Figure 1
From: Current status and emerging role of glutathione in food grade lactic acid bacteria
Schematic diagram showing a putative GSH system and its role in LAB. A putative GSH system in LAB is illustrated in the figure. Every component may or may not be present in every genus and species of LAB. Genes or proteins whose activity is not yet established in LAB are shown with dotted circle. Glutathione (GSH) is made-up of three amino acids viz. glutamic acid (E), cysteine (C) and glycine (G). The enzymes of glutathione system are γ-glutamylcystiene synthetase (GshA), glutathione synthetase (GshB), glutathione bifunctional fusion protein (GshAB/GshF), glutathione reductase (GshR/Gor), and glutathione peroxidase (Gpo). GshA catalyzes the formation of γ-glutamylcysteine from glutamic acid and cysteine. Some LAB have only GshA homologs making γ-glutamylcysteine as the major thiol. This molecule also serves as antioxidant in some species. The classical two step biosynthesis of glutathione is absent and it is carried by bifunctional fusion protein GshF in some LAB like S. thermophilus. Some other LAB also have this fusion protein. Besides its possible de-novo synthesis, GSH is also imported from the medium possibly by CydDC, a heterodimeric ATP-binding cassette type transporter. Gpo and GshR are the two main enzymes involved in metabolism of glutathione. Former catalyzes the conversion of reduced glutathione (GSH) to oxidized form (GSSG) and the latter enzyme regenerates the reduced form. Glutathione-S-transferases (GSTs) are a class of enzymes which are involved in cellular detoxification of xenobiotics (X) using reduced glutathione. The exact cellular role of GST in LAB is not yet established. GSH also carries glutathionylation of key proteins of EMP pathway (e.g. GAPDH) and helps to maintain ATP production at required levels during stress conditions. GSH supplementation upregulates activity of enzymes like β-phophoglucomutase (β-Pgm), phosphate acetyltransferase (Pat) etc. during stress conditions [81]. Both synthesized as well as imported GSH is involved in protection of cells from various stress conditions.