Porphyrins

porphyrinsMyoglobin/Hemoglobincytochromes,tryptophane pyrolase enzyme

Porphyrins: They are biologically important cyclic compounds composed of four pyrrole rings which are linked together by methenyl or methylene bridges and usually complexed with central atom to form biologically important compounds. If the porphyrin combines with ion "Fe+2 " then the molecule is the " heme " of hemoglobin, myoglobin or cytochrome in humans. But if the metal is " Mg+2 " the compound will be chlorophyll in plants . In nature, these metaloporphyrins are conjugated to proteins to form biological important compounds for example:

pyrrole

α β γ δ I
II
III
IV
1
2
3
4
5
6
7
8
The porphin molecular.rings are labeled I,II,III,IV.Substituent positions on rings are labeled 1,2,3,4,5,6,7,8. Methenyl bridges are labeled α ,β .γ ,δ 1
2
3
4
5
6
7
8
Fischers Fromula



1- Hemoglobin: " Hb= Heme + globin ". Heme : is the porphyrin & is called prosthetic group of globin. Hemoglobin combines with oxygen and it is responsible for transport of O2 in blood , CO2 and H+ from tissues to lungs and for buffering action. mol. wt. 65,000. In Hb A the protein molecule is composed of 2 alpha (141 a.a) & 2 beta(146 a.a) 2- Myoglobin: It is O2 storing molecule. It is the respiratory pigment of muscles. During excessive exercise O2 stored in myoglobin will be released and used by mitochondria to produce energy as ATP to continue exercise. The molecule is similar to a subunit of Hb. It consists of a single globin chain of 153 amino acids of α- helix, mol. wt. = 1700 .

3- Cytochromes : They are iron porphyrins. They transfer O2 in oxidative phosphorylation reactions. They are heme proteins. They are slightly different from hemoglobin in connection of iron . Hemoglobin can act with O2 freely , while the cytochromes can not act with O2 freely. Compound that act as electron transfer agents in oxidation-reduction reaction.An imp. Ex. Is cytochrome c, which has a M.wt.of about 13,000 and contains 1 gram-atom of iron per mole.


4- Tryptophan pyrrolase enzyme: This enzyme is found in the step of tryptophan metabolism as follows:

It is an iron porphyrinoprotein which catalyzes oxidation of tryptophan to forml kynurenine .
Tryptophan
Tryptophan
pyrrolase
formyl kynurenine ----------- nicotinic acid

The structure of porphyrins : They are composed of 4 pyrrole rings linked through methylene or methenyl bridges " α ,β,γ,δ" . Various side chains are attached to the 8 corners 1,2,3,4,5,6,7,8 of porphyrins.( C20H14N4) Naming of these compounds depends on the group attached to the 8 corners, the symmetry and the double bonds in corners α ,β,γ,δ. So if methenyl ــ Ch = " unsaturated " , double bond, and the name of this compound will end with ( in ) as uroporphyrin, but if methelyne - CH2 – " saturated " no double bond , the name of this compound will end with ( gen ) as uroporphyrinogen . If the ring No. IV has asymmetric substitution then the type is called type III. If ring No. IV has symmetric substitution the type is called type I. Only type III " sometimes called type IX " and type I are found in nature, but type III is more abundant.

pyrrole

α β γ δ I
II
III
IV
1
2
3
4
5
6
7
8
The porphin molecular.rings are labeled I,II,III,IV.Substituent positions on rings are labeled 1,2,3,4,5,6,7,8. Methenyl bridges are labeled α ,β .γ ,δ 1
2
3
4
5
6
7
8
Fischers Fromula

Uroporphyrin I

Uroporphyrin III
Coproporphyrin I
Coproporphyrin III
A: acetate CH2COO- P: propionate CH2CH2COO- M: methyl CH3- V: vinyl CH2=CH- Coproporphyrins were isolated from faces,but are also found in urine


