The high autoantibody activity of antibodies to rat skeletal muscle glycogen phosphorylase b produced in rabbits.

Antibodies to rat skeletal muscle glycogen phosphorylase b were prepared in six rabbits by weekly injection of the enzyme emulsified with complete Freund's adjuvant. All the antiserum preparations showed high autoantibody activities to react with the rabbit muscle enzyme in both inhibition of enzyme activity and precipitation. In Ouchterlony double diffusion in agar, the antiserum preparations were precipitable to give a distinct spur between the two precipitin lines formed with rat and rabbit enzymes. When the autoantibody index was taken as per cent cross-reactivity of rabbit enzyme with rat enzyme, the autoantibody indices of inhibition and precipitation of one of the antiserum preparations were as high as 98% and 52.3%, respectively.     

Acetylcholinesterase in Huntington''s and Alzheimer''s Diseases: Simultaneous Enzyme Assay and Immunoassay of Multiple Brain Regions

A newly developed enzyme-linked immunosorbent assay for acetylcholinesterase (AChE) protein was combined with conventional measures of enzyme activity in a study of 15 brain regions from six control cases (non-neurological deaths), six cases of Alzheimer's disease, and six cases of Huntington's disease. In the control brains, the mean AChE activity varied 100-fold from region to region (cortex lowest, striatum highest). The variation in enzyme activity was exactly paralleled by a variation in protein immunoreactivity. Overall, the homospecific activity of AChE averaged 0.26 +/- 0.007 mU/pg, close to the value for electrophoretically homogeneous enzyme isolated from red blood cells. Similar homospecific activities were observed in samples from Huntington's and Alzheimer's brains. Evidently, AChE that is immunoreactive but enzymatically inactive does not accumulate in any of the three conditions examined. Huntington's brain samples showed normal total contents of AChE, but Alzheimer's brains showed significant decreases of both enzyme activity and immunoreactivity in all seven cortical regions and in two out of the eight subcortical structures examined, hippocampus and nucleus accumbens.     

Purification and catalytic properties of a CO-oxidizing:H2-evolving enzyme complex from Carboxydothermus hydrogenoformans.

From the membrane fraction of the Gram-positive bacterium Carboxydothermus hydrogenoformans, an enzyme complex catalyzing the conversion of CO to CO2 and H2 was purified. The enzyme complex showed maximal CO-oxidizing:H2-evolving enzyme activity with 5% CO in the headspace (450 U per mg protein). Higher CO concentrations inhibited the hydrogenase present in the enzyme complex. For maximal activity, the enzyme complex had to be activated by either CO or strong reductants. The enzyme complex also catalyzed the CO- or H2-dependent reduction of methylviologen at 5900 and 180 U per mg protein, respectively. The complex was found to be composed of six hydrophilic and two hydrophobic polypeptides. The amino-terminal sequences of the six hydrophilic subunits were determined allowing the identification of the encoding genes in the preliminary genome sequence of C. hydrogenoformans. From the sequence analysis it was deduced that the enzyme complex is formed by a Ni-containing carbon monoxide dehydrogenase (CooS), an electron transfer protein containing four [4Fe-4S] clusters (CooF) and a membrane bound [NiFe] hydrogenase composed of four hydrophilic subunits and two membrane integral subunits. The hydrogenase part of the complex shows high sequence similarity to members of a small group of [NiFe] hydrogenases with sequence similarity to energy conserving NADH:quinone oxidoreductases. The data support a model in which the enzyme complex is composed of two catalytic sites, a CO-oxidizing site and a H2-forming site, which are connected via a different iron-sulfur cluster containing electron transfer subunits. The exergonic redox reaction catalyzed by the enzyme complex in vivo has to be coupled to energy conservation, most likely via the generation of a proton motive force.     

Properties of the multiple forms of the soluble 17alpha-hydroxy steroid dehydrogenase of rabbit liver.

The six forms of the 17alpha-hydroxy steroid dehydrogenase purified from rabbit liver cytosol have very similar physical properties. The molecular weights of all the enzymes were within 3% of the average mol.wt of 39 600. Only one of the six enzymes showed a significant difference in amino acid composition. All but one form of the 17alpha-hydroxy steroid dehydrogenases exhibited greater activities towards the androgen, epitestosterone, than towards oestrogen substrates. With oestrogen substrates one enzyme displayed a high specificity towards the substrate oestradiol-17alpha 3-glucuronide. This high activity was lost if the glucuronic acid moiety was removed or replaced by glucose or galacturonic acid. The other enzyme forms had approximately equal activity toward oestradiol-17alpha and its glucuronide or glucoside derivative. However, substitution of galacturonic acid at C-3 of oestradiol-17alpha substantially decreased the activity of all but one enzyme form.     

