Heme Synthesis Flashcards
highest rate of heme synthesis are in _____ cells in _____ and _____
erythroid cells of bone marrow and liver
in liver: heme is mainly used by cytochrome p450 enzymes, and levels depend on demand (CYP enzymes are inducible)
erythroid cells: heme used for hemoglobin synthesis, relatively constant
*note that virtually all tissues synthesis heme though
how is heme used in the liver
used by cytochrome p450 enzymes
heme levels depend on demand (CYP enzymes are inducible)
when does heme synthesis stop in erythrocyte life cycle
mature RBC lack organelles (including mitochondria), so heme synthesis stops when RBC mature
*reticulocytes: immature RBC that contain residual ribosomes/rRNA (account for 1-2% of total RBC
what is the rate-limiting step of heme biosynthesis?
first step:
succinyl CoA + glycine —> delta-aminolevulinic acid
via ALAS
ALAS (aminolevulinate synthase): requires PLP (vit B6 / pyridoxal phosphate)
ALAS, the rate limiting step of heme biosynthesis (in the first step), requires _____
PLP (pyridoxal phosphate), aka Vitamin B6
what is the difference between the 2 isozymes of ALAS?
ALAS - rate limiting first step enzyme of heme synthesis (requires PLP/Vitamin B6)
ALAS1: in liver, where heme levels change based on demand
ALAS2: in erythroid cells, where heme synthesis is relatively constant (for hemoglobin production)
what are 2 ways by which ALAS1 levels are regulated in the liver?
ALAS1: first step and rate-limiting step of heme synthesis in the liver (requires PLP/VitB6)
- negative feedback by heme (multiple mechanisms)
- insulin and glucose (carbohydrates) repress ALAS1 transcription
what are the 4 M’s that can induce ALAS1 transcription in the liver?
remember that ALAS1 is first and rate limiting step of heme synthesis (requires PLP/VitB6)
induce transcription of ALAS1:
1. medication
2. menstruation
3. malnutrition
4. maladies (being sick, stress, etc)
how do medications induce ALAS1 expression in the liver?
ALAS1 - first/rate limiting step of heme synthesis (requires VitB6/PLP)
medications induce synthesis of heme-containing cytochrome p450 —> increases heme demand
results in decrease in heme concentration in the cells
how is ALAS2 regulated in erythroid cells?
ALAS2 is first and rate limiting step of heme synthesis (requires PLP/VitB6)
low iron concentration: IRP (iron regulatory protein) binds IRE (iron response element), which blocks mRNA translation —> low production of ALAS2
high iron concentration: iron binds IRP (preventing it from binding IRE), uninhibiting translation —> increase ALAS2 production
[remember that ferritin is also regulated through IRE-IRP system, and downregulated when iron is low]
how would a vitamin B6 deficiency affect heme synthesis?
would decrease heme synthesis in both liver (ALAS1) and erythroid cells (ALAS2) because ALAS requires PLP/Vitamin B6
ALAS is first and also rate-limiting step of heme synthesis
sideroblastic anemia
ring sideroblasts: in bone marrow, nucleated erythroblasts with a ring of blue granules (due to iron-laden mitochondria) —> sign of impaired heme synthesis
mature RBC in circulation are microcytic (smaller) and hypochromic (pale) because of shortage of hemoglobin
[in blood smear, RBC look paler because the central pallor region is enlarged]
how does a blood smear of thalassemia look, and reminder of what thalassemia is caused by?
thalassemia: genetic mutation in either alpha or beta globin
blood smear - appears as hypochromic microcytic anemia (RBC are smaller and/or pale)
what is 1 hereditary and 3 acquired causes of sideroblastic anemias?
hereditary: X-linked ALAS2 mutation
acquired: drugs (isoniazid for TB), toxins (alcohol), Vitamin B6 deficiency (pyridoxine/PLP)
X-linked sideroblastic anemia
what is a possible complication resulting from the body’s attempt to compensate?
what is the treatment?
