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Questions and Answers
Which enzyme is the rate-limiting step in heme synthesis?
Which cofactor is required by ALAS for heme synthesis?
Which isozyme of ALAS is expressed in erythroid cells?
Which enzyme condenses two molecules of ALA to form porphobilinogen (PBG)?
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Which enzyme converts protoporphyrinogen IX to protoporphyrin IX?
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Which enzyme directs the stereochemistry of the reaction to form uroporphyrinogen III?
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Which porphyria is the most common?
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Which type of porphyria is characterized by increased synthesis of intermediates prior to the genetic block?
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Which type of porphyria is associated with photosensitivity?
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Which enzyme is inhibited by lead and can cause features similar to porphyrias?
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Which organelle is responsible for the synthesis of heme?
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What is the primary function of heme in the body?
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Which metal is chelated with the protoporphyrin IX ring to form heme?
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What is the main product of heme degradation?
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Which of the following is NOT a porphyria resulting from enzyme deficiencies in heme synthesis?
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What can heme rapidly autooxidize to?
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Which step in heme synthesis is catalyzed by 5-aminolevulinic acid (δALA) synthase (ALAS)?
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How does lead poisoning affect heme biosynthesis?
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Which of the following is NOT a primary site of heme synthesis and catabolism?
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What can accumulating upstream intermediates in porphyrias be cytotoxic in the presence of?
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Study Notes
Heme Synthesis Pathway
- The rate-limiting enzyme in heme synthesis is 5-aminolevulinic acid synthase (ALAS).
- ALAS requires pyridoxal phosphate as a cofactor for heme synthesis.
- The isozyme of ALAS expressed in erythroid cells is ALAS2.
Key Enzymatic Reactions
- Two molecules of 5-aminolevulinic acid (ALA) condense to form porphobilinogen (PBG) catalyzed by porphobilinogen synthase.
- Protoporphyrinogen IX is converted to protoporphyrin IX by protoporphyrinogen oxidase.
- Uroporphyrinogen III is formed with directed stereochemistry through the action of uroporphyrinogen synthase.
Porphyrias
- Acute intermittent porphyria is the most common type of porphyria.
- Porphyrias characterized by increased synthesis of intermediates before the genetic block are termed "acute" porphyrias.
- Cutaneous porphyria is associated with photosensitivity.
Lead and Heme Synthesis
- Lead inhibits the enzyme ferrochelatase, causing symptoms similar to porphyrias.
- Heme synthesis occurs primarily in the mitochondria.
Functions and Degradation of Heme
- Heme plays a crucial role in oxygen transport and electron transfer in the body.
- Heme is chelated with iron (Fe2+) in the protoporphyrin IX ring to form heme.
- The main product of heme degradation is biliverdin, which is further converted to bilirubin.
Miscellaneous
- Not all porphyrias result from enzyme deficiencies in heme synthesis; some can be caused by other factors.
- Heme can autooxidize rapidly to form methemoglobin.
- 5-aminolevulinic acid (δALA) synthase (ALAS) catalyzes the first step in heme synthesis, converting succinyl-CoA and glycine to ALA.
- Lead poisoning disrupts heme biosynthesis by inhibiting critical enzymes, leading to accumulation of toxic intermediates.
- Not all tissues are primary sites of heme synthesis and catabolism; the liver and bone marrow are the main sites.
- Accumulating upstream intermediates can be cytotoxic in the presence of reactive oxygen species.
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Description
Test your knowledge on the biochemistry of blood and heme synthesis. Learn about the regulation of the rate-limiting step, connections to the TCA cycle and amino acid metabolism, and the role of iron. Discover how lead poisoning can impact heme biosynthesis and resemble porphyria.
