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.

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

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.

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Biochemicals | Nitrogen heterocycles | Metabolism | Photosynthesis

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Types of porphyrins and related molecules

Porphyrin biosynthesis

Table

In brief

See also

External links