Porphyrin
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A porphyrin is a heterocyclic macrocycle made from 4 pyrrole subunits linked on opposite sides (α position) through 4 methine bridges (=CH-). The macrocycle, therefore, is more aromatic than the related corrins, chlorins (2,3-dihydroporphyrin) and bacteriochlorophylls (2,3,12,13-tetrahydroporphyrin). The extensive conjugated system makes the compound chromatic, hence the name porphyrin, from a Greek word for purple. The macrocycle has 22 pi electrons, 18 of which are active in the conjugated system. As they follow Hückel's rule, porphyrins have aromatic properties.
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[edit] Types of porphyrins and related molecules
Porphyrins combine readily with metals, coordinating with them in the central cavity. Iron- (heme), magnesium- (chlorophyll), zinc-, copper-, nickel-, and cobalt- containing porphyrins are known, and many other metals can be inserted. A porphyrin in which no metal is inserted in its cavity is called a free base.
Some iron-containing porphyrins are called hemes; and heme-containing proteins, or hemoproteins, are found extensively in biochemistry, e.g., hemoglobin. Hemoglobin iron is the actual transfer site for oxygen and can be preferentially bound up with carbon monoxide, thus poisoning by asphyxiation can occur. Some shellfish with green-colored blood have a copper-centered porphyrin.
If one of the four pyrrole subunits is reduced to pyrroline, a chlorin is produced, the ring structure found in chlorophyll. If two of the four pyrrole subunits are reduced, then either a bacteriochlorin (as found in some photosynthetic bacteria) or an isobacteriochlorin is formed, depending on the relative positions of the reduced pyrroles.
Practical uses of porphyrins include meso-tetraphenylporphyrin iron-(III) chloride (or ClFeTPP) as a catalyst in organic chemistry. Porphyrin-based compounds are also used in molecular memory. A common derivative, with benzene rings fused to the pyrrole rings, typically with tin as the metal center, is used as the ink commonly found in ballpoint pens.
[edit] Porphyrin biosynthesis
[edit] Table
This is a schematic representation of porphyrin biosynthesis, with references by EC number and the OMIM database. The porphyria associated with the deficiency of each enzyme is also shown:
| Enzyme | substrate | Product | Chromosome | EC | OMIM | porphyria |
| ALA synthase | Glycine, succinyl CoA | D-Aminolevulinic acid | 3p21.1 | 2.3.1.37 | 125290 | none |
| ALA dehydratase | D-Aminolevulinic acid | Porphobilinogen | 9q34 | 4.2.1.24 | 125270 | acute hepatic porphyria |
| PBG deaminase | Porphobilinogen | Hydroxymethyl bilane | 11q23.3 | 2.5.1.61 | 176000 | acute intermittent porphyria |
| Uroporphyrinogen III synthase | Hydroxymethyl bilane | Uroporphyrinogen III | 10q25.2-q26.3 | 4.2.1.75 | 606938 | congenital erythropoietic porphyria |
| Uroporphyrinogen III decarboxylase | Uroporphyrinogen III | Coproporphyrinogen III | 1q34 | 4.1.1.37 | 176100 | porphyria cutanea tarda |
| Coproporphyrinogen III oxidase | Coproporphyrinogen III | Protoporphyrinogen IX | 3q12 | 1.3.3.3 | 121300 | coproporphyria |
| Protoporphyrinogen oxidase | Protoporphyrinogen IX | Protoporphyrin IX | 1q22 | 1.3.3.4 | 600923 | variegate porphyria |
| Ferrochelatase | Protoporphyrin IX | Heme | 18q21.3 | 4.99.1.1 | 177000 | erythropoietic protoporphyria |
[edit] In brief
The committed step for porphyrin synthesis is the formation of D-Aminolevulinic acid from glycine (an abundant amino acid) and succinyl-CoA (from the citric acid cycle). Two dALA molecules are combined into porphobilinogen (PBG), which contains the pyrrole ring. Four PBGs are then combined through deamination into hydroxymethyl bilane (HMB), which is hydrolysed to form the circular tetrapyrrole uroporphyrinogen III. This molecule undergoes a number of further modifications. Intermediates are used in different species to form particular substances, but, in humans, the main end-product protoporphyrin IX is combined with iron to form heme. Bile pigments are the breakdown products of heme.
[edit] See also
- A porphyrin-related disease: porphyria
- Porphyrin coordinated to iron: heme
- Porphyrin coordinated to magnesium: chlorophyll
- The one-carbon-shorter analogues: corroles
- Corphins, the highly reduced porphyrin that binds the F430 active site in methyl coenzyme M reductase (MCR)
