Catabolism of heme

• Catabolism of heme is the process by which the heme molecule is broken down into simpler compounds.
• Catabolism of heme process primarily occurs in the liver and spleen and involves several key steps:

1. Heme Oxygenase (HO):

   • Reaction: Heme is broken down into biliverdin, carbon monoxide (CO), and free ferrous iron (Fe²⁺).
   • Enzyme: Heme oxygenase (HO).
   • Cofactors: Utilizes molecular oxygen and NADPH.
   • Mechanism: HO adds oxygen to the heme, cleaving the heme ring and releasing biliverdin, CO, and Fe²⁺.

2. Biliverdin Reductase (BVR):

   • Reaction: Biliverdin is reduced to bilirubin.
   • Enzyme: Biliverdin reductase (BVR).
   • Cofactors: Uses NADPH.
   • Mechanism: BVR reduces biliverdin to bilirubin by adding hydrogen atoms.

3. Conjugation of Bilirubin:

   • Process: In the liver, bilirubin is made more water-soluble by conjugation with glucuronic acid.
   • Enzyme: Uridine diphosphate-glucuronosyltransferase 1A1 (UGT1A1).
   • Product: Bilirubin diglucuronide.
   • Mechanism: UGT1A1 attaches glucuronic acid molecules to bilirubin, making it easier to excrete.

4. Excretion and Metabolism of Bilirubin:

   • Process: Conjugated bilirubin is excreted into bile and travels to the intestine.
   • Intestinal Metabolism: Gut bacteria convert bilirubin into urobilinogens.
   • Fate of Urobilinogens: Some are oxidized to stercobilins (excreted in feces), while others are reabsorbed and excreted as urobilins in urine.

• These steps are crucial for recycling and excreting heme components, with any disruptions potentially leading to health issues.

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