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.
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. 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 bezene rings fused to the pyrrole rings, typically with tin as the metal center, is used as the ink commonly found in ballpoint pens.
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:
In brief
Please see the articles on individual enzymesThe 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.
* A porphyrin-related disease:
porphyria* Porphyrin coordinated to iron:
heme* Porphyrin coordinated to magnesium:
chlorophyll* The one-carbon-shorter analogues:
corroles
*
The Porphyrin Page - an excellent introductory site*
Porphynet - an informative site about porphyrins and related structures
