Heme Synthesis & Hemoglobin Flashcards
1
Q
What are the functions of heme?
A
- Transport of oxygen (hemoglobin, myoglobin)
- Electron transport (respiratory cytochromes)
- Oxidation-reduction reactions (cytochrome P450 enzymes)
2
Q
- Where are the major sites of heme synthesis?
- Where else is heme synthesized?
- What cannot synthesize heme?
A
- Major Sites:
- bone marrow ⇒ hemoglobin (6-7g hemoglobin are synthesized each day)
- liver ⇒ cytochrome P450 enzymes (drug detoxificaton)
- However, heme is also required for other important cellular proteins and is synthesized in virtually all cells,
- mature erythrocytes do not synthesize heme (lack mitochondria)
3
Q
- What are porphyrins?
- What is the structure of heme?
A
- Porphyrins: cyclic tetrapyrroles capable of chelating to various metals to form essential prosthetic groups for various biological molecules
- Heme is predominantly a planar molecule
- porphyrin derrivative + a single ferrous ion (Fe2+ = reduced form of iron)
4
Q
Heme = ?
- What is heme oxidized to?
A
Heme = Ferroprotoporphyrin IX
- Ferroprotoporphyrin IX (heme) is rapidly autooxidized to ferriprotoporphyrin IX (“hemin”; contains ferric Fe3+ iron)
5
Q
7 major steps of heme biosynthesis:
- The 1st and last 3 steps occur in the ….
- The intermediate steps occur in the ….
A
- The 1st and last 3 steps occur in the mitochondrion
- The intermediate steps occur in the cytosol
6
Q
What is the committed step of heme synthesis?
A
Step 1: condensation of glycine and succinyl-CoA with decarboxylation, to yield 5-aminolevulinate (ALA)
7
Q
**Step 1 **of heme synthesis:
- **Reaction: **
- **Enzyme: **
- **Location: **
- **Cofactor: **
A
- Reaction: condensation of glycine and succinyl-CoA with decarboxylation, to yield 5-aminolevulinate (ALA)
- Enzyme: 5-aminolevulinate synthase (ALAS)
-
Location: ALAS is localized to the inner mitochondrial membrane
- encoded by a nuclear gene family
- must be imported into the mitochondrion
-
Cofactor: pyridoxal phosphate (PLP) dependent enzyme (vitamin B6)
- Condensation with succinyl-CoA takes place while the amino group of glycine is in Schiff base linkage to the PLP aldehyde
8
Q
What are the **isoforms **of ALAS?
A
Two isoforms of ALAS:
- ALAS1 is the liver isoform
- ALAS2 is the erythroid/reticulocyte isoform
9
Q
Describe the regulation of ALAS1:
A
- Feedback inhibition by heme or hemin regulates heme biosynthesis in the liver
- Heme (hemin) exerts multiple regulatory effects on hepatic heme biosynthesis by inhibiting ALAS1 synthesis at both transcriptional and translational levels, as well as its mitochondrial import
- drugs or metabolites can increase ALAS1 activity
- **increase the synthesis of cytochrome P450 enzymes **⇒ increasing the demand for heme
10
Q
Describe the regulation of ALAS2:
A
- Heme biosynthesis in erythroid cells is NOT regulated by feedback repression of ALAS2 by heme
- In reticulocytes (immature RBCs), **heme stimulates synthesis of globin **and ensures that heme & globin are synthesized in the correct ratio for assembly into hemoglobin
- Drugs that cause a marked elevation in ALAS1 activity, such as phenobarbital, do not affect ALAS2
11
Q
Step 2 of heme biosynthesis:
- Reaction:
- **Enzyme: **
- **Cofactor: **
- **Location: **
- Complication:
A
-
Reaction: condensation of two molecules of ALA to form one molecule of porphobilinogen (PBG)
- first pathway intermediate that includes a pyrrole ring
