenosylhomocysteine in most living organisms. The sahase gene was isolated from the genome of the highly thermostable anaerobic bacteria Thermotoga maritima, and then it was cloned, characterized, overexpressed using Escherichia coli, and partially purified by thermal precipitation. The thermal purification of the recombinant SAHase resulted in changes in the circular dichroism spectra. As a result of this analysis, it was possible to determine the structural changes in the composition of the α-helix and β-sheet content of the recombinant enzyme after purification. Moreover, a predicted secondary structure and 3D structural model was rendered by comparative molecular modeling to further understand the molecular function of this protein including its attractive biotechnological use.

Copper ions inhibit S-adenosylhomocysteine hydrolase by causing dissociation of NAD+ cofactor

S-Adenosylhomocysteine hydrolase (SAHH) regulates biomethylation and homocysteine metabolism and thus is an attractive target in drug design studies. SAHH has been shown to be a copper binding protein in vivo; however, the structure and catalytic mechanism of SAHH exclude a role for Cu2+. In the present work, we studied the mechanism of inhibition of SAHH activity by Cu2+. The experimental results showed that Cu2+ inhibited SAHH activity in a noncompetitive manner. Binding of Cu2+ to SAHH resulted in the release of NAD+ cofactors, explaining the loss of the enzymatic activity of SAHH. Further investigation by an ESR probe and computational simulation suggested that Cu2+ could bind at the central channel and interrupt the subunit interactions of SAHH, resulting in a large decrease in affinity to the NAD+ cofactor. This effect of Cu2+ resembled that of enzyme mutations at the C-terminal domain or Asp244 [Komoto, J., Huang, Y., Gomi, T., Ogawa, H., Takata, Y., Fujioka, M., and Takusagawa, F. (2000) Effects of site-directed mutagenesis on structure and function of recombinant rat liver S-adenosylhomocysteine hydrolase. Crystal structure of D244E mutant enzyme, J. Biol. Chem. 275, 32147-32156]. The mechanism of action of Cu2+ on SAHH suggested a possible regulative role for Cu2+ on the intracellular activity of SAHH. This could be helpful in understanding the biological effects of copper compounds and suggest a potential coupling mechanism between biomethylation and the redox states of cells.

Characterization ofS-adenosylhomocysteine hydrolase fromCryptosporidium parvum

The S-adenosylhomocysteine hydrolase from the apicomplexan Cryptosporidium parvum (CpSAHH) has been characterized. CpSAHH is a single-copy, intronless gene of 1479 bp encoding a protein of 493 amino acids with a molecular mass of 55.6 kDa. Reverse transcriptase-polymerase chain reaction analysis confirmed that CpSAHH is expressed both in intracellular stages (in C. parvum-infected HCT-8 cells 24 h after infection) and in sporozoites. CpSAHH was expressed in Escherichia coli TB1 cells as a fusion with maltose-binding protein. The recombinant fusion was cleaved by Factor Xa and the enzymatic activity of both the fusion protein and the purified separated CpSAHH was measured. The enzymatic activity of CpSAHH was inhibited by d-eritadenine, S-DHPA and Ara-A.

S-adenosylhomocysteine hydrolase from the archaeon Pyrococcus furiosus: biochemical characterization and analysis of protein structure by comparative molecular modeling

S-adenosylhomocysteine hydrolase (AdoHcyHD) is an ubiquitous enzyme that catalyzes the breakdown of S-adenosylhomocysteine, a powerful inhibitor of most transmethylation reactions, to adenosine and L-homocysteine. AdoHcyHD from the hyperthermophilic archaeon Pyrococcus furiosus (PfAdoHcyHD) was cloned, expressed in Escherichia coli, and purified. The enzyme is thermoactive with an optimum temperature of 95 degrees C, and thermostable retaining 100% residual activity after 1 h at 90 degrees C and showing an apparent melting temperature of 98 degrees C. The enzyme is a homotetramer of 190 kDa and contains four cysteine residues per subunit. Thiol groups are not involved in the catalytic process whereas disulfide bond(s) could be present since incubation with 0.8 M dithiothreitol reduces enzyme activity. Multiple sequence alignment of hyperthermophilic AdoHcyHD reveals the presence of two cysteine residues in the N-terminus of the enzyme conserved only in members of Pyrococcus species, and shows that hyperthermophilic AdoHcyHD lack eight C-terminal residues, thought to be important for structural and functional properties of the eukaryotic enzyme. The homology-modeled structure of PfAdoHcyHD shows that Trp220, Tyr181, Tyr184, and Leu185 of each subunit and Ile244 from a different subunit form a network of hydrophobic and aromatic interactions in the central channel formed at the subunits interface. These contacts partially replace the interactions of the C-terminal tail of the eukaryotic enzyme required for tetramer stability. Moreover, Cys221 and Lys245 substitute for Thr430 and Lys426, respectively, of the human enzyme in NAD-binding. Interestingly, all these residues are fairly well conserved in hyperthermophilic AdoHcyHDs but not in mesophilic ones, thus suggesting a common adaptation mechanism at high temperatures.

