porfirinas

Porphyrins are a family of heterocyclic macrocycles composed of four modified pyrrole rings linked by methine bridges at their meso positions to form a planar, highly conjugated ring system. The core porphyrin ring contains 18 π electrons and can bind a metal ion at its central cavity to give metalloporphyrins. Common natural metalloporphyrins include heme (iron-porphyrin) in hemoglobin and various cytochromes, and chlorophylls (magnesium-porphyrins) in photosynthetic organisms. Porphyrins without metal or with different metals appear in a variety of natural and synthetic contexts and can be pigmentary or catalytic.

Porphyrins are central in biological energy metabolism. In animals, the biosynthesis of heme proceeds from δ-aminolevulinic acid through porphobilinogen, hydroxymethylbilane, uroporphyrinogen III, and coproporphyrinogen III to protoporphyrin IX; insertion of iron completes heme. The pathway also yields chlorophylls in plants and bacteria. Porphyrins and porphyrinogens differ chemically; porphyrins are fully oxidized and typically colored; porphyrinogens are colorless and easily oxidized to porphyrins.

Porphyrins have intense electronic transitions giving strong absorption bands in the visible region, notably the Soret band near 400 nm, and exhibit fluorescence in many variants. They are used as photosensitizers in photodynamic therapy, in diagnostic imaging, and as catalysts or functional materials in chemistry and materials science. In nature, metalloporphyrins serve as active sites in redox reactions and energy transfer processes.

Disorders of heme biosynthesis, collectively called porphyrias, involve defects in enzymatic steps and lead to accumulation of porphyrins or their precursors, with symptoms ranging from skin photosensitivity to abdominal pain and neuropathy. Diagnosis relies on porphyrin quantification in blood, urine, and stool, and management focuses on avoiding triggers and treating symptoms.