Identification and characterization of a gamma-glutamyl transpeptidase from a thermo-alcalophile strain of Bacillus pumilus

Expression and characterization of C-terminal truncated mutants of γ-glutamyltranspeptidase II (PaGGTII) from Pseudomonas aeruginosa PAO1

The gene encoding γ-glutamyltranspeptidase II (PaGGTII) from Pseudomonas aeruginosa PAO1 was cloned in Escherichia coli. Recombinant PaGGTII showed a weak activity (0.0332 U/mg), and it can be easily inactivated. Multiple alignment of microbial GGTs showed the redundancy of the C-terminal of the small subunit of PaGGTII in length. The truncation of eight amino acid residues at the C-terminal of PaGGTII remarkably improved the activity and stability of the enzyme (PaGGTIIΔ8; 0.388 U/mg). Further truncation at the C-terminal also provided the enzyme relatively higher activity (PaGGTIIΔ9, -Δ10, -Δ11, and -Δ12). Among C-terminal truncated mutants, we focused on PaGGTIIΔ8 and examined the effect of C-terminal amino acid residues on the properties of PaGGTIIΔ8 because the activity of PaGGTII was found to be greatly improved when 8 amino acid residues were truncated. Various mutant enzymes with different C-terminal amino acid residues were constructed. They were expressed in E. coli and purified to homogeneity by ion-exchange chromatography. The properties of PaGGTIIΔ8 and the mutants obtained from mutation at E569 were characterized. K_m and k_cat of PaGGTIIΔ8 for γ-glutamyl-p-nitroanilide (γ-GpNA) were 8.05 mM and 15.49 s^-1, respectively. PaGGTIIΔ8E569Y showed the highest catalytic efficiency for γ-GpNA with a k_cat/Km of 12.55 mM^-1 s^-1. Mg²⁺, Ca²⁺, and Mn²⁺ exhibited positive effects on the catalytic activity for PaGGTIIΔ8 and its ten E569 mutants.

Biochemical Characterization of γ-Glutamyl Transpeptidase from Bacillus altitudinis IHB B1644 and Its Application in the Synthesis of l-Theanine

An extracellular γ-glutamyl transpeptidase (GGT) produced from Bacillus altitudinis IHB B1644 was purified to homogeneity employing ion-exchange chromatography. GGT comprised two subunits of 40 and 22 kDa determined by SDS-PAGE. The maximum enzyme activity was optimal at pH 9 and 37 °C. The purified enzyme was stable from pH 5-10 and <50 °C. Steady-state kinetic studies revealed a K_m value of 0.538 mM against γ-GpNA. For substrate specificity, GGT showed highest affinity for l-methionine. The inhibitors' effect demonstrated that serine or threonine and tryptophan residues are essential for enzyme activity. l-Theanine production was optimized by employing a one-variable-at-a-time approach with 60-65% conversion rate. The final reaction consisted of 20 mM l-glutamine, 200 mM ethylamine hydrochloride, and 10 U mL^-1 enzyme concentration at 37 °C in Tris-Cl (50 mM, pH 9) for 5 h. l-Theanine was purified using a Dowex 50W X 8 hydrogen form resin and confirmed by HPLC and ¹H NMR spectroscopies.

Affinity purification, identification, and biochemical characterization of Gamma-glutamyl transpeptidase, a membrane anchored enzyme of Gigantocotyle explanatum

Gamma-glutamyl transpeptidase is an enzyme that facilitates the transfer of glutamyl groups from glutamyl peptides to other peptides or water. Additionally, it also participates in important processes such as amino acid transport, cellular redox control, drug detoxification, apoptosis, and DNA fragmentation in a various organism. In the present study, GGT activity in Gigantocotyle explanatum was examined in order to characterize the enzyme in the helminth system. GGT is isolated using membrane solubilization and purified through affinity column chromatography (Con-A Sepharose column). Km and Vmax values, as well as the optimal pH, optimal temperature, and incubation period, are also determined using enzyme kinetics. The hetero-dimeric property of the enzyme is demonstrated by the purified GGT, which yielded two subunits of 65.5 and 55 kDa. The optimal pH and temperature are found to be 8.0 and 37 °C, respectively. While assessing the optimal incubation time of the enzyme, it was observed that the purified GGT not only retained its functional integrity up to 15 min but also reflected considerable thermostability at higher temperatures, by retaining 78% and 25% of its initial activities at 50 °C and 60 °C, respectively. One millimolar concentration of 6-Diazo-5-Oxo Nor-isoleucine (DON), a specific inhibitor of GGT, completely abolished GGT activity. These results suggest that GGT in these worms is a catalytically active enzyme with distinguishing characteristics that can be used for further study to comprehend its function in amphistome biology and in host-parasite relationships, especially since the potential therapeutic candidacy of the GGT enzyme has already been indicated in these groups of organisms.

