Wk 1 Heme Metabolism Flashcards
How is heme synthesized?
- In mitochondria: Glycine + Succinyl-CoA = δ-ALA (Amino-Levulinic Acid)
-Catalyzed by ALA synthase (rate-limiting enzyme)
-req’s pyridoxal-phosphate (vt B6) to activate glycine - ALA goes into cytosol:
ALA dehydratase (ALAD, aka porphobilinogen synthase) dimerizes 2xALA-> porphobilinogen (a pyrrole ring)
-ALAD req’s zinc (Zn2+)
How is heme degraded?
What is heme?
A porphyrin w/ a ferrous (Fe2+) iron in the center
How do forms of heme differ from each other?
the porphyrin side chains
What are 3 forms of heme in humans?
heme b - in Hb and myoglobin for O2 delivery
heme a and c are components of cytochromes in mitochondrial respiratory chain, cytochrome P450 detoxification enzymes (CYP450s) and some other enzymes
Which intermediates in heme synthesis and degradation are colored?
- Those that end in -in (eg bilirubin) are colored
- Those that end in -ogen (eg porphobilinogen are colorless
Where is heme produced?
In all cells with mitochondria, but BM and liver are the biggest producers
What is heme b made from?
The AA glycine and the TCA cycle intermediate succinyl- CoA with iron added in the final step
Where is heme made?
1st and last steps occur in the mitochondria, intermediary steps in the cytosol
What do deficiencies in heme cause?
Anemia or a group of unusual diseases called porphyrias (Gr for purpled pigment) that can be acquired or inherited
What is the first reaction in heme biosynthesis?
Need δ-ALA (Amino-Levulinic Acid):
In the mitochondria,
condensation of 1 glycine and 1 succinyl-CoA by vitamin B6, which requires ALAS (enzyme) to form gamma-aminolevulinic acid
= the committed step for biosynthesis of heme
What happens with vitamin B6 deficiency?
Prevents adequate heme synthesis -> microcytic hypochromic anemia
What happens when there is sufficient iron but insufficient protoporphyrin?
Excess iron accumulates in lysosomes or mitochondria -> formation of siderosomes (sideroblastic anemia)
What is X-linked sideroblastic anemia caused by?
Pathogenic variants of ALAS2 w/ reduced activity (the enzyme required by vitamin B6 in the biosynthesis of heme)
-some of the variants can be suppressed by pharmacologic doses of vit B6
What happens during (2nd step) heme biosynthesis after ALA is formed?
Need a pyrrole ring:
ALA leaves mitochondria into cytosol
ALA dehydratase (ALAD, AKA porphobilinogen synthase) dimerizes: ALA + ALA => porphobilinogen (a pyrrole ring)
-inhibited by lead (Pb)
What do pathogenic variants in the ALA gene result in?
Autosomal recessive hepatic porphyria
What is step 3 of heme biosynthesis?
Need 4 pyrrole rings:
condensation of 4x porphobilinogen => hydroxymethylbilane (Pre-uroporphyrinogen) (1st porphyrin in pathway - absorbs light and causes photosensitivity), which is 4 pyrrole rings (tetrapyrrole intermediate) arranged linearly
-By Porphobilinogen Deaminase (PBG deaminase, aka Hydroxymethylbilane synthase)
What are the effects of lead in heme biosynthesis?
Lead (Pb) inhibits 2 enzymes in the process:
1. ALA dehydratase
2. ferrochelatase
both -> buildup of ALA -> pathologic effects
Step 4 of heme biosynthesis?
Need a porphyrin ring:
Hydroxymethylbilane => Uroporphyrinogen III
Cyclization or ring closure by Uroporphyrinogen III synthase (UROS)
What happens when there are defects in the PBG deaminase gene?
Autosomal dominant hepatic porphyria called Acute Intermittent Porphyria
-symptoms: acute bouts of severe abdominal pain (neurologic) and psychological distress
What is something that induces acute intermittent porphyria bouts?
- Fasting or hypoglycemia
So, tx w/ glucose and hemin to inhibit ALAS activity - Hormone level changes (menstrual cycle)
- some drugs like barbituates
What happens with a defect in the UROS gene?