XLR mutation in ALAS2 (rate-limiting step in heme synthesis in erythroid cells)
sideroblasts seen in bone marrow due to iron excess (blue granules)
iron overload occurs from reduced usage and increased absorption (compensation) —> heart and liver cirrhosis
treatment: pyridoxine (B6), since it is cofactor for ALAS2
symptoms: fatigue, dizziness, conjunctival/palm/nail pallor
how does lead poisoning affect heme synthesis?
lead inhibits 2 key enzymes in heme synthesis:
1. ALAD (step 2) - more sensitive
2. Ferrochelatase (step 8)
*would see an increase in ALA (delta-aminolevulinic acid, product of ALAS - committed step) without increase in next product (PBG)
heme is a common prosthetic group. what is its structure?
derivative of porphyria - cyclic, formed by 4 pyrrole rings linked by methenyl bridges
iron/Fe2+ found in center of heme
heme is also known as Fe protoporphyrin IX
how does lead poisoning present?
headache, nausea, irritability, lethargy, anorexia, constipation/diarrhea (lead colic)
lead lines in gums, long bone
neuropathy (foot or wrist drop)
developmental delays, memory loss, learning difficulties
[remember that lead poisoning inhibits 2 enzymes in heme synthesis - ALAD and Ferrochelatase]
how is lead poisoning diagnosed? (5)
- measure blood lead levels (BLL)
- accumulation of ALA without increase in PBG (steps in heme synthesis - lead blocks ALAD enzyme in between these)
- zinc protoporphyrin in RBC
- basophilic stippling in peripheral smear (looks like spots - rRNA aggregates in RBC)
- microcytic anemia (small RBC)
other than removing the source of lead, what is a treatment of lead poisoning?
lead chelators
what are the enzymes in heme biosynthesis? and where does heme biosynthesis take place in the cell?
challenge question
mitochondria;
1. ALAS (rate limiting)
cytosol:
2. ALAD (inhibited by lead)
3. PBGD
4. UROS
5. UROD
mitochondria:
6. CPOX
7. PPOX
8. FECH (inhibited by lead)
porphyrias
rare inherited or acquired metabolic disorders caused by mutations in enzymes of heme biosynthesis
results in accumulation/excretion of heme precursors and/or porphyrins
clinical presentations differ - can be acute (triggered by something) or non-acute (constant symptoms), primary tissues involved differ
how does solubility change throughout the heme synthesis pathway?
earlier in the pathway, intermediary products are water soluble and can be excreted in urine (porphyrin precursors)
as more decarboxylation steps occur, water solubility degrees as heme biosynthesis pathway progression —> excretion shifts from urine to bile (porphyrins - become colored by oxygen, so urine or bile may be colored)
acute intermittent porphyria (AIP)
deficiency in hepatic PBG Deaminase (PBGD)
—> ALA and PBG accumulate (heme synthesis) —> neurotoxic
- acute, so presents episodically
- onset usually 20s/30s
- 2nd most common porphyria
abdominal pain, neuropsychiatric symptoms, peripheral neuropathy, red urine (port-wine)
*5Ps: Pain (abdominal), Psychiatric, Peripheral neuropathy, Port-wine urine, PBGD deficiency
what precipitating factors can cause an acute episode of AIP (acute intermittent porphyria)?
4 M’s that induce ALAS1 (first step of heme pathway, before PBG Deaminase which is deficient in AIP):
1. medication
2. menstruation
3. malnutrition
4. maladies
what are 3 treatment options of acute intermittent porphyria (AIP)?
remember that AIP is PBGD deficiency
- avoid precipitating factors (4 M’s that induce ALAS)
- glucose (carb) loading - glucose/insulin repress ALAS1
- hemin/hematin: pharmacologically stable form of heme (inhibits ALAS1 via negative feedback)
*keep in mind that AIP only affects heme in liver, so anemia will not ensue from these treatments
Porphyria Cutanea Tarda (PCT)
UROD (uroporphyrinogen decarboxylase) deficiency (heme synthesis pathway, step 5)
most common porphyria, onset 40’s/50’s
essentially an acquired disease, some people are just predisposed via genetic deficiency (“familial PCT”)
what are 6 factors that can precipitate acquired PCT (porphyria cutanea tarda)
- liver iron overload - such as patients with hereditary hemochromatosis
- chronic liver disease
- Hep B/C/HIV
- alcohol
also, but less important:
5. sunlight
6. hydrochlorobenzene (fungicide)
how does PCT (porphyria cutanea tarda) present?