A colorimetric assay for S-adenosylhomocysteine hydrolase

A colorimetric method for S-adenosyl-L-homocysteine hydrolase (SAHase) which uses S-adenosyl-L-homocysteine (SAH) as substrate is described. This method involves the hydrolytic conversion of SAH into adenosine (ADO) and L-homocysteine (HCY). The formation of HCY is quantified using Ellman's reagent and spectrophotometrical measured at 412 nm. Under these assay conditions, the product was followed continuously in a facile and quantitative manner until substrate conversion was complete. This method is an easy, cheap and shorter alternative to more complex methods and it is applicable to routine clinical analysis and in the assay and development of new S-nucleosidylhomocysteines to be used as therapeutic compounds.

Methylthioadenosine phosphorylase from the archaeon Pyrococcus furiosus. Mechanism of the reaction and assignment of disulfide bonds

The extremely heat-stable 5'-methylthioadenosine phosphorylase from the hyperthermophilic archaeon Pyrococcus furiosus was cloned, expressed to high levels in Escherichia coli, and purified to homogeneity by heat precipitation and affinity chromatography. The recombinant enzyme was subjected to a kinetic analysis including initial velocity and product inhibition studies. The reaction follows an ordered Bi-Bi mechanism and phosphate binding precedes nucleoside binding in the phosphorolytic direction. 5'-Methylthioadenosine phosphorylase from Pyrococcus furiosus is a hexameric protein with five cysteine residues per subunit. Analysis of the fragments obtained after digestion of the protein alkylated without previous reduction identified two intrasubunit disulfide bridges. The enzyme is very resistant to chemical denaturation and the transition midpoint for guanidinium chloride-induced unfolding was determined to be 3.0 M after 22 h incubation. This value decreases to 2.0 M in the presence of 30 mM dithiothreitol, furnishing evidence that disulfide bonds are needed for protein stability. The guanidinium chloride-induced unfolding is completely reversible as demonstrated by the analysis of the refolding process by activity assays, fluorescence measurements and SDS/PAGE. The finding of multiple disulfide bridges in 5'-methylthioadenosine phosphorylase from Pyrococcus furiosus argues strongly that disulfide bond formation may be a significant molecular strategy for stabilizing intracellular hyperthermophilic proteins.

A coupled fluorescent assay for histone methyltransferases

Histone methyltransferases (HMTs) catalyze the S-adenosylmethionine (AdoMet)-dependent methylation of lysines and arginines in the nucleosomal core histones H3 and H4 and the linker histone H1b. Methylation of these residues regulates either transcriptional activation or silencing, depending on the residue modified and its degree of methylation. Despite an intense interest in elucidating the functions of HMTs in transcriptional regulation, these enzymes have remained challenging to quantitatively assay. To characterize the substrate specificity of HMTs, we have developed a coupled-fluorescence-based assay for AdoMet-dependent methyltransferases. This assay utilizes S-adenosylhomocysteine hydrolase (SAHH) to hydrolyze the methyltransfer product S-adenosylhomocysteine (AdoHcy) to homocysteine (Hcy) and adenosine (Ado). The Hcy concentration is then determined through conjugation of its free sulfhydryl moiety to a thiol-sensitive fluorophore. Using this assay, we have determined the kinetic parameters for the methylation of a synthetic histone H3 peptide (corresponding to residues 1-15 of the native protein) by Schizosaccharomyces pombe CLR4, an H3 Lys-9-specific methyltransferase. The fluorescent SAHH-coupled assay allows rapid and facile determination of HMT kinetics and can be adapted to measure the enzymatic activity of a wide variety of AdoMet-dependent methyltransferases.

S-adenosyl-L-homocysteine hydrolase in yeast: key enzyme of methylation metabolism and coordinated regulation with phospholipid synthesis

S-adenosyl-L-homocysteine hydrolase (Sah1p, EC 3.3.1.1.) is a key enzyme of methylation metabolism. It catabolizes S-adenosyl-L-homocysteine, which is formed after donation of the activated methyl group of S-adenosyl-L-methionine (AdoMet) to an acceptor, and which acts as strong competitive inhibitor of all AdoMet-dependent methyltransferases. Sah1p is an essential enzyme in yeast and one of the most highly conserved proteins with up to 80% sequence homology throughout all kingdoms of life. SAH1 expression in yeast is subject to the general transcriptional control of phospholipid synthesis. Profound changes in cellular lipid composition upon depletion of Sah1p support the notion of a tight interaction between lipid metabolism and Sah1p function.

