Heme Synthesis, Degradation and Porphyrias - Part 1 Flashcards
In which cells is heme mainly synthesized?
Heme synthesis takes place in all cells but in larger scale mainly (85 %) in bone marrow (in erythroid cells, not mature RBC) and the rest mainly in the liver.
What is the difference between heme and hemin?
- Heme contains ferrous iron and hemin contains ferric iron.
- Hemin is formed from heme when it cannot be incorporated into a protein.
What is the general concept of heme synthesis?
The general concept of heme synthesis is to use succinyl CoA and glycine, form ALA, and porphobilinogen as building blocks and then hyddroxymethylbilane.
- Ring closure needs the switch to asymmetric pyrrol ring D.
- Finally the side chains are modified and the correct double bonds arranged.
Where does Heme synthesis take place in the cells? Are the reactions reversible? What is special about the intermediates?
The synthesis starts and ends in mitochondria, other steps take place in cytosol. All reactions are irreversible. The enzymes have to act fast in order to prevent the spontaneous formation of abnormal porphyrins from intermediates.
Give six examples of proteins or enzymes that need heme.
Hemoglobin, myoglobin, cytochromes P450, cytochromes of the ETC, catalase, nitric oxide synthase
Describe the reaction catalyzed by ALA synthase: cellular location, substrates, coenzyme.
ALA synthase is a mitochondrial enzyme that uses succinyl CoA and glycine.
-The formation of gamma-aminolevulinic acid (ALA) includes the decarboxylation of
glycine
-ALA synthase needs PLP (vitamin B6) as coenzyme.
Where in the cell is porphobilinogen formed?
ALA leaves from mitochondria into the cytosol and porphobilinogen is formed in cytosol
During the formation of porphobilinogen, what is used?
two ALAs with release of water
What are two other names for ALA synthase?
- ALA dehydratase
- Porphobilinogen synthase
How many porphobilinogens are needed for synthesis of hydroxymethylbilane (HMB)?
Four porphobilinogens are used for the synthesis of one hydroxymethylbilane
During the synthesis of hydroxymethylbilane (HMB), what is released in this reaction?
4 ammonia
What scavenges the ammonia released during the synthesis of HMB?
Glutamine Synthetase
What is another name for Porphobilinogen deaminase?
Hydromethylbilane synthase
What enzyme catalyzes the conversion of 4 porphobilinogens to HMB?
Porphobilinogen deaminase (Hydromethylbilane synthase)
How many succinyl CoAs and glycines are needed for the synthesis of one heme?
Succinyl CoA and glycine are needed to form ALA.
Two ALAs are needed to form porphobilinogen.
Four porphobilinogens are needed to form hydroxymethylbilane.
Totally 8 succinyl CoAs and 8 glycines are needed for the synthesis of one heme
What is special about pyrrol ring D in the heme porphyrin ring system?
Pyrrol ring D is asymmetric in the porphyrin ring system.
What is the first porphyrin ring leading to heme?
The first porphyrin leading to heme is uroporphyrinogen III
The liver isoform for ALA synthase, which is down-regulated by heme/hemin accumulation, is known as what?
ALAS1
The erythroid cell isoform for ALA synthase, which is down-regulated at low amounts of intracellular iron, is known as what?
ALAS2
Hereditary deficiency of the ALAS2 gene leads to what?
X-linked sideroblastic anemia
Do both ALAS1 and 2 need PLP?
- Yes
- Both ALAS1 and ALAS2 decarboxylate glycine and need PLP as coenzyme
Which enzyme in heme synthesis is inhibited by lead?
- Cytosolic ALA dehydratase (porphobilinogen synthase)
- Mitochondrial ferrochelatase
Which cofactors do Cytosolic ALA dehydratase (porphobilinogen synthase) and Mitochondrial ferrochelatase need?
Zinc
Lead binds to what of ALA dehydratase?
Sulfhydryl groups
