Functional role of the conserved glycine residues, Gly481 and Gly482, of the γ-glutamyltranspeptidase from Bacillus licheniformis

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.

Mutational Analysis of a Highly Conserved PLSSMXP Sequence in the Small Subunit of Bacillus licheniformis γ-Glutamyltranspeptidase

A highly conserved ⁴⁵⁸PLSSMXP⁴⁶⁴ sequence in the small subunit (S-subunit) of an industrially important Bacillus licheniformis γ-glutamyltranspeptidase (BlGGT) was identified by sequence alignment. Molecular structures of the precursor mimic and the mature form of BlGGT clearly reveal that this peptide sequence is in close spatial proximity to the self-processing and catalytic sites of the enzyme. To probe the role of this conserved sequence, ten mutant enzymes of BlGGT were created through a series of deletion and alanine-scanning mutagenesis. SDS-PAGE and densitometric analyses showed that the intrinsic ability of BlGGT to undergo autocatalytic processing was detrimentally affected by the deletion-associated mutations. However, loss of self-activating capacity was not obviously observed in most of the Ala-replacement mutants. The Ala-replacement mutants had a specific activity comparable to or greater than that of the wild-type enzyme; conversely, all deletion mutants completely lost their enzymatic activity. As compared with BlGGT, S460A and S461S showed greatly enhanced k_cat/K_m values by 2.73- and 2.67-fold, respectively. The intrinsic tryptophan fluorescence and circular dichroism spectral profiles of Ala-replacement and deletion mutants were typically similar to those of BlGGT. However, heat and guanidine hydrochloride-induced unfolding transitions of the deletion-associated mutant proteins were severely reduced as compared with the wild-type enzyme. The predictive mutant models suggest that the microenvironments required for both self-activation and catalytic reaction of BlGGT can be altered upon mutations.