Heme Metabolism and Hemoproteins Quiz

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Questions and Answers

Which type of hemoprotein is primarily responsible for oxygen transport in blood?

  • Myoglobin
  • Cytochrome P450
  • Hemoglobin (correct)
  • Catalase

What is the first step in the heme synthesis process?

  • Conversion of Porphobilinogen to Heme
  • Formation of δ-ALA (correct)
  • Cleavage of Heme into Bilirubin
  • Formation of Heme from Protoporphyrin IX

Which of the following statements about porphyrins is true?

  • Porphyrins consist of four pyrrole rings linked by methenyl bridges. (correct)
  • Porphyrins are linear compounds formed by a single pyrrole ring.
  • Porphyrins lack nitrogen in their structure.
  • Porphyrins can form complexes with other organic molecules.

Where does the majority of heme synthesis occur in the body?

<p>Liver and bone marrow (A)</p> Signup and view all the answers

What inhibits the formation of Porphobilinogen in heme synthesis?

<p>Lead (B)</p> Signup and view all the answers

What is the primary enzyme responsible for catalyzing the final step in heme synthesis?

<p>Ferrochelatase (A)</p> Signup and view all the answers

Which substance inhibits the activity of Ferrochelatase, leading to acquired porphyria?

<p>Lead (C)</p> Signup and view all the answers

In liver cells, which mechanism does heme NOT use to regulate ALA synthase activity?

<p>Direct activation (B)</p> Signup and view all the answers

What is the primary clinical manifestation of Acute Intermittent Porphyria (AIP)?

<p>Severe abdominal pain (A)</p> Signup and view all the answers

Which enzyme is deficient in Porphyria Cutanea Tarda (PCT)?

<p>Uroporphyrinogen decarboxylase (B)</p> Signup and view all the answers

What effect do barbiturates and grisofulvin have on ALA synthase activity in the liver?

<p>Decrease heme concentration, thus stimulating ALA synthase (D)</p> Signup and view all the answers

In erythroid cells, which enzyme primarily regulates heme formation?

<p>Uroporphyrinogen synthase (B)</p> Signup and view all the answers

What triggers the symptoms in Acute Intermittent Porphyria?

<p>Consumption of certain medications (B)</p> Signup and view all the answers

What type of inheritance is exhibited by most types of porphyrias?

<p>Autosomal dominant (D)</p> Signup and view all the answers

Which diagnostic technique is best for demonstrating porphyrins in urine?

<p>Ultraviolet fluorescence (C)</p> Signup and view all the answers

Flashcards

Heme Formation

The last step in heme synthesis, where protoporphyrin IX reacts with ferrous iron to form heme, occurring in mitochondria.

Ferrochelatase

An enzyme that catalyzes the reaction of protoporphyrin IX and ferrous iron to form heme.

ALA Synthase Regulation in Liver

The rate-controlling enzyme for heme biosynthesis in liver cells, regulated by feedback inhibition and other mechanisms.

Porphyrias

A group of disorders caused by genetic defects in heme synthesis, resulting in accumulation of porphyrin precursors.

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Erythroid Heme Regulation

In erythroid cells, heme synthesis is primarily regulated by Uroporphyrinogen synthase and ferrochelatase, not ALA synthase.

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Acute Intermittent Porphyria (AIP)

A hepatic porphyria caused by a deficiency in hydroxymethylbilane synthase, leading to abdominal pain and neuropsychiatric symptoms.

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Porphyria Cutanea Tarda (PCT)

Chronic cutaneous porphyria caused by uroporphyrinogen decarboxylase deficiency, resulting in photosensitivity.

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Inheritance of Porphyrias

All types exhibit autosomal dominant inheritance except congenital erythropoietic porphyrias, which are autosomal recessive.

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Diagnosis of Porphyrias

Demonstrating porphyrins using UV fluorescence to detect strong red fluorescence in urine.

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Heme Synthesis

The process of creating heme, essential for hemoproteins, involving multiple steps.

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δ-ALA Formation

First step in heme synthesis, forming δ-aminolevulinate from succinyl-CoA and glycine, requiring vitamin B6.

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Lead Poisoning

Inhibits the formation of porphobilinogen (PBG) during heme synthesis, resulting in symptoms of porphyria.