A water-soluble fluorescent probe for the determination of γ-glutamyltransferase activity and its application in tumor imaging

γ-glutamyltransferase (GGT), an important tumor marker, is highly expressed in tumor tissues, and precise detection of its activity provides a vital indicator for the diagnosis and treatment. In this work, a "lighting-on" probe (TCF-GGT) was elaborated to detect endogenous GGT with high selectivity and sensitivity. Dicyanomethyldifuranyl (TCF-OH) was employed as the fluorescence reporter and short peptide glutathione (GSH) worked as the GGT-active trigger, the introduction of which prevented the initial proton transfer of TCF-OH contributing to a blank sensing background. A bright red fluorescence could be switched on upon GGT catalytic hydrolysis, avoiding the potential interference from background. There displayed an excellent water-solubility, and little organic solvent was required during the exploration, which otherwise avoided the potential damage to enzyme and organism. TCF-GGT has been proved to be workable at cellular and organism level with highly effective imaging and a short metabolic cycle, which is expected to offer an alternative solution or reference to the early diagnosis and treatment of tumor.

Identification and Activity Characterization of γ-Glutamyltransferase from Bovine Milk

It is well known that bovine milk contains γ-glutamyltransferase (GGT) activity. To verify the identity of the GGT and further to characterize the generation of γ-glutamyl peptides, identification of GGT from bovine milk and quantification of kokumi peptides and free amino acids were performed. GGT was purified from skim milk and identified as the bovine protein (G3N2D8), and it reveals that it is composed of two subunits. Sequence alignment with human GGT and molecular mass determination showed that the bovine GGT was glycosylated and contained an N-terminal transmembrane part. Further activity characterization was performed in comparison with GGT from Bacillus amyloliquefaciens in terms of the ability to generate γ-glutamyl peptides from casein hydrolysates. During the transpeptidation reaction catalyzed by both GGT, γ-glutamyl peptides significantly (P < 0.05) increased after γ-glutamylation; addition of glutamine contributed to the generation of γ-glutamyl peptides, suggesting that glutamine could act as a γ-glutamyl donor. This study reveals that the GGT of skim milk membranes is a glycosylated membrane protein that can generate γ-glutamyl peptides.

Effects of pH and NaCl on hydrolysis and transpeptidation activities of a salt-tolerant γ-glutamyltranspeptidase from Bacillus amyloliquefaciens S0904

Bacillus amyloliquefaciens S0904 was selected as a hyperproducer of a glutamine-hydrolyzing enzyme which was identified as a γ-glutamyltranspeptidase catalyzing both hydrolysis and transpeptidation of glutamyl substrates. The signal peptide-truncated recombinant enzyme (rBAGGT) showed two-fold enhanced specific activity for hydrolysis and optimum pH shift to pH 7 from pH 6 compared with the wild type. The hydrolysis activity of rBAGGT was tolerant against NaCl up to 2.5 M, whereas the transpeptidation activity decreased by NaCl. At pH 6, the addition of 1.5 M NaCl not only enhanced the hydrolysis activity but also inhibited the transpeptidation activity to be ignorable. By contrast, at pH 9 in the absence of NaCl, the alkaline pH-favored transpeptidation activity was 99% of the maximum with only 15% of the maximum hydrolysis activity. In conclusion, the hydrolysis and transpeptidation activities of the recombinant BAGGT is controllable by changing pH and whether or not to add NaCl.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-021-00928-6.

