Abstract
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.
Highlights
The enzyme γ-glutamyl transpeptidase (GGT; E.C.2.3.3.2) is conserved throughout all three domains of life, ranging from single-celled prokaryotes to multicellular higher eukaryotes (Rawlings et al, 2010)
The current review focuses mainly on bacterial Gamma-glutamyl transpeptidase (GGT) with emphasis on their physiological significance, structural and molecular aspects, autoprocessing, and enzyme catalysis along with their relevance in biotechnology and biomedicine sectors
Gamma-glutamyl transpeptidase enzyme has been studied extensively over the past few decades owing to its universal existence and high sequence similarity among the prokaryotes and eukaryotes, indicating evolutionary conservation, the reason for which is not clear yet
Summary
The enzyme γ-glutamyl transpeptidase (GGT; E.C.2.3.3.2) is conserved throughout all three domains of life, ranging from single-celled prokaryotes to multicellular higher eukaryotes (Rawlings et al, 2010). Gamma-glutamyl transpeptidase was first reported in some Gram-negative proteobacteria, such as Proteus mirabilis and E. coli, as a periplasmic protein along with glutathione as the most abundant thiol (Fahey et al, 1978; Nakayama et al, 1984; Suzuki et al, 1986; Masip et al, 2006) In these organisms, deletion or inhibition of the GGT enzyme led to increased extracellular leakage of glutathione (Nakayama et al, 1984; Suzuki et al, 1986). Constitutive periplasmic expression of H. pylori GGT (HpGGT) allowed metabolism of extracellular glutathione and glutamine present in the host cytosol as a source of glutamate by H. pylori cells (Chevalier et al, 1999; Shibayama et al, 2007) This has been suggested to be the key physiological role played by HpGGT resulting in profuse growth of the pathogen in the gastric mucosa (Gong and Ho, 2004). Glutathione degrading GGT – mainly present in glutathione producing Gram-negative bacteria
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