Damage of biological components by UVB can be manifested in two ways: (1) direct absorption ofUVB by organic molecules, or (2) oxidation of organic molecules by reactive oxygen species (ROS) and other radicals that are produced by the UV photolysis of water (e.g., hydroxyl ions, peroxyl ions, singlet oxygen, superoxide) or dissolved substances in the extra- or intracellular environment.
Many organic molecules absorb UVB and undergo conformational modifications that interfere with physiological processes. There are a number of UVB-induced DNA photoproducts such as cyclobutane pyrimidine dimers (CPDs), 6-4 pyrimidine-pyrimidone adducts, pyrimidine hydrates, and DNA-protein crosslinks. CPDs are the most abundantly formed type of DNA damage, but the presence of any lesions in the DNA molecule can interfere with DNA replication and RNA transcription processes, affecting physiological rates, cell growth, and viability. ROS and other radicals also cause specific types of oxidative damage to DNA that can be detrimental. UVB-induced DNA damage can result in debilitating, mutagenic, and lethal effects.
UVB can also damage proteins by direct absorption or oxidation by UVB-induced radicals. Aromatic amino acids (tyrosine, phenylalanine, and tryptophan) absorb strongly at the lower end of the UVB range (280 nm) and account for the UV-absorbing properties of polypep-tides. UVB exposure can alter rates of protein synthesis and turnover. Since proteins have diverse functions (e.g., enzymes, protective and structural components, energy storage, molecular motors, hormones, etc.), UVB-induced damage can have a wide range of effects.
Lipid molecules can also be damaged by UVB with the greatest biological hazard manifested in damage to membranes. Peroxidation of lipids by the action of UVB is a significant stress for cells to overcome.
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