Purification and characteristics of glutamate dehydrogenase (GDH) from triticale roots

In the investigated 14 day old triticale seedlings a much higher GDH activity was observed in roots than in leaves. The enzyme from the roots was purified up to the state of homogeneity (about 400 fold). The purified enzyme showed a higher activity in the presence of reduced coenzyme forms (NAD(P)H) than their oxidated forms. In the presence of NAD(P)H the enzyme showed absolute specificity to 2-oxoglutarate and in cooperation with NAD(P)⁺ to L-glutamate. The Km values determined for particular substrates indicate a high affinity of NADPH-GDH to ammonium ions. Optimum pH, temperature and thermostability of GDH depended on the type and form of the coenzyme. Molecular mass of purified enzyme was 257 kDa. It seems that native GDH is composed of six identical subunits of the molecular mass 42.5 kDa.     

Analysis of oxidation sensitivity of maleate cis-trans isomerase from Serratia marcescens

The maleate cis-trans isomerase gene (maiA) from Serratia marcescens IFO3736 was cloned and sequenced. Serratia MaiA has 62.4% amino acid identity with Alcaligenes faecalis IFO13111 MaiA and 64.9% with Bacillus stearothermophilus MI-102 MaiA. All known ten amino acid sequences of MaiA had significant conserved regions containing cysteine residues, which were previously suggested to be involved in an active site of the enzyme. The maiA gene was expressed in Escherichia coli, and expressed products MaiA was purified and characterized. The purified enzyme of strain IFO3736 showed high activity at room temperature and high heat stability. It also showed higher activity in the presence of high concentration of aspartic acid than the enzyme of A. faecalis IFO13111, but it was also sensitive to chemical oxidation. By amino acid composition analysis, cysteine, methionine, and tyrosine residues were suggested to be oxidized to inactivate the enzyme by chemical oxidation. To investigate the mechanism of chemical oxidation of the enzyme, six methionine residues in the conserved regions of S. marcescens MaiA were replaced with cysteine residues by site-directed mutagenesis. The analysis of the constructed mutants suggested that the Met201 residue near the Cys198 residue is involved in the sensitivity of the enzyme to chemical oxidation.     

Characterisation of a 1,4-beta-fucoside hydrolase degrading colanic acid

A novel colanic acid-degrading enzyme was isolated from a mixed culture filtrate obtained by enrichment culturing of a compost sample using colanic acid as carbon source. The enzyme was partially purified resulting in a 17-fold increase in specific activity. Further purification by Native PAGE revealed that the enzyme is part of a high-molecular weight multi protein complex of at least six individual proteins. The enzyme showed a temperature optimum at 50 degrees C while after 5h at 50 degrees C and pH7 still 70% of the total activity was left. The pH optimum was found to be pH7. Analysis of the degradation products showed that the enzyme is a novel 1,4-beta-fucoside hydrolase that liberates repeating units of colanic acid with varying degrees of acetylation. Km and Vmax of the enzyme were determined against the native substrate as well as its de-O-acetylated and depyruvated forms. Compared to the native substrate the affinity of the enzyme for the modified substrates was much lower. However, for the de-O-acetylated sample a dramatic increase in catalytic efficiency was observed. The native form of the substrate showed substrate inhibition at high concentrations, probably due to the formation of nonproductive substrate complexes. Removal of the acetyl groups probably prevents this effect resulting in a higher catalytic efficiency.     