Purification and characterization of 5'-methylthioadenosine phosphorylase from the hyperthermophilic archaeon Pyrococcus furiosus: substrate specificity and primary structure analysis

5'-Methylthioadenosine phosphorylase (MTAP) was purified to homogeneity from the hyperthermophilic archaeon Pyrococcus furiosus. The protein is a homoexamer of 180 kDa. The enzyme is highly thermoactive, with an optimum temperature of 125 degrees C, and extremely thermostable, retaining 98% residual activity after 5 h at 100 degrees C and showing a half-life of 43 min at 130 degrees C. In the presence of 100 mM phosphate, the apparent T(m) (137 degrees C) increases to 139 degrees C. The enzyme is extremely stable to proteolytic cleavage and after incubation with protein denaturants, detergents, organic solvents, and salts even at high temperature. Thiol groups are not involved in the catalytic process, whereas disulfide bond(s) are present, since incubation with 0.8 M dithiothreitol significantly reduces the thermostability of the enzyme. N-Terminal sequence analysis of the purified enzyme is 100% identical to the predicted amino acid sequence of the gene PF0016 from the partially sequenced P. furiosus genome. The deduced amino acid sequence of the gene revealed a high degree of identity (52%) with human MTAP. Nevertheless, unlike human MTAP, MTAP from P. furiosus is not specific for 5'-methylthioadenosine, since it phosphorolytically cleaves adenosine, inosine, and guanosine. The calculated k(cat)/ K(m) values for 5'-methylthioadenosine and adenosine, about 20-fold higher than for inosine and guanosine, indicate that 6-amino purine nucleosides are preferred substrates of MTAP from P. furiosus. The structural features and the substrate specificity of MTAP from P. furiosus document that it represents a 5'-methylthioadenosine-metabolizing enzyme different from those previously characterized among Archaea, Bacteria, and Eukarya. The functional and structural relationships among MTAP from P. furiosus, human MTAP, and two putative MTAPs from P. furiosus and Sulfolobus solfataricus are discussed here for the first time.

Expression, purification, and characterization of recombinant S-adenosylhomocysteine hydrolase from the thermophilic archaeon Sulfolobus solfataricus

S-Adenosylhomocysteine hydrolase from Sulfolobus solfataricus was expressed in Escherichia coli by inserting the genomic fragment containing the gene encoding for S-adenosylhomocysteine hydrolase downstream the isopropyl-beta-d-thiogalactoside-inducible promoter of pTrc99A expression vector. An ATG positioned 25 bp upstream of the gene which is in frame with a stop codon was utilized as the initiation codon. This construct was used to transform E. coli RB791 and E. coli JM105 strains. The recombinant protein, purified by a fast and efficient two-step procedure (yield of 0.4 mg of enzyme per gram of cells), does not appear homogeneous on SDS-PAGE because of the presence of a protein contaminant corresponding to a "truncated" S-adenosylhomocysteine hydrolase subunit lacking the first 24 amino acid residues. The recombinant enzyme shows the same molecular mass, optimum temperature, and kinetic features of S-adenosylhomocysteine hydrolase isolated from S. solfataricus but it is less thermostable. To construct a vector which presents a correct distance between the ribosome-binding site and the start codon of S-adenosylhomocysteine hydrolase gene, a NcoI site was created at the translation initiation codon using site-directed mutagenesis. The expression of the homogeneous mutant S-adenosylhomocysteine hydrolase was achieved at high level (1.7 mg of mutant protein per gram of cells). The mutant S-adenosylhomocysteine hydrolase and the native one were indistinguishable in all physicochemical and kinetic properties including thermostability, indicating that the interactions involving the NH(2)-terminal sequence of the protein play a role in the thermal stability of S. solfataricus S-adenosylhomocysteine hydrolase.

N, Antranikian G. Extremely thermophilic microorganisms and their polymer-hidrolytic enzymes

Thermophilic and hyperthermophilic microorganisms are found as normal inhabitants of continental and submarine volcanic areas, geothermally heated sea-sediments and hydrothermal vents and thus are considered extremophiles. Several present or potential applications of extremophilic enzymes are reviewed, especially polymer-hydrolysing enzymes, such as amylolytic and hemicellulolytic enzymes. The purpose of this review is to present the range of morphological and metabolic features among those microorganisms growing from 70oC to 100°C and to indicate potential opportunities for useful applications derived from these features.

