Heme biosynthetic pathway
The pathway for the formation of heme starts with eight molecules of D-Aminolevulinate.
Condensation with succinyl-CoA takes place while the amino group of glycine is in Schiff base linkage to the aldehyde of PLP. Coenzyme A and the carboxyl of glycine are lost following the condensation reaction.
D-aminolevulinate synthase (ALA Synthase) is the committed step of the heme synthesis pathway, and is usually rate-limiting for the overall pathway. Regulation occurs through control of gene transcription. Heme functions as a feedback inhibitor, repressing transcription of the gene for D-aminolevulinate synthase in most cells. A variant of ALA Synthase expressed only in developing erythrocytes is regulated instead by availability of iron in the form of iron-sulphur clusters.
As each of the two d-aminolevulinate (ALA) substrates binds at the active site, its keto group initially reacts with the side-chain amino group of one of the two lysine residues to form a Schiff base. These Schiff base linkages promote the C-C and C-N condensation reactions that follow, assisted by the metal ion that coordinates to the ALA amino groups.
The Zn++ binding sites in the homo-octomeric mammalian Porphobilinogen Synthase, which include cysteine S ligands, can also bind Pb++ (lead). Inhibition of Porphobilinogen Synthase by Pb++ results in elevated blood ALA, as impaired synthesis of heme results in de-repression of transcription of the gene for ALA Synthase.
High ALA is thought to cause some of the neurological effects of lead poisoning, although Pb2+ also may directly affect the nervous system. ALA (d-aminolevulinate) is toxic to the brain. This may be due in part to the fact that ALA is somewhat similar in structure to the neurotransmitter GABA (g-aminobutyric acid). In addition, autoxidation of ALA generates reactive oxygen species (oxygen radicals).
PBG Synthase - ALA
PBG Synthase - PBG
Porphobilinogen (PBG) is the first pathway intermediate that includes a pyrrole ring.
The porphyrin ring is formed by condensation of four molecules of porphobilinogen.
Porphobilinogen Deaminase catalyzes successive condensations of PBG, initiated in each case by elimination of the amino group.
Porphobilinogen Deaminase enzyme has a dipyrromethane prosthetic group, linked at the active site via a cysteine S.
The enzyme itself catalyzes formation of this prosthetic group.
PBG units are added to the dipyrromethane until a linear hexapyrrole has been formed.
Porphobilinogen Deaminase is organized in 3 domains. Predicted interdomain flexibility may accommodate the growing polypyrrole in the active site cleft.
Hydrolysis of the link to the enzyme's dipyrromethane releases the tetrapyrrole hydroxymethylbilan
Uroporphyrinogen III Synthase converts the linear tetrapyrrole hydroxymethylbilane to the macrocyclic uroporphyrinogen III.
Uroporphyrinogen III Synthase catalyzes ring closure, and flipping over one of the pyrroles, to yield an asymmetric tetrapyrrole. Note the distribution of acetyl and propionyl side chains in the diagram above.
This rearrangement is thought to proceed via a spiro intermediate, as depicted at right and in the animation below.
The active site of Uroporphyrinogen III Synthase is located in a cleft between two domains of the enzyme. The structural flexibility inherent in this arrangement is proposed to be essential to catalysis.
Uroporphyrinogen III is the precursor for synthesis of vitamin B12, chlorophyll, and heme, in organisms that produce these compounds.
Conversion of uroporphyrinogen III to protoporphyrin IX (above) occurs in several steps.
These steps include:
- decarboxylation of all 4 acetyl side chains, converting them to methyl groups (catalyzed by Uroporphyrinogen Decarboxylase).
- oxidative decarboxylation of 2 of the 4 propionyl side chains, converting them to vinyl groups (catalyzed by Coproporphyrinogen Oxidase)
- oxidation adds more double bonds (catalyzed by Protoporphyrinogen Oxidase), yielding protoporphyrin IX.
Fe2+ is added to protoporphyrin IX via Ferrochelatase. This enzyme in mammals is homodimeric and contains two [2Fe-2S] iron-sulfur clusters.
A conserved active site histidine, along with a chain of anionic residues, may conduct released protons away, as Fe2+ binds from the other side of the porphyrin ring, to yield heme.