- Enzyme: ALA dehydratase (ALAD)
-
Cofactor: Zn2+
- lead and other heavy metals can displace the Zn2+ and eliminate catalytic activity
- Location: cytosol
-
Complication: lead poisoning
- increase ALA in urine
- clinical manifestations that mimic acute porphyrias
12
Q
Effects of **lead poisoning: **
- Heme synthesis:
- Neurologic symptoms:
A
-
Inhibition of ALA dehydratase (aka, porphobilinogen synthase) by lead (Pb2+) results in elevated blood ALA
- as impaired heme synthesis leads to de-repression of transcription of the ALAS gene
-
ALA is toxic to the brain, perhaps due to:
- Similar ALA & neurotransmitter GABA (γ-aminobutyric acid) structures
- ALA autoxidation generates reactive oxygen species (ROS)
13
Q
Step 3 of heme synthesis:
- Reaction:
- Enzyme:
- Coenzyme:
- **Location: **
A
- **Reaction: **
-
Step 1: head-to-tail condensation of 4 porphobilinogen molecules to form hydroxymethylbilane (linear tetrapyrrole)
- Each condensation ⇒ liberation of one ammonium ion
- Step 2: hydroxymethylbilane ⇒ uroporphyrinogen III
-
Step 1: head-to-tail condensation of 4 porphobilinogen molecules to form hydroxymethylbilane (linear tetrapyrrole)
- Enzyme: porphobilinogen deaminase (PBGD) or uroporphyrinogen I synthase
- Coenzyme: uroporphyrinogen III cosynthase
- Location: cytosol
14
Q
What is the role of uroporphrinogen III cosynthase?
A
- The tetrapyrrole can spontaneously cyclize to form uroporphyrinogen I (nonenzymatic) which IS NOT in the normal pathway for heme biosynthesis
- However, PBGD is tightly associated with a second enzyme uroporphyrinogen III cosynthase (UROS)
- no enzymatic activity alone
- serves to direct the stereochemistry of the condensation reaction to yield the uroporphyrin ogen III isomer which IS on the pathway for heme biosynthesis
15
Q
**Step 4 **of heme synthesis:
- Reaction:
- **Enzyme: **
- Location:
A
-
Reaction: uroporphyrinogen III ⇒ coprophorphyrinogen III
- decarboxylation of acetate side chains to methyl groups
- Enzyme: Uroporphyrinogen decarboxylase (UROD)
- Location: cytosol
16
Q
Step 5 of heme synthesis:
- Reaction:
-
Enzyme:
- What is being converted?
-
Location:
- What does this imply?
A
-
Reaction: Coproporphyrinogen III ⇒ protoporphyrinogen IX
- transported into the intermembrane space
-
Enzyme: coproporphyrinogen III oxidase (CPO)
- converts specific propionic acid side chains to vinyl groups
-
Location: intermembrane space of the mitochondrion
- implying that its product or protoporphyrin IX must cross the inner mitochondrial membrane because heme is formed within the inner membrane
17
Q
Step 6 of heme synthesis:
- **Reaction: **
- **Enzyme: **
- **Location: **
A
- **Reaction: **protoporphyrinogen IX ⇒ protoporphyrin IX (moving double bonds)
- Enzyme: protoporphyrinogen IX oxidase (PPO)
- Location: mitochondrion
18
Q
Step 7 of heme synthesis:
- **Reaction: **
- **Enzyme: **
- **Location: **
A
- Reaction: Insertion of Fe2+ into protoporphyrin IX to generate HEME
- Enzyme: ferrochelatase
- Location: mitochondrion
19
Q
- What can inhibit Step 7 of heme synthesis?
- What will result in a brillant flourescent complex?
A
- Ferrochelatase is inhibited by lead (lead poisoning; increase protoporphyrin in urine) and is also inhibited during iron deficiency (anemia)
-
In the absence of Fe2+:
- ferrochetalase can insert Zn2+ into the protoporphyrin ring to yield a brilliantly fluorescent complex