Molecular cloning and expression of a full-length cDNA encoding acetylcholinesterase in optic lobes of the squid Loligo opalescens: a new member of the cholinesterase family resistant to diisopropyl fluorophosphate

Acetylcholinesterase cDNA was cloned by screening a library from Loligo opalescens optic lobes; cDNA sequence analysis revealed an open reading frame coding for a protein of 610 amino acids that showed 20-41% amino acid identity with the acetylcholinesterases studied so far. The characteristic structure of cholinesterase (the choline binding site, the catalytic triad, and six cysteines that form three intrachain disulfide bonds) was conserved in the protein. The heterologous expression of acetylcholinesterase in COS cells gave a recovery of acetylcholinesterase activity 20-fold higher than in controls. The enzyme, partially purified by affinity chromatography, showed molecular and kinetic features indistinguishable from those of acetylcholinesterase expressed in vivo, which displays a high catalytic efficiency. Both enzymes are true acetylcholinesterase corresponding to phosphatidylinositol-anchored G2a dimers of class I, with a marked substrate specificity for acetylthiocholine. The deduced amino acid sequence may explain some particular kinetic characteristics of Loligo acetylcholinesterase, because the presence of a polar amino acid residue (S313) instead of a nonpolar one [F(288) in Torpedo] in the acyl pocket of the active site could justify the high substrate specificity of the enzyme, the absence of hydrolysis with butyrylthiocholine, and the poor inhibition by the organophosphate diisopropyl fluorophosphate.     

Regulation of proline 3-hydroxylation and prolyl 3-hydroxylase and 4-hydroxylase activities in transformed cells.

Prolyl 3-hydroxylase activity and the extent of collagen proline 3-hydroxylation were studied in six transformed and three control human cell lines. In the transformed cell lines, the enzyme activity was markedly high in two, similar to that in control cells in two and significantly low in two. The extent of proline 3-hydroxylation was markedly high in cell lines with high enzyme activity, but it was also significantly high in some transformed cell lines with enzyme activities similar to those in the controls. The results thus suggest that, in addition to the amount of enzyme activity present, the rate of collagen synthesis also affects the extent of proline 3-hydroxylation in the newly synthesized collagen. The effect of acute cell transformation on prolyl 3-hydroxylase and 4-hydroxylase activities was studied by infecting chick-embryo fibroblasts with Rous sarcoma virus mutant NY68, temperature-sensitive for transformation. At the permissive temperature prolyl 3-hydroxylase activity showed a more rapid increase and decrease than did prolyl 4-hydroxylase activity, the maximal activity for both enzymes being about 2.5 times that in the control chick fibroblasts. When the transformed cells were shifted to the non-permissive temperature the decays in the elevated enzyme activities were similar, suggesting identical half-lives.     

Population genetic analysis of Helicobacter pylori by multilocus enzyme electrophoresis: extensive allelic diversity and recombinational population structure.

Genetic diversity and relationships in 74 Helicobacter pylori isolates recovered from patients assigned to distinct clinical categories were estimated by examination of allelic variation in six genes encoding metabolic housekeeping enzymes by multilocus enzyme electrophoresis. Seventy-three distinct allele profiles, representing multilocus chromosomal genotypes, were identified. All six loci were highly polymorphic, with an average of 11.2 alleles per locus. The mean genetic diversity in the sample was 0.735, a value that exceeds the level of diversity recorded in virtually all bacterial species studied by multilocus enzyme electrophoresis. A high frequency of occurrence of null alleles (lack of enzyme activity) was identified and warrants further investigation at the molecular level. Lack of linkage disequilibrium (nonrandom association (of alleles over loci) indicates that horizontal transfer and recombination of metabolic enzyme genes have contributed to the generation of chromosomal diversity in H. pylori. In this sample of isolates, there was no statistically significant association of multilocus enzyme electrophoretic types or cluster of related chromosomal types and disease category.     

