Influence of signal-peptide truncations on the functional expression of Escherichia coli gamma -glutamyltranspeptidase

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.

Biophysical characterization of Bacillus licheniformis and Escherichia coli γ-glutamyltranspeptidases: A comparative analysis

The oligomeric states of Bacillus licheniformis and Escherichia coli γ-glutamyltranspeptidases (BlGGT and EcGGT) in solution have been investigated by analytical ultracentrifugation. The results showed that BlGGT has a sedimentation coefficient of 5.12S, which can be transformed into an experimental molecular mass of approximately 62,680Da. The monomeric conformation is conserved in EcGGT. SDS-PAGE analysis and cross-linking studies further proved that the autocatalytically processed BlGGT and EcGGT form a heterodimeric association. Unfolding analyses using circular dichroism and tryptophan emission fluorescence revealed that these two proteins had a different sensitivity towards temperature- and guanidine hydrochloride (GdnHCl)-induced denaturation. BlGGT and EcGGT had a T(m) value of 59.5 and 49.2°C, respectively, and thermal unfolding of both proteins was found to be highly irreversible. Chemical unfolding of BlGGT was independent to the pH value ranging from 5 to 10, whereas the pH environment was found to significantly influence the GdnHCl-induced denaturation of EcGGT. Both enzymes did not reactivate from the completely unfolded states, accessible at 6M GdnHCl. BlGGT was active in the presence of 4M NaCl, whereas the activity of EcGGT was significantly decreased at the high-salt condition. Taken together, these findings suggest that the biophysical properties of the homologous GGTs from two mesophilic sources are quite different.

Role of the conserved Thr399 and Thr417 residues of Bacillus licheniformis gamma-Glutamyltranspeptidase as evaluated by mutational analysis

Role of the conserved Thr399 and Thr417 residues of Bacillus licheniformis gamma-glutamyltranspeptidase (BlGGT) was investigated by site-directed mutagenesis. Substitutions of Thr399 and Thr417 of BlGGT with Ser resulted in a dramatic reduction in enzymatic activity. A complete loss of the GGT activity was observed in T399A, T399C, T417A, and T417K mutant enzymes. Furthermore, mutations on these two residues impaired the capability of autocatalytic processing of the enzyme. In vitro maturation experiments showed that BlGGT mutant precursors, pro-T399S, pro-T417S, and pro-T417A, could precede a time-dependent autocatalytic process to generate the 44.9- and 21.7-kDa subunits; however, the processed T417A had no enzymatic activity. Measurement of intrinsic tryptophan fluorescence revealed alteration of the microenvironment of aromatic amino acid residues, while Far-UV circular dichroism spectra were nearly identical for wild-type and mutant enzymes. These results suggest that residues Thr399 and Thr417 are important for BlGGT in the enzymatic maturation and reaction.