Succinyl-CoA synthetase structure-function relationships and other considerations

Precursor Supply for Erythromycin Biosynthesis: Engineering of Propionate Assimilation Pathway Based on Propionylation Modification

Erythromycin is necessary in medical treatment and known to be biosynthesized with propionyl-CoA as direct precursor. Oversupply of propionyl-CoA induced hyperpropionylation, which was demonstrated as harmful for erythromycin synthesis in Saccharopolyspora erythraea. Herein, we identified three propionyl-CoA synthetases regulated by propionylation, and one propionyl-CoA synthetase SACE_1780 revealed resistance to propionylation. A practical strategy for raising the precursor (propionyl-CoA) supply bypassing the feedback inhibition caused by propionylation was developed through two approaches: deletion of the propionyltransferase AcuA, and SACE_1780 overexpression. The constructed Δ acuA strain presented a 10% increase in erythromycin yield; SACE_1780 overexpression strain produced 33% higher erythromycin yield than the wildtype strain NRRL2338 and 22% higher erythromycin yield than the industrial high yield Ab strain. These findings uncover the role of protein acylation in precursor supply for antibiotics biosynthesis and provide efficient post-translational modification-metabolic engineering strategy (named as PTM-ME) in synthetic biology for improvement of secondary metabolites.

Mechanistic Aspects of Phosphate Diester Cleavage Assisted by Imidazole. A Template Reaction for Obtaining Aryl Phosphoimidazoles

Phosphoimidazole-containing compounds are versatile players in biological and chemical processes. We explore catalytic and mechanistic criteria for the efficient formation of cyclic aryl phosphoimidazoles in aqueous solution, viewed as a template reaction for the in situ synthesis of related compounds. To provide a detailed analysis for this reaction a series of o-(2'-imidazolyl)naphthyl (4-nitrophenyl) phosphate isomers were examined to provide a basis for analysis of both mechanism and the influence of structural factors affecting the nucleophilic attack of the imidazolyl group on the phosphorus center of the substrate. Formation of the cyclic aryl phosphoimidazoles was probed by NMR and ESI-MS techniques. Kinetic experiments show that cyclization is faster under alkaline conditions, with an effective molarity up to 2900 M for the imidazolyl group, ruling out competition from external nucleophiles. Heavy atom isotope effect and computational studies show that the reaction occurs through a S_N2(P)-type mechanism involving a pentacoordinated phosphorus TS, with apical positions occupied by the incoming imidazolyl nucleophile and the p-nitrophenolate leaving group. The P-O bond to the leaving group is about 50-60% broken in the transition state.

Tricarboxylic Acid Cycle and Glyoxylate Bypass

The tricarboxylic acid (TCA) cycle plays two essential roles in metabolism. First, under aerobic conditions the cycle is responsible for the total oxidation of acetyl-CoA that is derived mainly from the pyruvate produced by glycolysis. Second, TCA cycle intermediates are required in the biosynthesis of several amino acids. Although the TCA cycle has long been considered a "housekeeping" pathway in Escherichia coli and Salmonella enterica, the pathway is highly regulated at the transcriptional level. Much of this control is exerted in response to respiratory conditions. The TCA cycle gene-protein relationship and mutant phenotypes have been well studied, although a few loose ends remain. The realization that a "shadow" TCA cycle exists that proceeds through methylcitrate has cleared up prior ambiguities. The glyoxylate bypass has long been known to be essential for growth on carbon sources such as acetate or fatty acids because this pathway allowsnet conversion of acetyl-CoA to metabolic intermediates. Strains lacking this pathway fail to grow on these carbon sources, since acetate carbon entering the TCA cycle is quantitatively lost as CO2 resulting in the lack of a means to replenish the dicarboxylic acids consumed in amino acid biosynthesis. The TCA cycle gene-protein relationship and mutant phenotypes have been well studied, although the identity of the small molecule ligand that modulates transcriptional control of the glyoxylate cycle genes by binding to the IclR repressor remains unknown. The activity of the cycle is also exerted at the enzyme level by the reversible phosphorylation of the TCA cycle enzyme isocitrate dehydrogenase catalyzed by a specific kinase/phosphatase to allow isocitratelyase to compete for isocitrate and cleave this intermediate to glyoxylate and succinate.