新疆和云南番茄潜叶蛾种群对六种杀虫剂的敏感性及其与解毒酶活性的关系

【目的】番茄潜叶蛾Tuta absoluta 是新入侵我国的对番茄具有毁灭性危害的入侵害虫,目前入侵我国的番茄潜叶蛾种群对杀虫剂的抗性尚无报道。本研究旨在明确新疆和云南番茄潜叶蛾田间种群对6种常用杀虫剂的敏感性及其与解毒酶活性的关系。【方法】采用浸叶法测定6种常用杀虫剂对番茄潜叶蛾新疆和云南种群2龄幼虫的室内毒力。通过对2龄幼虫的生物测定确定3种增效剂［CYP450抑制剂胡椒基丁醚(PBO)、酯酶抑制剂磷酸三苯酯(TPP)和GST抑制剂丁烯二酸二乙酯(DEM)］对氯虫苯甲酰胺的增效作用。采用酶活性分析测定室内敏感种群和田间抗性种群(新疆种群) 2龄幼虫体内解毒酶［细胞色素P450酶(CYP450)、谷胱甘肽S-转移酶(GST)和羧酸酯酶(CarE)］活性,以确定杀虫剂抗性与解毒酶活性的关系。【结果】番茄潜叶蛾云南种群对6种杀虫剂的敏感性由高到低依次为甲维盐、溴虫腈、多杀菌素、茚虫威、氯虫苯甲酰胺和高效氯氰菊酯。新疆种群对6种杀虫剂的敏感性由高到低依次为甲维盐、溴虫腈、氯虫苯甲酰胺、多杀菌素、茚虫威和高效氯氰菊酯。与室内敏感种群相比,云南和新疆种群对氯虫苯甲酰胺的抗性水平最高,抗性倍数分别为212.7和169.3倍。生物测定结果表明,3种增效剂PBO, TPP和DEM均对氯虫苯甲酰胺无明显增效作用。酶活性测定结果表明,番茄潜叶蛾室内敏感种群和田间抗性种群之间2龄幼虫中CYP450, GST和CarE活性无显著差异。【结论】番茄潜叶蛾新疆和云南种群对测试的6种杀虫剂产生不同程度的抗性,对氯虫苯甲酰胺的抗性最高,番茄潜叶蛾对杀虫剂的抗性与解毒酶活性无关。本研究的结果对番茄潜叶蛾的田间防治和杀虫剂抗性治理具有指导意义。     

Isolation,characterization and function of laccase from Trichoderma

Of fourteen natural isolates of Trichoderma, no correlation was found between substrate weight loss and phenol oxidase (PO) activity in rice straw cultures. The highest PO producer from these laccase-positive strains was subjected to UV mutagenesis in order to select high and low PO activity mutants. There was no significant difference in substrate weight loss for mutant strains with six times higher and six times lower PO activity than the parent strain. Nor did the enzyme activity result in decreased growth inhibition by inhibitory phenolic compounds. PO enzyme from the parent Trichoderma and one of its high-PO-activity mutants was subsequently purified by ethanol precipitation from liquid cultures optimized by supplementation with copper sulphate and cycloheximide. Protein staining and activity staining of disc electrophoresis gels showed that only one PO enzyme of approximately 71 000 Da was produced. The enzyme could be defined as a laccase (benzenediol: oxygen oxidoreductase E.C. 1.10.3.2) because it catalysed the oxidation of syringaldazine and p-phenylenediamine in the absence of hydrogen peroxide, and because it was inhibited by cetyltrimethylammonium because but not by cinnamic acid. No specific in-vivo function could be assigned to this enzyme.Correspondence to: T. W. Flegel     

Homotropic effects in aspartate transcarbamoylase: What happens when the enzyme binds a single molecule of the bisubstrate analog N-phosphonacetyl-l-aspartate?

The active sites of aspartate transcarbamoylase from Escherichia coli were titrated by measuring the decrease in the enzyme-catalyzed arsenolysis of N-carbamoyl-L-aspartate caused by the addition of the tight-binding inhibitor, N-phosphonacetyl-L-aspartate. Because the enzyme is a poor catalyst for this non-physiological reaction, high concentrations are required for the assays (more than 1000-fold the dissociation constant of the reversibly bound inhibitor) and, therefore, virtually all of the bisubstrate analog is bound. From the endpoint of the titration, 5.7 active sites were calculated, in excellent agreement with the number, six, based on the structure of the enzyme. Simple inhibition was observed only when the molar ratio of inhibitor to enzyme exceeded five; under these conditions, as shown in earlier physical chemical studies, the R-conformational state of the enzyme is the sole or predominant species. At low ratios of inhibitor to enzyme, the addition of inhibitor caused an increase in activity which is attributable to the conversion of the enzyme from the low-activity T-state to the much more active R-state. Comparison of the linear increase in activity as a function of inhibitor concentration at the low molar ratio (0.01, i.e. 1 inhibitor/600 active sites) with the activity lost at the high ratio provided a direct value for the mean number of active sites converted from the T-state to the R-state as a result of the binding of one bisubstrate analog to an enzyme molecule. This number was four with Mg X ATP or carbamoyl phosphate present and 4.7 for the enzyme in the presence of Mg X PPi, values approaching or identical to the theoretical maximum, 4.7, for a concerted transition with all of the active sites of the molecule changing from the T- to R-state upon the formation of a binary complex of hexameric enzyme with a single inhibitor. With the enzyme in the absence of effectors or with Mg X CTP present, the titrations showed that an average of two and one sites, respectively, of 4.7 possible, changed conformation upon ligand binding. These results were interpreted as a manifestation of an equilibrium between a sub-population of T- and R-state enzyme complexes containing one bound inhibitor molecule. The R-state species would represent 40% of the population for aspartate transcarbamoylase in the absence of extraneous ligands.(ABSTRACT TRUNCATED AT 400 WORDS)     

