Probing the metabolism of γ-glutamyl peptides in cyanobacteria via metabolite profiling and 13 C labeling.

Cyanobacteria are attractive model organisms for studies of photosynthesis and diurnal metabolism and as hosts for photoautotrophic production of chemicals. Exposure to bright light, environmental pollutants, and diurnal lifestyle of these prokaryotes may result in significant oxidative stress. Glutathione is a widely studied gamma-glutamyl peptide that plays a key role in managing oxidative stress and detoxification of xenobiotics in cyanobacteria. The functional role and biosynthetic pathways of this tripeptide have been studied in detail in various phyla, including cyanobacteria. However, other γ-glutamyl peptides remain largely unexplored. We use an integrated approach to identify a number of γ-glutamyl peptides based on signature mass fragments and mass shifts in them in 13 C and 15 N enriched metabolite extracts. The newly identified compounds include γ-glutamyl dipeptides and derivatives of glutathione. Carbon backbones of the former turn over much faster than that of glutathione suggesting that they follow a distinct biosynthetic pathway. Further, transients of isotopic 13 C enrichment show positional labeling in these peptides which allow us to delineate between the alternative biosynthetic pathways. Importantly, the amino acid of γ-glutamyl dipeptides shows much faster turnover compared to the glutamate moiety. The significant accumulation of γ-glutamyl dipeptides under slow-growth conditions combined with the results from dynamic 13 C labeling suggests that these compounds may act as reservoirs of amino acids in cyanobacteria.