Crystal structure of S-adenosylhomocysteine hydrolase from rat liver

The crystal structure of rat liver S-adenosyl-L-homocysteine hydrolase (AdoHcyase, EC 3.3.1.1) which catalyzes the reversible hydrolysis of S-adenosylhomocysteine (AdoHcy) has been determined at 2.8 A resolution. AdoHcyase from rat liver is a tetrameric enzyme with 431 amino acid residues in each identical subunit. The subunit is composed of the catalytic domain, the NAD+-binding domain, and the small C-terminal domain. Both catalytic and NAD+-binding domains are folded into an ellipsoid with a typical alpha/beta twisted open sheet structure. The C-terminal section is far from the main body of the subunit and extends into the opposite subunit. An NAD+ molecule binds to the consensus NAD+-binding cleft of the NAD+-binding domain. The peptide folding pattern of the catalytic domain is quite similar to the patterns observed in many methyltransferases. Although the crystal structure does not contain AdoHcy or its analogue, there is a well-formed AdoHcy-binding crevice in the catalytic domain. Without introducing any major structural changes, an AdoHcy molecule can be placed in the catalytic domain. In the structure described here, the catalytic and NAD+-binding domains are quite far apart from each other. Thus, the enzyme appears to have an "open" conformation in the absence of substrate. It is likely that binding of AdoHcy induces a large conformational change so as to place the ribose moiety of AdoHcy in close proximity to the nicotinamide moiety of NAD+. A catalytic mechanism of AdoHcyase has been proposed on the basis of this crystal structure. Glu155 acts as a proton acceptor from the O3'-H when the proton of C3'-H is abstracted by NAD+. His54 or Asp130 acts as a general acid-base catalyst, while Cys194 modulates the oxidation state of the bound NAD+. The polypeptide folding pattern of the catalytic domain suggests that AdoHcy molecules can travel freely to and from AdoHcyase and methyltransferases to properly regulate methyltransferase activities. We believe that the crystal structure described here can provide insight into the molecular architecture of this important regulatory enzyme.

Trichomonas vaginalis: expression and characterisation of recombinant S-adenosylhomocysteinase

The gene encoding S-adenosylhomocysteinase activity (S-adenosylhomocysteine hydrolase, SAHH; EC 3.3.1.1) in Trichomonas vaginalis has been expressed in Escherichia coli to facilitate the characterisation of the enzyme. Expression of this gene using the pQE-30 (6xHis N-terminal tag) expression system (QIAGEN) has enabled the one-step purification of 6 mg of active recombinant enzyme from a 100-ml bacterial culture by affinity chromatography using a nickel-NTA matrix. The recombinant enzyme has a molecular weight of approximately 56,000 and identification of tryptic peptides by matrix-assisted laser desorption ionisation (MALDI) mass spectrometry has shown that the purified recombinant protein is identical in primary structure to the predicted sequence. The presence of the N-terminal 6xHis tag in the recombinant enzyme did not appear to affect its kinetic and other properties, which are similar to those exhibited by the "native" enzyme present in cell-free extracts of T. vaginalis. These properties include a similar apparent Km for adenosine (20-25 microM for the recombinant and 5-10 microM for the native enzymes, respectively) and similar inhibition/inactivation patterns exhibited by adenosine analogues such as arabinosyl adenine (ara-A).

Cloning and sequencing of the gene coding for S-adenosylhomocysteine hydrolase in the thermophilic archaeon Sulfolobus solfataricus

The gene from the thermophilic archaeon Sulfolobus solfataricus (Ss), encoding the S-adenosylhomocysteine hydrolase (AdoHcyHD), has been cloned. Two degenerate oligodeoxyribonucleotide (oligo) probes, synthesized on the basis of amino acid (aa) sequence of cyanogen bromide-peptide fragments of the purified protein, were used to screen a genomic library of Ss cloned into the pGEM7Zf(+) vector. The AdoHcyHD gene (adohcyhd) comprises 1254 nucleotides (nt) and encodes a polypeptide of 417 aa with a deduced molecular mass of 46 kDa, in good agreement with the value directly measured for the purified enzyme. The identity of more than 32% of the deduced aa sequence was confirmed by Edman degradation of peptides. Putative regulatory elements which are in good agreement with the archaeal promoter consensus sequences were identified in the flanking regions. Comparison of the aa sequences of AdoHcyHD from different sources shows a remarkable degree of conservation. Surprisingly, several aa residues, thought important in substrate binding and catalysis, show non-conserved replacements in Ss AdoHcyHD.