Bacterial Gamma-Glutamyl Transpeptidase, an Emerging Biocatalyst: Insights Into Structure-Function Relationship and Its Biotechnological Applications

Gamma-glutamyl transpeptidase (GGT) enzyme is ubiquitously present in all life forms and plays a variety of roles in diverse organisms. Higher eukaryotes mainly utilize GGT for glutathione degradation, and mammalian GGTs have implications in many physiological disorders also. GGTs from unicellular prokaryotes serve different physiological functions in Gram-positive and Gram-negative bacteria. In the present review, the physiological significance of bacterial GGTs has been discussed categorizing GGTs from Gram-negative bacteria like Escherichia coli as glutathione degraders and from pathogenic species like Helicobacter pylori as virulence factors. Gram-positive bacilli, however, are considered separately as poly-γ-glutamic acid (PGA) degraders. The structure-function relationship of the GGT is also discussed mainly focusing on the crystallization of bacterial GGTs along with functional characterization of conserved regions by site-directed mutagenesis that unravels molecular aspects of autoprocessing and catalysis. Only a few crystal structures have been deciphered so far. Further, different reports on heterologous expression of bacterial GGTs in E. coli and Bacillus subtilis as hosts have been presented in a table pointing toward the lack of fermentation studies for large-scale production. Physicochemical properties of bacterial GGTs have also been described, followed by a detailed discussion on various applications of bacterial GGTs in different biotechnological sectors. This review emphasizes the potential of bacterial GGTs as an industrial biocatalyst relevant to the current switch toward green chemistry.

Characterization and engineering of two new GH9 and GH48 cellulases from a Bacillus pumilus isolated from Lake Bogoria

OBJECTIVES

To search for new alkaliphilic cellulases and to improve their efficiency on crystalline cellulose through molecular engineering RESULTS: Two novel cellulases, BpGH9 and BpGH48, from a Bacillus pumilus strain were identified, cloned and biochemically characterized. BpGH9 is a modular endocellulase belonging to the glycoside hydrolase 9 family (GH9), which contains a catalytic module (GH) and a carbohydrate-binding module belonging to class 3 and subclass c (CBM3c). This enzyme is extremely tolerant to high alkali pH and remains significantly active at pH 10. BpGH48 is an exocellulase, belonging to the glycoside hydrolase 48 family (GH48) and acts on the reducing end of oligo-β1,4 glucanes. A truncated form of BpGH9 and a chimeric fusion with an additional CBM3a module was constructed. The deletion of the CBM3c module results in a significant decline in the catalytic activity. However, fusion of CBM3a, although in a non native position, enhanced the activity of BpGH9 on crystalline cellulose.

CONCLUSIONS

A new alkaliphilic endocellulase BpGH9, was cloned and engineered as a fusion protein (CBM3a-BpGH9), which led to an improved activity on crystalline cellulose.

Statistical optimization of culture conditions of mesophillic gamma-glutamyl transpeptidase from Bacillus altitudinis IHB B1644

Microbial gamma-glutamyl transpeptidase (GGT) is a key enzyme in production of several γ-glutamyl compounds with food and pharmaceutical applications. Bacterial GGTs are not commercially available in the market owing to their low production from various sources. Thus, the study was focused on achieving the higher GGT production from B. altitudinis IHB B1644 by optimizing the culture conditions using one-variable-at-a-time (OVAT) strategy. A mesophillic temperature of 28 °C, agitation 200 rpm and neutral pH 7 were found to be optimal for higher GGT titre. Among the medium components, the monosaccharide glucose served as the best carbon source over disaccharides, and yeast extract was the preferred organic nitrogen source over inorganic nitrogen sources. The statistical approaches (Plakett-Burman and response surface methodology) were further employed for the optimization of medium components. Medium composition: 0.1% w/v glucose, 0.3% w/v yeast extract, 0.03% w/v magnesium sulphate, 0.20% w/v potassium dihydrogen phosphate and 2.5% w/v sodium chloride with inoculum size (1% v/v) was suitable for higher GGT titres (449 U ml^-1). Time kinetics showed the stability of enzyme up to 96 h of incubation suggesting its application in the industrial use. The proposed strategy resulted in 2.6-fold increase in the GGT production compared to that obtained in the unoptimized medium. The results demonstrated that RSM was fitting to identify the optimum production conditions and this finding should be of great importance for commercial GGT production.