Novel regulatory roles of omega-3 fatty acids in metabolic pathways: a proteomics approach

Background

Omega-3 polyunsaturated fatty acids (n-3 PUFA) have been shown to alleviate the symptoms of metabolic disorders, such as heart disease, diabetes, obesity and insulin resistance. Several putative mechanisms by which n-3 PUFA elicit beneficial health effects have been proposed; however, there is still a shortage of knowledge on the proteins and pathways that are regulated by n-3 PUFA.

Methods

Using two dimensional polyacrylamide gel electrophoresis (2D-PAGE) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis, we investigated the effects of diets high or low in n-3 PUFA on hepatic proteomic profile of C57BL/6 mice.

Results

The findings show for the first time that high dietary n-3 PUFA reduced the expression of regucalcin, adenosine kinase and aldehyde dehydrogenase. On the other hand, diets high in n-3 PUFA increased the expression of apolipoprotein A-I, S-adenosylmethionine synthase, fructose-1, 6-bisphosphatase, ketohexokinase, malate dehydrogenase, GTP-specific succinyl CoA synthase, ornithine aminotransferase and protein disulfide isomerase-A3.

Conclusions

Our findings revealed for the first time that n-3 PUFA causes alterations in several novel functional proteins involved in regulating lipid, carbohydrate, one-carbon, citric acid cycle and protein metabolism, suggesting integrated regulation of metabolic pathways. These novel proteins are potential targets to develop therapeutic strategies against metabolic disorders.

Identification of the kinetic mechanism of succinyl-CoA synthetase

The kinetic mechanism of SCS [succinyl-CoA (coenzyme A) synthetase], which participates in the TCA (tricarboxylic acid) cycle, ketone body metabolism and haem biosynthesis, has not been fully characterized. Namely, a representative catalytic mechanism and associated kinetic parameters that can explain data on the enzyme-catalysed reaction kinetics have not been established. To determine an accurate model, a set of putative mechanisms of SCS, proposed by previous researchers, were tested against experimental data (from previous publication) on SCS derived from porcine myocardium. Based on comparisons between model simulation and the experimental data, an ordered ter–ter mechanism with dead-end product inhibition of succinate against succinyl-CoA is determined to be the best candidate mechanism. A thermodynamically constrained set of parameter values is identified for this candidate mechanism.

Biochemical and structural characterization of the GTP-preferring succinyl-CoA synthetase from Thermus aquaticus

Succinyl-CoA synthetase (SCS) from Thermus aquaticus was characterized biochemically via measurements of the activity of the enzyme and determination of its quaternary structure as well as its stability and refolding properties. The enzyme is most active between pH 8.0 and 8.4 and its activity increases with temperature to about 339 K. Gel-filtration chromatography and sedimentation equilibrium under native conditions demonstrated that the enzyme is a heterotetramer of two α-subunits and two β-subunits. The activity assays showed that the enzyme uses either ADP/ATP or GDP/GTP, but prefers GDP/GTP. This contrasts with Escherichia coli SCS, which uses GDP/GTP but prefers ADP/ATP. To understand the nucleotide preference, T. aquaticus SCS was crystallized in the presence of GDP, leading to the determination of the structure in complex with GDP-Mn(2+). A water molecule and Pro20β in T. aquaticus take the place of Gln20β in pig GTP-specific SCS, interacting well with the guanine base and other residues of the nucleotide-binding site. This leads to the preference for GDP/GTP, but does not hinder the binding of ADP/ATP.

Novel reaction of succinyl coenzyme A (Succinyl-CoA) synthetase: activation of 3-sulfinopropionate to 3-sulfinopropionyl-CoA in Advenella mimigardefordensis strain DPN7T during degradation of 3,3'-dithiodipropionic acid