Rare autosomal recessive erythroid porphyria called Congeintal Erythropoietic Porphyria
-with this, patients exposed to sunlight get skin lesions and excessive body hair growth as a protective response
Step 5 of heme biosynthesis
Need a proto-porphyrin IX:
Uroporphyrinogen III => Coproporphyrinogen III
Ring modification (removal of 4 CO2) by Uroporphyrinogen Decarboxylase
What does deficiency of uroporphyrinogen decarboxylase cause?
Porphyria Cutanea Tarda
-most common porphyria
-most often acquired by hepatotoxins, ethanol, estrogen therapy, hepatitis, HIV
-~20% cases are familial caused by the UROD gene
-> excessive heme synthesis or inhibition of uroporphyrinogen decarboxylase activity -> excess substrates in hepatocytes-> spontaneous oxidation to photoactive porphyrins that leave liver & circulate -> skin lesions, reddish urine that’s fluorescent in UV light
Steps 6, 7, and 8 of heme synthesis
Need proto-porphyrin IX: steps in mitochondra:
- Coproporphyrinogen III leaves cytosol and enters back into mitochondria
- Coproporphyrinogen III => Proto-porphyrin IX
- final step catalyzed by ferrochelatase, the enzyme that adds an Fe2+ to the center of the protoporphyrin IX ring and forms Heme b
By oxidation reactions
-proto-porhyrin IX + Fe2+ = Heme
-inhibited by lead (Pb)
What are the 2 main categories of porphyrias?
- Erythroid class (development of RBCs affected)
- Hepatic class (heme synthesis in the liver affected)
What are 3 porphyrias in the erythroid class?
- X-linked sideroblastic anemia
- congenital erythropoietic porphyria (Gunther disease)
- erythropoietic protophyria (EPP)
What are 6 porphyrias in the hepatic class?
- ALAD deficient porphyria (ADP)
- Acute intermittent porphyria, AIP
- Hereditary coproporphyria, HCP
- Variegate porphyria, VP
- Porphyria Cutanea Tarda Type I, PCT or sporadic type PCT
- Porphyria Cutanea Tarda Type II, or Familial Type
What happens to the parts of a RBC and heme molecule when catabolized?
- globin amino acids and iron mostly recycled
- Heme is oxidized, ring is open by ER enzyme (heme oxygenase)
- protoporphyrin rings broken down to bilirubin -> eliminiated via bile/feces and urine
Where does most heme catabolism occur?
In macrophages, primarily of the spleen but also liver, lymph and BM
What is the shape of the major form of bilirubin?
Folded so hydrophilic atoms are hydrogen bonded in the center and hydrophobic regions are exposed (opp of typical proteins)
-very hydrophobic
What happens to bilirubin so that it can be eliminated?
- It’s conjugated to 2 molecules of oxidized glucose (glucuronic acid) in the liver
- glucuronic acid is first activated by attachment of the nucleotide UDP by UDP-glucuronosyl transferases, which are part of the Phase II detox system
-covalent attachment of 2 glucuronic acids makes bilirubin more water soluble (hydrophilic)
What is the glucuronic acid molecule?
A glucose oxidized to a caboxylic acid (COOH) at C6
=uronic acid b/c first isolated from urine
What are effects of hyperbilirubinemia?
- Inhibit DNA synthesis
- uncouple oxidative phosphorylation
- inhibit ATPase activity in brain mitochondria
- Inhibits other enzymes - dehydrogenases, electron transport proteins, hydrolyases, enzymes of RNA synthesis, protein and CHO metabolism
What reverses toxic effects of bilirubin?