Trypanosoma cruzi: subcellular distribution of glycolytic and some related enzymes of epimastigotes

The first six glycolytic enzymes in epimastigote Trypanosoma cruzi were shown to behave similarly during differential centrifugation, when maximum relative specific activity was found in the small granule fraction, and by isopycnic centrifugation, when the bulk of each activity coequilibrated on sucrose gradients with a modal density of 1.23 g/ml. All six showed substantial detergent latency in whole cell homogenates. Electron microscopic examination of fractions from a sucrose gradient with modal density 1.23 g/ml showed the presence of single membrane bound vesicles of diameter 0.2-0.8 micron. It was concluded that these six enzymes were contained in a microbodylike organelle, termed the "glycosome." Phosphoglucose isomerase (EC 5.3.1.9) also possessed substantial soluble activity. No microbody marker enzyme described in other sources could be detected. Peroxidase (EC 1.11.1.7) had an insignificant glycosomal component. Enzymes of amino acid and fatty acid metabolism were not detected in microbody fractions. Marker enzymes for the flagellar pocket and plasma membrane were suggested.     

Binding studies with bovine liver UPD-D-glucose dehydrogenase

The direct binding of uridine 5′-(α-d-glucopyranosyluronic acid pyrophosphate) (UDP-d-glucuronic acid) and of uridine 5′-(α-d-xylopyranosyl pyrophosphate) (UDP-d-xylose) to bovine liver UDP-d-glucose:NAD oxidoreductase (EC 1.1.1.22) has been measured by equilibrium dialysis. At saturation, the hexameric enzyme binds six molecules of UDP-d-glucuronic acid to noninteracting sites. UDP-d-xylose binds to 2 distinct classes of sites, each class binding six molecules of ligand. UDP-d-xylose is able to displace either UDP-d-glucose or UDP-d-glucuronic acid and UDP-d-glucose is able to displace UDP-d-glucuronic acid from the enzyme. It is proposed that the enzyme displays half-of-the-sites reactivity toward both substrate (UDP-d-glucose) and cosubstrate (NAD) and all-of-the-sites reactivity toward UDP-d-glucuronic acid. UDP-d-xylose is considered to bind cooperatively with high affinity to six regulatory sites and independently with lower affinity to six catalytic sites on the enzyme. Active-enzyme centrifugation studies show that UDP-d-glucose: NAD oxidoreductase is hexameric at concentrations corresponding to those used in steady-state kinetic measurements.     

Analysis of Oxidation Sensitivity of Maleate cis-trans Isomerase from Serratia marcescens

The maleate cis-trans isomerase gene (maiA) from Serratia marcescens IFO3736 was cloned and sequenced. Serratia MaiA has 62.4% amino acid identity with Alcaligenes faecalis IFO13111 MaiA and 64.9% with Bacillus stearothermophilus MI-102 MaiA. All known ten amino acid sequences of MaiA had significant conserved regions containing cysteine residues, which were previously suggested to be involved in an active site of the enzyme. The maiA gene was expressed in Escherichia coli, and expressed products MaiA was purified and characterized. The purified enzyme of strain IFO3736 showed high activity at room temperature and high heat stability. It also showed higher activity in the presence of high concentration of aspartic acid than the enzyme of A. faecalis IFO13111, but it was also sensitive to chemical oxidation. By amino acid composition analysis, cysteine, methionine, and tyrosine residues were suggested to be oxidized to inactivate the enzyme by chemical oxidation. To investigate the mechanism of chemical oxidation of the enzyme, six methionine residues in the conserved regions of S. marcescens MaiA were replaced with cysteine residues by site-directed mutagenesis. The analysis of the constructed mutants suggested that the Met201 residue near the Cys198 residue is involved in the sensitivity of the enzyme to chemical oxidation.     

Aspartokinase of Streptococcus mutans: purification, properties, and regulation.