The sucCD gene of Advenella mimigardefordensis strain DPN7(T) encodes a succinyl coenzyme A (succinyl-CoA) synthetase homologue (EC 6.2.1.4 or EC 6.2.1.5) that recognizes, in addition to succinate, the structural analogues 3-sulfinopropionate (3SP) and itaconate as substrates. Accumulation of 3SP during 3,3'-dithiodipropionic acid (DTDP) degradation was observed in Tn5::mob-induced mutants of A. mimigardefordensis strain DPN7(T) disrupted in sucCD and in the defined deletion mutant A. mimigardefordensis ΔsucCD. These mutants were impaired in growth with DTDP and 3SP as the sole carbon source. Hence, it was proposed that the succinyl-CoA synthetase homologue in A. mimigardefordensis strain DPN7(T) activates 3SP to the corresponding CoA-thioester (3SP-CoA). The putative genes coding for A. mimigardefordensis succinyl-CoA synthetase (SucCD(Am)) were cloned and heterologously expressed in Escherichia coli BL21(DE3)/pLysS. Purification and characterization of the enzyme confirmed its involvement during degradation of DTDP. 3SP, the cleavage product of DTDP, was converted into 3SP-CoA by the purified enzyme, as demonstrated by in vitro enzyme assays. The structure of 3SP-CoA was verified by using liquid chromatography-electrospray ionization-mass spectrometry. SucCD(Am) is Mg²⁺ or Mn²⁺ dependent and unspecific regarding ATP or GTP. In kinetic studies the enzyme showed highest enzyme activity and substrate affinity with succinate (V(max) = 9.85 ± 0.14 μmol min⁻¹ mg⁻¹, K(m) = 0.143 ± 0.001 mM). In comparison to succinate, activity with 3SP was only ca. 1.2% (V(max) = 0.12 ± 0.01 μmol min⁻¹ mg⁻¹) and the affinity was 6-fold lower (K(m) = 0.818 ± 0.046 mM). Based on the present results, we conclude that SucCD(Am) is physiologically associated with the citric acid cycle but is mandatory for the catabolic pathway of DTDP and its degradation intermediate 3SP.

Drosophila ortholog of succinyl-CoA synthetase {beta} subunit: a novel modulator of Drosophila KCNQ channels

Voltage-gated KCNQ potassium channels are responsible for slowly activating potassium currents in heart, brain, and other tissues. Functional defects of KCNQ channels are linked with many diseases, including epilepsy and cardiac arrhythmias. Therefore KCNQ potassium channels have been widely studied, especially in the CNS. We have identified Drosophila CG11963, which encodes a protein orthologous to the beta subunit of mammalian succinyl-CoA synthetase (SCS, also known as succinate thiokinase), as a novel modulator of Drosophila KCNQ channels. Direct interaction of CG11963 and dKCNQ was demonstrated by yeast two-hybrid screen and coimmunoprecipitation. Cell surface biotinylation experiments further confirmed that CG11963 resides on the plasma membrane of tsA-201 cells. Coexpression of CG11963 with dKCNQ shifts the conductance-voltage (G-V) relationship of dKCNQ channels to more positive membrane potentials in Chinese hamster ovary (CHO) cells. Moreover, directly dialyzing glutathione S-transferase fusion CG11963 protein into CHO cells also shifts the dKCNQ G-V curve rightward. The effect of CG11963 persists in the presence of 1 mM adenosine triphosphate (ATP), a substrate of SCS. Taken together, our data define CG11963 as a new dKCNQ-binding protein capable of modulating the properties of the channel. Our evidence suggests that this modulation is mediated by direct interaction of CG11963 with the channel and is not dependent on ATP.

Cloning, expression, purification, crystallization and preliminary X-ray analysis of Thermus aquaticus succinyl-CoA synthetase

Succinyl-CoA synthetase (SCS) is an enzyme of the citric acid cycle and is thus found in most species. To date, there are no structures available of SCS from a thermophilic organism. To investigate how the enzyme adapts to higher temperatures, SCS from Thermus aquaticus was cloned, overexpressed, purified and crystallized. Attempts to crystallize the enzyme were thwarted by proteolysis of the beta-subunit and preferential crystallization of the truncated form. Crystals of full-length SCS were grown after the purification protocol was modified to include frequent additions of protease inhibitors. The resulting crystals, which diffract to 2.35 A resolution, are of the protein in complex with Mn2+-GDP.

