Chapter 15 - Ultraviolet-screening compound mycosporine-like amino acids in cyanobacteria: biosynthesis, functions, and applications

Abstract

Photosynthetic efficiency of photoautotrophs such as algae, cyanobacteria, and higher plants is severely affected by the solar ultraviolet (UV) radiation (UVR), but the mechanisms/strategies employed by these photoautotrophs, including cyanobacteria, against lethal UVR exposure are still not well understood. On exposure to UVR, cellular damage is triggered directly by affecting proteins and DNA as well as indirectly via the generation of damaging doses of reactive oxygen species (ROS). For their survival and sustenance and overcoming the harmful effects of UVR, cyanobacteria synthesize their own UV-blockers such as mycosporine-like amino acids (MAAs) and scytonemin, which help in protecting them against UVR. MAAs are low molecular weight (<400Da), water-soluble nitrogenous compounds having maximum absorption in the UV range (310–360nm). Their structure consists of a cyclohexenimine or cyclohexenone ring attached to an amino acid, amino alcohol, or an amino group. However, the precise biosynthetic route of these multipurpose compounds is still unclear. Evidence suggests their origin from the pentose phosphate pathway as well as from the shikimate pathway with 4-deoxygadusol acting as a common precursor of all types of MAAs. Photochemical and photophysical studies evidenced that MAAs absorb UVR and release it as heat, without generation of free radicals.