Aspartokinase from Streptococcus mutans BHT was purified to homogeneity and characterized. The molecular weight of the native enzyme was estimated to be 242,000 by gel filtration. Cross-linking of aspartokinase with dimethyl suberimidate and polyacrylamide gel electrophoresis of the amidinated enzyme in the presence of sodium dodecyl sulfate showed the enzyme to be composed of six identical subunits with a molecular wieght of 40,000. The optimal pH range for enzyme activity was 6.5 to 8.5. The apparent Michaelis-Menten constants for aspartate and ATP were 5.5 and 2.2 mM, respectively. The enzyme was stable within the temperature range of 10 to 35 degrees C. Aspartokinase was not feedback inhibited by individual amino acids, but was concertedly inhibited by L-lysine and L-threonine (93.5% inhibition at 10 mM each). The inhibition was noncompetitive with respect to aspartate (Ki = 10 mM) and mixed with respect to ATP. L-Threonine methyl ester and L-threonine amide were able to substitute for L-threonine in feedback inhibition, but the requirement for L-lysine uas strict. The feedback inhibitor pair protected the enzyme against heat denaturation. Aspartokinase synthesis was repressed by L-threonine; this repression was enhanced by L-lysine, but was slightly attenuated by L-methionine.     

Purification, partial amino acid sequence and structure of the product of raucaffricine-O-beta-D-glucosidase from plant cell cultures of Rauwolfia serpentina

Plant cell suspension cultures of Rauwolfia produce within 1 week approximately 250 nkat/l of raucaffricine-O-beta-D-glucosidase. A five step procedure using anion exchange chromatography, chromatography on hydroxylapatite, gel filtration and FPLC-chromatography on Mono Q and Mono P delivered in a yield of 0.9% approximately 1200-fold enriched glucosidase. A short protocol employing DEAE sepharose, TSK 55 S gel chromatography and purification on Mono Q gave a 5% recovery of glucosidase which was 340-fold enriched. SDS-PAGE showed a Mr for the enzyme of 61 kDa. The enzyme is not glycosylated. Structural investigation of the enzyme product, vomilenine, demonstrated that the alkaloid exists in aqueous solutions in an equilibrium of 21(R)- and 21(S)-vomilenine in a ratio of 3.4:1. Proteolysis of the pure enzyme with endoproteinase Lys C revealed six peptide fragments with 6-24 amino acids which were sequenced. The two largest fragments showed sequences, of which the motif Val-Thr-Glu-Asn-Gly is typical for beta-glucosidases. Sequence alignment of these fragments demonstrated high homologies to linamarase from Manihot esculenta (81% identity) or to beta-glucosidase from Prunus avium (79% identity). Raucaffricine-O-beta-D-glucosidase seems to be a new member of the family 1 of glycosyl hydrolases.     

Enzymatic hydrolysis of cellulose: Evaluation of cellulase culture filtrates under use conditions

Culture filtrates from three mutant strains of Trichoderma reesei grown on lactose and on cellulose were compared under use conditions on four cellulose substrates. Cellulose culture filtrates contained five to six times as much cellulase as lactose culture filtrates. Unconcentrated cellulose culture filtrates produced up to 10% sugar solutions from 15% cellulose in 24 h. Specific activity in enzyme assays and efficiency in saccharification tests were low for enzymes from all the mutants. Over a wide range the percent saccharification of a substrate in a given times was directly proportional to the logarithm of the ratio of initial concentrations of enzyme and substrate. As a result of this, dilute enzyme is more efficient than concentrated enzyme, but if high sugar concentrations are desired, very large quantities of enzyme are required. Since the slopes of these plots varied, the relative activity of cellulase on different substrates may be affected by enzyme concentration.     

Human carbamoyl phosphate synthetase I (CPSI): insights on the structural role of the unknown function domains

Carbamoyl phosphate synthetase (CPS) is an ancient protein. In mammals it intervenes in the urea cycle. This enzyme is organized into six domains, three of which have no established role in the mammalian enzyme. Taking advantage of the high degree of conservation between the human and the Escherichia coli homologue a comparative study was carried out in order to infer about the biological role of these less characterized domains. We show that among the residues involved in the maintenance of quaternary structure of the E. coli enzyme, several are highly conserved between human and bacterial enzyme and match the homologous positions of the "unknown function" domains in human enzyme, suggesting they are involved in the structural stability of the human enzyme as they are in bacteria.