احسان نظیفی

Mycosporine-like amino acids (MAAs) are water-soluble pigments that absorb specific UV-B radiation in the range of 280 to 320 nm. They are also thought to be the strongest UVA-absorbing compounds in nature. MAAs are found in various organisms including cyanobacteria and algae from tropical to polar regions and structurally distinct MAAs are known in taxonomically diverse organisms. In this study novel glycosylated MAAs were found in the terrestrial cyanobacterium Nostoc commune. An MAA with an absorption maximum at 334 nm was identified as a hexose-bound porphyra-334 derivative with a molecular mass of 508 Da. The other MAAs with an absorption maximum at 322 nm were identified as the palythine-threonine derivatives linked to one or two hexoses with the molecular masses of 450 and 612 Da, respectively. These purified MAAs have radical scavenging activities in vitro, which suggests multifunctional roles as sunscreens and antioxidants.According to the structures and fragmentation patterns of these glycosylated MAAs, a MAA biosynthetic pathway is predicted in N. commune. The simultaneous occurrence of palythine-threonine and porphyra-334 as scaffolds in N. commune suggests that porphyra-334 could be a metabolic precursor of palythine-threonine. Supporting this idea, the glycosylated palythine-threonine accounted for 60% of the total MAAs and was the most abundant one in N. commune. In our predicted pathway, porphyra-334 could be generated via the addition of threonine to the core ring of mycosporine-glycine and the glycosylation of porphyra-334 could produce the 508-Da MAA. The 450-Da MAA could be formed via the elimination of a portion of glycine from C3 of the 508-Da MAA. The addition of another hexose to the 450-Da MAA could produce the 612-Da MAA.This biosynthetic pathway for the production of the glycosylated palythine-threonine and porphyra-334 derivatives could occur particularly in genotype D of N. commune; however, the enzymes and genes involved in the biosynth