Porphyrin

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 bacteriochlorins (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.

Contents

1 Types of porphyrins and related molecules
2 Porphyrin biosynthesis

2.1 Table
2.2 In brief

3 See also
4 External links

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- (vitamin B12) 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.

If one of the three pyrrole subunits is reduced to pyrroline, a chlorin is produced, the ring structure found in chlorophyll. If two of the three 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.

Porphyrin biosynthesis

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
PBG deaminase Porphobilinogen Hydroxymethyl bilane 11q23.3 [2.5.1.61] [176000] acute intermittent
Uroporphyrinogen III synthase Hydroxymethyl bilane Uroporphyrinogen III 10q25.2-q26.3 [4.2.1.75] [606938] congenital erythropoietic
Uroporphyrinogen III decarboxylase Uroporphyrinogen III Coproporphyrinogen III 1q34 [4.1.1.37] [176100] 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
Ferrochelatase Protoporphyrin IX Heme 18q21.3 [4.99.1.1] [177000] protoporphyria

In brief

Please see the articles on individual enzymes

Heme synthesis - note that some reactions occur in the cytoplasm and some in the mitochondrion (yellow)

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.

External links

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