When it binds to albumin
Describe X-linked sideroblastic anemia (XLSA)
- erythroid class
- Defect in: δ-aminolevulinic acid synthase 2: ALAS2
- microcytic hypochromic anemia; erythroblast present with sidersomes; wide variability in age of presentation; progressive iron accumulation, fatal if not treated
Describe Congenital erythropoietic porphyria, CEP (Gunther disease)
- erythroid class
- defect in: uroporphyrinogen III synthase: UROS
- photosensitivity evidenced by blistering on the back of the hands and other sun-exposed areas of skin, skin friability after minor trauma, facial hypertrichosis (excessive hair growth), skin hyperpigmentation, reddish discoloration of the teeth (erythrodontia); mild to severe hemolytic anemia; wide variability in phenotypic presentation from fetal lethality to late onset
Describe Erythropoietic protoporphyria, EPP
- erythroid class
- defect in: ferrochelatase: FECH
- hypersensitivity to sunlight and fluorescent lighting resulting in burning and itching sensations in skin, severe blistering and scarring
Describe ALA dehydratase deficient porphyria, ADP
- hepatic class
- defect in: ALA dehydratase (also called porphobilinogen synthase): ALAD
- neurovisceral symptoms that are very similar to those experienced by acute intermittent porphyria patients
Describe Acute intermittent porphyria, AIP
- hepatic class
- defect in: PBG deaminase (also called hydroxymethylbilane synthase or rarely uroporphyrinogen I synthase): HMBS
- neurovisceral symptoms including severe abdominal pain, nausea, vomiting, tachycardia, hypertension, anxiety, depression, convulsions, peripheral neuropathy; chronic complications include hepatocellular carcinoma (HCC) and renal failure
Describe Hereditary coproporphyria, HCP
- hepatic class
- defect in: coproporphyrinogen III oxidase: CPOX
- neurovisceral symptoms similar to those experienced by acute intermittent porphyria patients; seizures; peripheral neuropathy with ascending paralysis; some photosensitivity
Describe Variegate porphyria, VP
- hepatic class
- defect in: protoporphyrinogen IX oxidase: PPOX
- neurovisceral symptoms and photosensitivity; most commonly adult-onset; cutaneous blistering skin on photoexposed surfaces; crusty slowly healing skin lesions; occasional facial hypertrichosis (excessive hair growth) and hyperpigmentation; abdominal pain; constipation; back, chest, and extremity pain; anxiety; seizures; peripheral neuropathy associated with progressive muscle weakness that may progress to respiratory paralysis
Describe Porphyria cutanea tarda type I, PCT type I, also called the sporadic type PCT
- hepatic class
- defect in: hepatic uroporphyrinogen decarboxylase activity
- photosensitivity; referred to as the sporadic type of PCT; associated with reduced UROD activity in liver; not associated with direct mutations in the UROD gene; most likely due to multifactorial causes
Describe Porphyria cutanea tarda type II, PCT type II, also called the familial type PCT, may also be referred to as hepatoerythropoietic porphyria, HEP
- hepatic class
- defect in: uroporphyrinogen decarboxylase in non-hepatic tissues: UROD
- photosensitivity evidenced by blistering on the back of the hands and other sun-exposed areas of skin, skin friability after minor trauma, facial hypertrichosis (excessive hair growth), skin hyperpigmentation, severe thickening of affected skin areas (pseudoscleroderma)
3 types of microcytic anemias with similar presentations to porphyrias
- B6 deficiency
- iron deficiency
- heavy metal poisoning
What does gluconeogenesis induce?
heme synthesis - reason why fasting is troublesome w/ porphyrias
What can be determined from measuring the amount of CO we exhale?
There is only 1 enzyme in the body that produces CO, hem oxygenase, which is what creates linear biliverdin (green) (breaks the ring, eventually breaks down to bilirubin, yellow) in heme catabolism.
So can determine the approximate number of Hb catabolized
Heme degradation on metabolic map
What is an important characteristic of bilirubin?
hydrophobic
-> Hepatic UDP-glucoronosyl transferases add glucoronic acids (oxidized glucose), making bilirubin water-soluble, and therefore excretable.
What happens at time of birth?
High heme catabolism and delayed expression of UDP-glucuronosyl transferases -> high bilirubin
How does phototherapy help w/ hyperbilirubinemia?
Blue-green light (460-490 nm) - photons have right amount of energy to unfold bilirubin, exposing hydrophobic atoms so it can be excreted