Interaction between succinyl CoA synthetase and the heme-biosynthetic enzyme ALAS-E is disrupted in sideroblastic anemia

The first and the rate-limiting enzyme of heme biosynthesis is delta-aminolevulinate synthase (ALAS), which is localized in mitochondria. There are 2 tissue-specific isoforms of ALAS, erythroid-specific (ALAS-E) and nonspecific ALAS (ALAS-N). To identify possible mitochondrial factors that modulate ALAS-E function, we screened a human bone marrow cDNA library, using the mitochondrial form of human ALAS-E as a bait protein in the yeast 2-hybrid system. Our screening led to the isolation of the beta subunit of human ATP-specific succinyl CoA synthetase (SCS-betaA). Using transient expression and coimmunoprecipitation, we verified that mitochodrially expressed SCS-betaA associates specifically with ALAS-E and not with ALAS-N. Furthermore, the ALAS-E mutants R411C and M426V associated with SCS-betaA, but the D190V mutant did not. Because the D190V mutant was identified in a patient with pyridoxine-refractory X-linked sideroblastic anemia, our findings suggest that appropriate association of SCS-betaA and ALAS-E promotes efficient use of succinyl CoA by ALAS-E or helps translocate ALAS-E into mitochondria.

Succinyl coenzyme A synthetase of Pseudomonas aeruginosa with a broad specificity for nucleoside triphosphate (NTP) synthesis modulates specificity for NTP synthesis by the 12-kilodalton form of nucleoside diphosphate kinase

Pseudomonas aeruginosa secretes copious amounts of an exopolysaccharide called alginate during infection in the lungs of cystic fibrosis patients. A mutation in the algR2 gene of mucoid P. aeruginosa is known to exhibit a nonmucoid (nonalginate-producing) phenotype and showed reduced activities of succinyl-coenzyme A (CoA) synthetase (Scs) and nucleoside diphosphate kinase (Ndk), implying coregulation of Ndk and Scs in alginate synthesis. We have cloned and characterized the sucCD operon encoding the alpha and beta subunits of Scs from P. aeruginosa and have studied the role of Scs in generating GTP, an important precursor in alginate synthesis. We demonstrate that, in the presence of GDP, Scs synthesizes GTP using ATP as the phosphodonor and, in the presence of ADP, Scs synthesizes ATP using GTP as a phosphodonor. In the presence of inorganic orthophosphate, succinyl-CoA, and an equimolar amount of ADP and GDP, Scs synthesizes essentially an equimolar amount of ATP and GTP. Such a mechanism of GTP synthesis can be an alternate source for the synthesis of alginate as well as for the synthesis of other macromolecules requiring GTP such as RNA and protein. Scs from P. aeruginosa is also shown to exhibit a broad NDP kinase activity. In the presence of inorganic orthophosphate (P(i)), succinyl-CoA, and either GDP, ADP, UDP or CDP, it synthesizes GTP, ATP, UTP, or CTP. Scs was previously shown to copurify with Ndk, presumably as a complex. In mucoid cells of P. aeruginosa, Ndk is also known to exist in two forms, a 16-kDa cytoplasmic form predominant in the log phase and a 12-kDa membrane-associated form predominant in the stationary phase. We have observed that the 16-kDa Ndk-Scs complex present in nonmucoid cells, synthesizes all three of the nucleoside triphosphates from a mixture of GDP, UDP, and CDP, whereas the 12-kDa Ndk-Scs complex specifically present in mucoid cell predominantly synthesizes GTP and UTP but not CTP. Such regulation may promote GTP synthesis in the stationary phase when the bulk of alginate is synthesized by mucoid P. aeruginosa.

Archael phosphoproteins. Identification of a hexosephosphate mutase and the alpha-subunit of succinyl-CoA synthetase in the extreme acidothermophile Sulfolobus solfataricus

When soluble extracts from the extreme acidophilic archaeon Sulfolobus solfataricus were incubated with [gamma-32P]ATP, several radiolabeled polypeptides were observed following SDS-PAGE. The most prominent of these migrated with apparent molecular masses of 14, 18, 35, 42, 46, 50, and 79 kDa. Phosphoamino acid analysis revealed that all of the proteins contained phosphoserine, with the exception of the 35-kDa one, whose protein-phosphate linkage proved labile to strong acid. The observed pattern of phosphorylation was influenced by the identity of the divalent metal ion cofactor used, Mg2+ versus Mn2+, and the choice of incubation temperature. The 35- and 50-kDa phosphoproteins were purified and their amino-terminal sequences determined. The former polypeptide's amino-terminal sequence closely matched a conserved portion of the alpha-subunit of succinyl-CoA synthetase, which forms an acid-labile phosphohistidyl enzyme intermediate during its catalytic cycle. This identification was confirmed by the ability of succinate or ADP to specifically remove the radiolabel. The 50-kDa polypeptide's sequence contained a heptapeptide motif, Phe/Pro-Gly-Thr-Asp/Ser-Gly-Val/Leu-Arg, found in a similar position in several hexosephosphate mutases. The catalytic mechanism of these mutases involves formation of a phosphoseryl enzyme intermediate. The identity of p50 as a hexosephosphate mutase was confirmed by (1) the ability of sugars and sugar phosphates to induce removal of the labeled phosphoryl group from the protein, and (2) the ability of [32P]glucose 6-phosphate to donate its phosphoryl group to the protein.