Biosynthesis of porphyrins and heme formation: In living cells, porphyrin compounds like heme in animals and chlorophyll in plants are synthesized thru a common pathway . Synthesis occurs in all cells except cells which do not contain mitochondria, for example mature erythrocytes. The starting substances are succinyl CoA and glycine. Succinyl CoA is derived from citric acid ( or Krebs cycle ) in carbohydrate metabolism. Pyridoxal phosphate ( B6 ) and magnesium are required. The enzyme which catalyzes the reaction is amino levulinic acid ( ALA ) synthetase . This reaction occurs in the mitochondria. ALA synthetase enzyme is a rate limiting enzyme .

It is inhibited by heme and activated when there's demand for hemoglobin , α- amino – β- ketoadipic acid is formed. Then this α- amino - β- ketoadipic acid loses CO2 molecule forming δ- aminolevulinic acid , this reaction is also catalyzed by aminolevulic synthetase enzyme.

α-Amino-β-ketoadipate δ-Aminolevulinate ALA Scheme for the synthesis of heme :

Further reaction, two molecules of δ - aminolevulinic acid will lose two molecules of water in the cytosol in the presence of ALA dehydratase ( which contains zinc and is inhibited by lead ) . porphobilinogen (PBG) will be produced as the first precursor of pyrrole , and condense together to form uroporphyrinogen (UPG).

δ-Aminolevulinic Porphobilinogen ( PBG )

In the cytoplasm, four molecules of porphobilinogen will lose four ammonia ( NH3 ) forming uroporphyrinogen I using uroporphyrinogen I synthetase enzyme and uroporphyrinogen III by using two enzymes, uroporphyrinogen I synthetase and uroporphyrinogen III cosynthetase.

Four molecules of porphobilinogen

Uroporphyrinogen I synthase Uroporphyrinogen III cosynthase
Uroporphyrinogen I synthase
Uroporphyrin III
Uroporphyrin I
Uroporphyrinogen III
Uroporphyrinogen I
Auto oxid.
Auto oxid.
6H
III
III
IV
IV


Then, both type I and III ( UPG ) will be decarboxylated losing 4 molecules of CO2 with the help of uroporphyrinogen-decarboxylase enzyme , producing coproporphyrinogen I and III in cytosol .

Uroporphyrinogen I

Uroporphyrinogen III
Coproporphyrinogen I
Coproporphyrinogen III
---- copro in I
---- copro in III
Uroporphyrinogen Decarboxylase



Coproporphyrinogen will move to the mitochondria again. Coproporphyrinogen III is oxidized by molecular oxygen and lose 2 molecules of CO2 with the help of coproporphyrinogen oxidase enzyme and it will be converted to protoporphyrinogen III ( or IX ) . Further oxidation of protoporphyrinogen III in the mitochondria with the help of protoporphyrinogen oxidase enzyme,

Coproporphyrinogen III

Protoporphyrinogen III ( IX )
Protoporphyrin III ( IX )
Coproporphyrinogen oxidase
Protoporphyrinogen oxidase

protoporphyrinogen III will be converted to protoporphyrin III. Protoporphyrin III is the parent porphyrin of the heme. Protoporphyrin III with the help of ferrochelatase and the presence of ferrous ion (Fe+2 ) , heme will be produced which is the prosthetic group of hemoglobin. About 85% of heme will be used for hemoglobin synthesis; 10% will be used for the synthesis of myoglobin and 5% for the synthesis of cytochromes.

Ferrochelatase

protoporphyrin III ( IX ) (parent porphyrin of heme )
Heme (Prosthetic group of hemoglobin )

Uroporphyrinogen III cosynthase and Uroporphyrinogen I synthase

Uroporphyrinogen I synthase
Uroporphyrinogen Decarboxylase
Coproporphyrinogen oxidase
Protoporphyrinogen oxidase
Ferrochelatase
CYTOSOL
MITOCHONDRIA
l
Or light in vitro