Secretion of Bacillus amyloliquefaciens γ-Glutamyltranspeptidase from Bacillus subtilis and Its Application in Enzymatic Synthesis of l-Theanine

In this study, the gene of γ-glutamyltranspeptidase (GGT) from Bacillus amyloliquefaciens (BaGGT) controlled by the P_lac promoter was cloned into Bacillus subtilis to construct two recombinant vectors with either one or two signal peptides to drive extracellular secretion. After optimization, 90 ± 0.2 mg/L BaGGT was obtained when the inducing conditions were 24 h and 80 μM (IPTG). The properties of BaGGT were measured, showing that the optimal reaction conditions were 40 °C and pH 9.0 with 55.0 ± 0.5 U/mg enzymatic activity. K_m and V_max were 0.214 mM and 88.13 μmol/min/mg. BaGGT could be stored for 72 h with 90% of the initial activity at 40 °C and retained more than 50% of the initial activity after being maintained at different pH values for 24 h. Finally, enzymatic synthesis of l-theanine was performed with the optimal conditions: 20 mM l-Gln, 100 mM ethylamine HCl, 0.5 U/mL BaGGT, incubated at 40 °C for 6 h, 200 rpm.

γ-Glutamyltranspeptidase-Triggered Intracellular Gadolinium Nanoparticle Formation Enhances the T2-Weighted MR Contrast of Tumor

Magnetic resonance imaging (MRI) is advantageous in the diagnosis of deep internal cancers, but contrast agents (CAs) are always needed to improve MRI sensitivity. Gadolinium (Gd)-based agents are routinely used as T₁-dominated CAs in clinic but using intracellularly formed Gd nanoparticles to enhance the T₂-weighted MRI of tumor in vivo at high magnetic field has not been reported. Herein, we rationally designed a "smart" Gd-based probe Glu-Cys(StBu)-Lys(DOTA-Gd)-CBT (1), which was subjected to γ-glutamyltranspeptidase (GGT) cleavage and an intracellular CBT-Cys condensation reaction to form Gd nanoparticles (i.e., 1-NPs) to enhance the T₂-weighted MR contrast of tumor in vivo at 9.4 T. Living cell experiments indicated that the 1-treated HeLa cells had an r₂ value of 27.8 mM^-1 s^-1 and an r₂/r₁ ratio of 10.6. MR imaging of HeLa tumor-bearing mice indicated that the T₂ MR contrast of the tumor enhanced 28.6% at 2.5 h post intravenous injection of 1. We anticipate that our probe 1 could be employed for T₂-weighted MRI diagnosis of GGT-related cancers in the future when high magnetic field is available in clinic.

A novel mitochondrial-targeting near-infrared fluorescent probe for imaging γ-glutamyl transpeptidase activity in living cells

γ-Glutamyl transpeptidase (GGT) plays an essential role in regulating cellular glutathione and cysteine homeostasis, and its abnormal elevation is associated with different diseases including cancers. Here a novel mitochondrial-targeting near-infrared fluorescent probe was designed for GGT by conjugating glutamate acid to a newly synthesized amine hemicyanine fluorophore. The fluorescent probe was initially non-emissive due to the formation of an amide bond which destroyed the electronic-donating ability of the amine moiety and disrupted the push-pull structure. GGT-mediated cleavage of the γ-glutamyl bond regenerated the initial fluorophore with distinct intramolecular charge transfer (ICT) and activated the fluorescence signal. The fluorescent probe displayed a linear relationship to the concentration of GGT in the range of 1.0-90 U L-1, with an estimated limit of detection (LOD) of 0.4 U L-1. Its ability to target and image mitochondrial GGT activity was demonstrated in living cells with high specificity and fast response. We believe our near-infrared fluorescent probe could have great potential in imaging mitochondrial GGT activity and elucidating GGT-associated pathological consequences in living cells and even small animal models.

Dual-channel near-infrared fluorescent probe for real-time tracking of endogenous γ-glutamyl transpeptidase activity

We developed a curcuminoid difluoroboron-based fluorescent probe for tracking endogenous GGT activity with dual-channel light-up near-infrared (NIR) imaging.

Bioluminescence Sensing of γ-Glutamyltranspeptidase Activity In Vitro and In Vivo

γ-Glutamyltranspeptidase (GGT) is an important tumor biomarker but using a bioluminescence (BL) probe to real time monitor its activity has not been reported. Herein, we rationally designed two GGT-cleavable BL probes Glu-AmLH₂ (1) and Glu-p-aminobenzyloxycarbonyl-AmLH₂ (2), and successfully applied them for sensing GGT activity with high sensitivity and excellent selectivity both in vitro and in vivo. The results indicated that, although 2 had lower background BL signal than 1, GGT had higher catalytic efficiency for 1 than 2, and 1 was superior to 2 for sensing GGT activity in living cells and tumors. We envision that our probe 1 could be widely applied for the diagnosis of important GGT-related diseases in animal models in the near future.