A diverse superfamily of enzymes with ATP-dependent carboxylate-amine/thiol ligase activity

The recently developed PSI-BLAST method for sequence database search and methods for motif analysis were used to define and expand a superfamily of enzymes with an unusual nucleotide-binding fold, referred to as palmate, or ATP-grasp fold. In addition to D-alanine-D-alanine ligase, glutathione synthetase, biotin carboxylase, and carbamoyl phosphate synthetase, enzymes with known three-dimensional structures, the ATP-grasp domain is predicted in the ribosomal protein S6 modification enzyme (RimK), urea amidolyase, tubulin-tyrosine ligase, and three enzymes of purine biosynthesis. All these enzymes possess ATP-dependent carboxylate-amine ligase activity, and their catalytic mechanisms are likely to include acylphosphate intermediates. The ATP-grasp superfamily also includes succinate-CoA ligase (both ADP-forming and GDP-forming variants), malate-CoA ligase, and ATP-citrate lyase, enzymes with a carboxylate-thiol ligase activity, and several uncharacterized proteins. These findings significantly extend the variety of the substrates of ATP-grasp enzymes and the range of biochemical pathways in which they are involved, and demonstrate the complementarity between structural comparison and powerful methods for sequence analysis.

Novel mechanisms of Escherichia coli succinyl-coenzyme A synthetase regulation

Low concentrations of ADP are shown to increase the rate of phosphoenzyme formation of E. coli succinyl-coenzyme A (CoA) synthetase (SCS) without altering the fraction of phosphorylated enzyme. This is true when either ATP or succinyl-CoA and Pi are used to phosphorylate the enzyme. The stimulatory effect of ADP is not altered by sample dilution, is retained upon partial purification of the enzyme, and reflects the binding of ADP to a site other than the catalytic site. GDP also alters the phosphorylation of the E. coli SCS but does so primarily by enhancing the level of the phosphoenzyme and only when ATP is used as the phosphate donor. GDP appears to function by neutralizing the action of a specific inhibitory protein. This inhibitor of SCS allows for interconversion of succinate and succinyl-CoA in a manner dissociated from changes in ATP-ADP metabolism. These previously unidentified and varied mechanisms by which SCS is regulated focus attention on this enzyme as an important control point in determining the cell's potential to meet its metabolic demands.

Cloning, characterization, and expression of the beta subunit of pig heart succinyl-CoA synthetase

The form of succinyl-CoA synthetase found in mammalian mitochondria is known to be an alpha beta dimer. Both GTP- and ATP-specific isozymes are present in various tissues. We have isolated essentially identical complementary DNA clones encoding the beta subunit of pig heart succinyl-CoA synthetase from both newborn and adult tissues. These cDNAs include a 1.4-kb sequence encoding the cytoplasmic precursor to the beta subunit comprised of 417 amino acid residues including a 22-residue mitochondrial targeting sequence. The cDNA encoding the 395-amino acid, 42,502-Da mature protein was confirmed to be the succinyl-CoA synthetase beta subunit by agreement with the N-terminal protein sequence and by high homology to prokaryotic forms of the beta subunit that were previously cloned (about 45% identical to beta from Escherichia coli). In contrast to a previous report (Nishimura, J.S., Ybarra, J., Mitchell, T., & Horowitz, P.M., 1988, Biochem. J. 250, 429-434), we found no tryptophan residue to be encoded in the sequence for the mature beta subunit, and this finding is corroborated by the fact that highly purified pig heart succinyl-CoA synthetase shows no tryptophan fluorescence or tryptophan content in amino acid compositional analysis. The cDNA clones encoding the mature pig heart beta subunit and its counterpart alpha subunit were coexpressed in a deletion mutant strain of E. coli. Recovery of succinyl-CoA synthetase activity demonstrated that this combination of subunits forms a productive enzymatic complex having GTP specificity.

