Porphyrin

A porphyrin is a heterocyclic macrocycle made from 4 pyrrole subunits linked on opposite sides through 4 methine bridges. The macrocycle, therefore, is completely aromatic, unlike the related corrins or chlorins.

Table of contents

1 Types of porphyrins
2 Porphyrin synthesis

2.1 Table
2.2 In brief

3 See also
4 External links

Types of porphyrins

Porphyrins combine readily with metals, coordinating with them in the central cavity. Iron, 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 "free base".

Some iron containing porphyrins are called hemes, and heme-containing proteins, or hemoproteins, are found extensively in biochemistry.

If one of the four pyrrole subunits is reduced, 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, depending on the relative positions of the reduced pyrroles, is formed.

Porphyrin synthesis

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.

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