The maturation mechanism of γ-glutamyl transpeptidases: Insights from the crystal structure of a precursor mimic of the enzyme from Bacillus licheniformis and from site-directed mutagenesis studies

γ-Glutamyl transpeptidases (γ-GTs) are members of N-terminal nucleophile hydrolase superfamily. They are synthetized as single-chain precursors, which are then cleaved to form mature enzymes. Basic aspects of autocatalytic processing of these pro-enzymes are still unknown. Here we describe the X-ray structure of the precursor mimic of Bacillus licheniformis γ-GT (BlGT), obtained by mutating catalytically important threonine to alanine (T399A-BlGT), and report results of autoprocessing of mutants of His401, Thr415, Thr417, Glu419 and Arg571. Data suggest that Thr417 is in a competent position to activate the catalytic threonine (Thr399) for nucleophilic attack of the scissile peptide bond and that Thr415 plays a major role in assisting the process. On the basis of these new structural results, a possible mechanism of autoprocessing is proposed. This mechanism, which guesses the existence of a six-membered transition state involving one carbonyl and two hydroxyl groups, is in agreement with all the available experimental data collected on γ-GTs from different species and with our new Ala-scanning mutagenesis data.

L-theanine synthesis using γ-glutamyl transpeptidase from Bacillus licheniformis ER-15

Recombinant γ-glutamyl transpeptidase (rBLGGT) from Bacillus licheniformis ER-15 was purified to homogeneity by ion-exchange chromatography. Molecular masses of large and small subunits were 42 and 22 kDa, respectively. The enzyme was optimally active at pH 9.0 and 60 °C and was alkali stable. K(m) and V(max) for γ-glutamyl-p-nitroanilide hydrochloride were 45 μM and 0.34 mM/min, respectively. L-Theanine synthesis was standardized using a one variable at a time approach followed by response surface methodology, which resulted in approximately 85-87% conversion of L-glutamine to L-theanine within 4 h. The standardized reaction contained 80 mM L-glutamine, 600 mM ethylamine, and 1.0 U/mL rBLGGTin 50 mM Tris-Cl (pH 9.0) at 37 °C. Similar conversions were also obtained with the enzyme immobilized in calcium alginate. Using immobilized enzyme, 35.2 g of L-theanine was obtained in three cycles of 1 L each. The product was purified by Dowex 50W X 8 hydrogen form resin and was confirmed by HPLC and proton NMR spectroscopy.

γ-Glutamyltranspeptidases: sequence, structure, biochemical properties, and biotechnological applications

γ-Glutamyltranspeptidases (γ-GTs) are ubiquitous enzymes that catalyze the hydrolysis of γ-glutamyl bonds in glutathione and glutamine and the transfer of the released γ-glutamyl group to amino acids or short peptides. These enzymes are involved in glutathione metabolism and play critical roles in antioxidant defense, detoxification, and inflammation processes. Moreover, γ-GTs have been recently found to be involved in many physiological disorders, such as Parkinson's disease and diabetes. In this review, the main biochemical and structural properties of γ-GTs isolated from different sources, as well as their conformational stability and mechanism of catalysis, are described and examined with the aim of contributing to the discussion on their structure-function relationships. Possible applications of γ-glutamyltranspeptidases in different fields of biotechnology and medicine are also discussed.

