Heme Synthesis and Degradation Flashcards
What is the function of the gene ALAS1 and on which chromosome is it located?
It is a ubiquitous/housekeeping gene located on chromosome 3
What is the rate-limiting step in heme synthesis? What are the reactants and product? Where is it produced?
Synthesis of δ-aminolevulinic acid (δ-ALA) from succinyl-CoA and glycine. It is produced in the bone marrow
Which part of the cell do the steps for heme synthesis take place?
The first and last three steps takes place in the mitochondria. The intermediates take place in the cytosol
The accumulation of hydroxymethylbilane and intermediates after leads to what?
The accumulation of hydroxymethylbilane leads to photosensitivity
What enzyme catalyzes the synthesis of δ-aminolevulinic acid (δ-ALA)?
δ-ALA synthase
The synthesis of vitamin B6 and mutations in the ALAS2 gene can cause what?
X-linked sideroblastic anemia
Out of ALAS1 and ALAS2, which is erythroid specific and which one is ubiquitous?
ALAS1 is ubiquitous, ALAS2 is erythroid-specific
Where is ALAS2 expressed?
In which chromosome is it located?
When is it activated and what regulates its activation?
ALAS2 is expressed in RBC precursors. It is located in chromosome X
Expression is induced only during active heme synthesis. Iron availability regulates it, which prevents the accumulation of porphyrin intermediates
What are sideroblasts?
They are abnormal nucleated erythroblasts
What is the treatment for X-linked sideroblastic anemia?
Pyridoxine (vitamin B6), which is a cofactor of ALA-synthase
What is the function of PGC-1α?
It is a coactivator of nuclear receptors and transcription factors, regulates mitochondrial biogenesis and oxidative metabolism.
It induces the synthesis of enzymes involved in FA oxidation and gluconeogenesis. Insulin and glucose inhibit PGC-1α synthesis because of the decreased demand in cytochromes required for the production of ATP in the liver
This disease is due to mutations in ALAD gene
ALAD porphyria
This disease leads to the accumulation of succinylacetone, which is a potent competitive inhibitor of ALAD
Hepatorenal tyrosinemia
This disease leads to anemia and accumulation of δ-ALA
Lead poisoning
What is the action of lead?
It inhibits ALAD activity by displacing zinc atoms from the enzyme
What happens in elevated levels of δ-ALA?
Elevated levels of δ-ALA is neurotoxic that can affect both central and peripheral nerve systems, resulting in memory loss and confusion. It can cause hypertension and hyponatremia via increased production of ADH
How is hydroxymethylbilane (HMB) produced? What catalyzes the reaction?
HMB is produced by condensation of four porphobilinogen molecules
The reaction is catalyzed by PBG-deaminase (PBGD)/HMB synthase
What happens to the final step of the production of protoporphyrin IX? What enhances the reaction?
Introduction of Fe2+. It occurs spontaneously, but very slow. The reaction is enhanced by ferrochelatase
This recessive disease is caused by a deficiency of ALA dehydratase in RBC
ALA dehydratase porphyria
This disease is caused by the gain of function mutation in ALAS2 gene
X-linked dominant protoporphyria
This disease is the most subtype of porphyria. Patients often develop photosensitivity, cutaneous sensitivity is caused by accumulationo f porphyrins under skin, which become phototoxic upon activation by light
Porphyria Cutanea Tardea (PCT)
What are the precipitating factors of porphyria?
Illicit drugs: barbituates, sulfa-containing antibiotics, some antiepileptic drugs, progestogens, and synthetic estrogens
Excessive alcohol consumption, heavy exercise schedule, infections, reproductive hormones, smoking, and severe calorie restriction or fasting
What is the function of haptoglobin?
Hemoglobin-haptoglobin complex is readily metabolized in the liver and spleen forming an iron-globin complex and bilirubin. Prevents loss iron in urine
What is the function of hemopexin?
Binds free heme. The heme-hemopexin complex is taken up by the liver and the iron is stored bound to ferritin