Porphyrins

• Porphyrins  are cyclic molecules  formed  by fusion of four pyrrole  rings linked  by  methenyl  bridges.
• side  chains  may be  acetate (A), propinonate  (P), methyl  (M)  or vinyl  (V)  groups.
• The technical name for 4 pyrroles connected together is a tetrapyrrole.
• There  are  two  types  of porphyrins,  porphyrin  I  and  porphyrin  III.
•  Heme is the most important porphyrin III.
• Type  III  porphyrins  are far more abundant.
• The  important  porphyrins are: 

1. Uroporphyrins  (contain acetate  and  propionate as  substituent groups),
2. coproporphyrizs  (contain  methyl  and  propionate  as  substituent  groups)
3. protoporphyrin (contain  vinyl, methyl  and propionate  as  substituent groups).

• Heme  is  protoporphyrin.
• Uroporphyrin, the most water-soluble of the porphyrins, is excreted predominantly in urine, coproporphyrin in urine and bile, whereas protoporphyrin the least soluble, is excreted only in the bile
• Reduced form  of porphyrins  is  called  porphyrinogens.
• porphyrins are  coloured compounds and have luminous  property.
• reduced porphyrins (porphyrinogens)  are  colourless  and  nonluminous.
• In porphyrinogens Pyrrole  rings  are  joined  by methylene bridges (-CH2-), whereas in porphyrins Pyrole  rings  are joined  by  methenyl  bridges(-HC=).
• All porphyrias are autosomal dominant except ALA dehydratase deficiency and Congenital Erythropoetic Porphyria, which are autosomal recessive^e, and X-linked sideroblastic anemia, which is X-linked recessive in inheritance.
• Photosensitivity manifest as fluid vesicle and bulla develop on sun exposed areas.
• Hyperpigmentation and hypertrichosis especially on face in porphyria cutanea tarda and congenital erythropoetic porphyria
  

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