Porphyrins are cyclic tetrapyrroles that are important in producing heme in animals and chlorophyll in plants. They contain four pyrrole rings linked by methylene bridges that fluoresce red when exposed to light. The main types that have clinical significance are uroporphyrin, coproporphyrin, and protoporphyrin, which differ in their side chain substituents. Heme synthesis occurs mostly in the liver and bone marrow and involves multiple steps in the mitochondria and cytoplasm, and is regulated by a negative feedback loop involving free intracellular heme. Porphyrins play important roles in oxygen transport and activation in hemoproteins.

1. PORPHYRINS

2. Synthesis of porphyrins occurs in
animals for haem production and in
plants for chlorophyll production.

3. STRUCTURE
 Porphyrins are cyclic tetrapyrroles in which the pyrrole rings,
conventionally designated as A, B, C and D, are linked through
their carbon atoms by methene (–CH=) bridges*.

4.  Porphyrins emit intense red fluorescence when
exposed to light at around 400 nm. Thus,
spectrofluorometric methods provide very
sensitive detection and quantification of
porphyrins
 The porphyrin isomers differ in the arrangement
of the side-chain substituents (e.g. acetyl and
propionyl in uroporphyrins; methyl and propionyl
in coproporphyrins) *.

5.  Porphyrins are differentiated by the kind and
order of substitutes occupying the 8 peripheral
position of pyrole rings of porphyrins .
 Of many kinds of porphyrins found in nature,
only uroporphyrin caproporphyrin and
protoporphyrin have clinical significance. *

6. SYNTHESIS *

7.  In mammalian cells, haem synthesis occurs mostly in liver and
erythropoietictissues (the erythropoietic cells (80%) and the liver
parenchymal cells (20%))
 The biosynthetic pathway starts in the mitochondrion and, after
passing through three cytoplasmic stages, re-enters the
mitochondrion for the final steps of haem formation.
 Haemoproteins play very important roles in electron and oxygen
transport, in the activation of oxygen or hydrogen peroxide and,
finally, in hydrogen peroxide decomposition

8. **

14. REGULATION OF HAEM SYNTHESIS *
Regulation of haem biosynthesis in the liver

15. Retroinhibition of hepatic ALAS1 activity via a negative
feedback regulation mediated by the free intracellular
haem pool.

16. BIBLIOGRAPHY
* Puy, Hervé and Deybach, Jean-Charles. "Haem biosynthesis and
excretion of porphyrins", [Documento en línea]. Gastrohep, online
Resource for Gastroenterology, Hepatology and Endoscopy.
Dirección URL:
<http://www.gastrohep.com/ebooks/rodes/Rodes_2_3_10.pdf>.
** King, Michael W. Ph.D. Iron, Heme and Porphyrin Metabolism,
[Documento en línea]. The Medical Biochemistry Page. Dirección
URL: <http://themedicalbiochemistrypage.org/heme-
porphyrin.html>.