γ-Glutamyl transpeptidase from Bacillus pumilus KS 12: decoupling autoprocessing from catalysis and molecular characterization of N-terminal region

Gamma glutamyl transpeptidase from Bacillus pumilus KS12 (GGTBP) was cloned, expressed in pET-28-E. coli expression system as a heterodimeric enzyme with molecular weights of 45 and 20 kDa for large and small subunit, respectively. It was purified by nickel affinity chromatography with hydrolytic and transpeptidase activity of 1.82 U/mg and 4.35 U/mg, respectively. Sequence analysis revealed that GGTBP was most closely related to Bacillus licheniformis GGT and had all the catalytic residues and nucleophiles for autoprocessing recognized from E. coli. It was optimally active at pH 8 and 60°C. It exhibited pH stability from pH 6-9 and high thermostability with t(1/2) of 15 min at 70°C. It had K(m), V(max) of 0.045 mM, 4.35 μmol/mg/min, respectively. Decoupling of autoprocessing by co-expressing large and small subunit in pET-Duet1-E. coli expression system yielded active enzyme with transpeptidase activity of 5.31 U/mg. Though N-terminal truncations of rGGTBP upto 95 aa did not affect autoprocessing of GGT however activity was lost with truncation beyond 63 aa.

Purification and characterization of γ-glutamyltranspeptidase from Bacillus subtilis SK11.004

An extracellular γ-glutamyltranspeptidase (GGT) with a specific activity of 683.4 U/mg was purified to homogeneity from a culture filtrate of Bacillus subtilis SK11.004 in three steps and then characterized. The GGT is composed of one large subunit of 40 kDa and one small subunit of 21 kDa that was determined by SDS-PAGE and a molecular mass of 62 kDa that was determined by gel-filtration chromatography. The purified GGT had an optimal pH and temperature of 10 and 37 °C, respectively, and it was stable at pH 4.0-11.0 or <50 °C. The enzyme exhibited the highest affinity to imino acids (L-Pro) and then decreasing affinities for aromatic amino acids, ethylamine and basic amino acids. The K(m) values of hydrolysis and of transpeptidation for L-Gln were 3.16 mM and 0.83 mM, respectively, suggesting that the GGT likely synthesizes valuable γ-glutamyl peptides using L-Gln as γ-glutamyl donor. The effects of inhibitors on the enzyme suggested that the tryptophan residues and hydroxy groups of Ser or Thr are essential to enzyme activity. Based on the biochemical characteristics of the enzyme and lack of homology to previously identified proteins, it can be concluded that the GGT from B. subtilis SK11.004 is a novel enzyme.

Development of efficient enzymatic production of theanine by γ-glutamyltranspeptidase from a newly isolated strain of Bacillus subtilis, SK11.004

BACKGROUND

Theanine, a unique amino acid found almost exclusively in tea plants, has various favourable physiological and pharmacological functions in humans. Gamma-glutamyltranspeptidase (GGT, EC 2.3.2.2) is considered to be the most effective enzyme for the production of theanine. In fact, GGT can catalyse the transfer of γ-glutamyl moieties from γ-glutamyl compounds to water (hydrolysis) or to amino acids and peptides (transpeptidation).

RESULTS

A novel strain, SK11.004, which produces GGT with high theanine-forming ability was isolated from fermented shrimp paste and identified as Bacillus subtilis through its physiological and biochemical properties as well as its 16S rDNA sequence analysis. Theanine (18.9 mmol L(-1)) was synthesised by GGT (0.06 U mL(-1)) through transfer reaction in the presence of glutamine (20 mmol L(-1)) as a donor and ethylamine HCl (50 mmol L(-1)) as an acceptor at pH 10 and 37 °C for 4 h, the conversion rate being up to 94%.

CONCLUSION

The enzymatic synthesis of theanine using GGT from a newly isolated strain Bacillus subtilis SK11.004 was found to be an efficient method. Moreover, compared with others, the GGT from B. subtilis SK11.004 exhibited the highest ratio of transferring activity to hydrolytic activity using glutamine, suggesting a high potential application in the production of theanine and other functional γ-glutamyl compounds.

Improved catalytic efficiency of a monomeric gamma-glutamyl transpeptidase from Bacillus licheniformis in presence of subtilisin

Monomeric 30 kDa gamma-glutamyl transpeptidase (GGT(30)) was purified from culture broth of Bacillus licheniformis ER-15 along with a heterodimeric 67 kDa GGT (GGT(67)). In presence of subtilisin, GGT(30) had improved catalytic efficiency (V(max)/K(m)) of 59 min(-1), altered pH and temperature optima of pH 11 and 70 degrees C and had salt-tolerant glutaminase activity. Glutaminase activity was retained even in protease-inhibited condition in presence of 2 mM PMSF. GGT(30) and subtilisin complexation was also confirmed by relative electrophoretic mobility and fluorescence quenching experiment.