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Hemoproteins

Proteins that contain heme, important for various functions such as oxygen transport and detoxification.

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Study Notes

Heme Metabolism Objectives

  • Study Heme Synthesis
  • Understand Heme Catabolism
  • Examine the Biochemical Basis and Clinical Implications of Porphyrias
  • Explore Heme Catabolism into Bilirubin

Hemoproteins

  • Hemoglobin: Oxygen transport in blood
  • Myoglobin: Oxygen storage in muscles
  • Cytochromes: Electron transport in the respiratory chain
  • Cytochrome P450: Hydroxylation reactions
  • Catalase and peroxidases: Degradation of hydrogen peroxide
  • Tryptophan pyrrolase: Oxidation of tryptophan
  • Nitric oxide synthase: Synthesis of nitric oxide

Heme Structure

  • Heme is a Metaloporphyrin
  • Heme = porphyrin ring (Protoporphyrin IX) + iron
  • Porphyrins are cyclic compounds formed by fusion of four pyrrole rings linked by methenyl bridges
  • Porphyrins form complexes with metal ions

Heme Synthesis Sites

  • Heme synthesis occurs in all cells due to the requirement for heme as a prosthetic group of enzymes and electron transport chain
  • Major sites of heme synthesis are the liver, and erythroid progenitor cells of the bone marrow
  • Liver involves formation of hemoproteins (e.g., cytochrome P450) for detoxification
  • Erythroid cells involve hemoglobin synthesis
  • Steps 1, 6, 7, and 8 occur in mitochondria
  • Steps 2, 3, 4, and 5 occur in cytoplasm

Overview of Heme Synthesis

  • Heme synthesis involves 8 steps
  • Succinyl CoA + Glycine → 8-aminolevulinic acid
  • Step 1: Formation of 8-ALA
    • Occurs in the mitochondria
    • Requires Pyridoxal phosphate (B6)
    • Catalyzed by d-amino levulenic acid synthase
    • Condensation reaction of succinyl-CoA and glycine
  • Step 2: Formation of Porphobilinogen (PBG)
    • Occurs in the cytoplasm
    • Catalyzed by ALA dehydratase
    • Inhibited by Lead (Acquired porphyria)
  • Last Step: Formation of heme
    • Occurs in the mitochondria
    • Catalyzed by Ferrochelatase
    • Inhibited by Lead (Acquired porphyria)

Regulation of Heme Synthesis

  • Heme biosynthesis is regulated differently in erythroid and liver cells
  • Rate-controlling enzyme for heme biosynthesis is ALA synthase
  • Heme or its Fe(III) oxidized (Hematin and Hemin) products control the enzyme's activity via feedback inhibition
  • Inhibiting ALA synthase transport from the cytosol to the mitochondria
  • Repressing ALAS synthesis

Effect of Drugs on ALA Synthase Activity

  • Drugs like barbiturates and grisofulvin increase ALA synthase activity, leading to more heme consumption.
  • Consequently, heme concentration reduced, stimulating ALA synthase

Porphyrias

  • Porphyria is an inherited genetic defect in enzyme of heme synthesis resulting in accumulation of porphyrin precursors
  • Precursors increase in blood and appear in urine.
  • Porphyrias are classified as erythropoietic or hepatic, depending on the major site where the deficiency manifests
  • Porphyrias exhibit autosomal dominant (AD) inheritance, except for congenital erythropoietic porphyrias (AR).

Types of Porphyria

  • Many types with different defects and symptoms
    • Acute Intermittent Porphyria (AIP): Usually expresses after puberty, abdominal pain, neuropsychiatric disturbances;triggered by drugs, infections, menstrual cycle; not photosensitive.
    • Porphyria cutanea tarda (PCT): Chronic cutaneous hepatic porphyria, the most common; caused by deficiency of the enzyme uroporphyrinogen decarboxylase; increased excretion of uroporphyrins; cutaneous photosensitivity.

Degradation of Heme

  • Red blood cells (RBCs) live for 120 days.
  • RBC degradation occurs in the Reticulo-endothelial System (RES) in the liver and spleen.
  • Heme is released from hemoglobin; Globin reused
  • Heme -> Biliverdin -> Bilirubin -> Excreted by liver into Bile.

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