Overexpression and site-directed mutagenesis of the succinyl-CoA synthetase of Escherichia coli and nucleotide sequence of a gene (g30) that is adjacent to the suc operon

The succinyl-CoA synthetase of Escherichia coli is encoded by two genes, sucC (beta subunit) and sucD (alpha subunit), which are distal genes in the sucABCD operon. They are expressed from the suc promoter, which also expresses the dehydrogenase and dihydrolipoyl succinyl-transferase subunits of the 2-oxoglutarate dehydrogenase complex. Strategies have now been devised for the site-directed mutagenesis and independent expression of the succinyl-CoA synthetase (alpha 2 beta 2 tetramer) and the individual subunits. These involve (1) subcloning a promoterless sucCD fragment downstream of the lac promoter in M13mp10, and (2) precise splicing of the suc coding regions with the efficient atpE ribosome-binding site and expression from the thermoinducible lambda promoters in the pJLA503 vector. Succinyl-CoA synthetase specific activities were amplified 40-60-fold within 5 h of thermoinduction of the lambda promoters, and the alpha and beta subunits accounted for almost 30% of the protein in supernatant fractions of the cell-free extracts. Site-directed mutagenesis of potential CoA binding-site residues indicated that Trp-43 beta and His-50 beta are essential residues in the beta-subunit, whereas Cys-47 beta could be replaced by serine without inactivating the enzyme. No activity was detected after the histidine residue at the phosphorylation site of the alpha-subunit was replaced by aspartate (His-246 alpha----Asp), but this alteration seemed to have a deleterious effect on the accumulation of the enzyme in cell-free supernatant extracts. The nucleotide sequence of an unidentified gene (g30) that is adjacent to the sucABCD operon was defined by extending the sequence of the citric acid cycle gene cluster by 818 bp to 13379 bp: gltA-sdhCDAB-sucABCD-g30. This gene converges on the suc operon and encodes a product (P30) that contains 230 amino acids (Mr 27,251). Highly significant similarities were detected between the N-terminal region of P30 and those of GENA [the product of another unidentified gene (geneA) located upstream of the aceEF-lpd operon], and GNTR (a putative transcriptional repressor of the gluconate operon of Bacillus subtilis). Possible roles for GENA and P30 as transcriptional regulators of the adjacent operons encoding the pyruvate and 2-oxoglutarate dehydrogenase complexes are discussed.

Isolation, amino acid analyses and refolding of subunits of pig heart succinyl-CoA synthetase

For the first time, pig heart succinyl-CoA synthetase has been refolded from its isolated subunits after denaturation. Amino acid analyses of pig heart succinyl-CoA synthetase and its subunits were performed. Subunits were isolated by gel filtration in neutral 6 M-urea. The amino acid composition of the native enzyme bears a strong resemblance to that of the Escherichia coli enzyme. Application of the various methods for comparing amino acid compositions [Cornish-Bowden (1983) Methods Enzymol. 91, 60-75] shows that the degree of relatedness between the alpha-subunits of the pig heart and E. coli enzymes and between the beta-subunits of the two synthetases is intermediate between 'strong' and 'weak'. As for the E. coli synthetase, it is unlikely that the alpha-subunit arises from the larger beta-subunit by post-translational modification. The pig heart enzyme contains a single tryptophan residue, which is located in the beta-subunit. Excitation of the enzyme at 295 nm resulted in a typical tryptophan emission spectrum. Refolding of enzyme denatured in 6 M-guanidine hydrochloride or of alpha- and beta-subunits isolated in this solvent required the presence of either ethylene glycol or glycerol, optimally at 20-25% (v/v). GTP-Mg2+ did not stimulate reactivation of the enzyme, in contrast with the result obtained with ATP-Mg2+ in the reconstitution of the enzyme from E. coli. Yields of 60% and 40% were obtained in the refolding of denatured enzyme and isolated subunits respectively. The fluorescence spectrum of the refolded protein was essentially the same as that of native enzyme. Unrecovered activity could not be accounted for in the form of protein aggregates. The specific activity of refolded enzyme that had been separated from inactive protein on a Bio-Sil TSK 250 column was the same as that of native enzyme. Km values for GTP of 27 microM and 14 microM were determined for native and refolded enzyme respectively.