how is it treated?
bullae filled with clear fluid on skin, chronic blistering, phototoxicity
phototoxicity due to tetrapyrrole intermediates of heme pathway that are light reactive —> ROS formation
treatments: avoid environmental exposures (alcohol, tobacco), sunscreen, iron chelator, phlebotomy (bleeding) - reduces iron load
how is PCT diagnosed? (porphyria cutanea tarda) - 4
- coral red urine
- isocoproporhyrin in feces (remember later in heme pathway intermediates are not water soluble)
- 8-carboxyl or 7-carboxyl porphyrins in serum (indicates UROD defect)
- UROD enzyme activity assay or genetic analysis
erythropoietic protoporphyria (EPP)
mutations in ferrochelatase (last step of heme pathway) —> increased levels of free protoporphyrin IX in RBC
early childhood presentation
severe cutaneous photosensitivity (non-blistering), burning/stinging pain with sunlight
chronic liver disease later in life
may have elevated protoporphyrin in stool, but not urine (at this part of pathway, intermediates no longer water soluble)
deficiencies of enzymes earlier in the heme synthesis pathway are more likely to cause ______ symptoms, while deficiencies in enzymes later in the pathway are more likely to cause ______ symptoms
earlier in the pathway: neurological symptoms
later in the pathway: photosensitivity and sun explorer produces more severe cutaneous symptoms
A blood smear shows microcytic and hypochromic RBC. In the bone marrow, erythroblasts show a ring of blue granules
this is indicative of what?
sideroblastic anemia - impaired heme synthesis
A patient presents with conjunctival and nail bed pallor, fatigue, and dizziness. The patient has moderate cirrhosis of the heart and liver, which is determined to be caused from an iron overload. Erythroblasts in the bone marrow appear to have a ring of blue granules.
What is going on? How will you treat them?
X-linked sideroblastic anemia: XLR mutation in ALAS2 (in erythroid cells)
iron overload due to reduced usage and increased compensatory iron absorption (from diet)
treat them with vitamin B6 (pyridoxine)
A patient presents with diarrhea, staining in their gums, foot drop, memory loss, and irritability. Urine analysis shows an increase in ALA. What is going on?
lead poisoning - causes increase in ALA without increase in PBG, because lead blocks ALAD enzyme in heme synthesis
staining in gums - lead lines
foot drop - neuropathy
diarrhea - lead colic
Pt is a 32yo M presenting sudden onset of with neurovisceral pain, hallucinations and seizures, and peripheral neuropathy. Urine sample appears port-wine colored.
What is going on, and what is one dietary change that can be made to alleviate these symptoms?
AIP (acute intermittent porphyria) - deficiency in hepatic PBGD (step 3 of heme synthesis), onset in 20/30s
neurovisceral pain - abdominal
hallucinations/seizures - neuropsychiatric symptoms
urine would show accumulation of ALA or PBG
acute attack due to precipitating factors (4 M’s that induce ALAS1)
treat with carb loading - glucose/insulin repress ALAS1 transcription
(can also administer Hemin/Hematin)
Pt is a 47yo F presenting with skin bullae filled with clear liquid and phototoxicity with skin fragility. Urine sample has coral red color and high level of uroporphyrins. Feces sample has high levels of isocoproporphyrin.
What is going on, and what is causing skin damage?
PCT (porphyria cutanea tarda): UROD deficiency (heme synthesis), presents 40/50s (essentially acquired)
precipitating factors: liver iron overload, chronic liver disease, Hep B/C/HIV, alcohol
tetrapyrrole intermediates react with light in skin —> ROS, skin damage
Pt is a 4yo M with severe cutaneous photosensitivity, non-blistering. Pt complains of burning/stinging pain with sunlight. Urine porphyrin levels are normal.
What may be going on?
EPP (erythropoietic protoporphyria): mutations in ferrochelatase (heme synthesis, last step), presents in early childhood
chronic liver disease later in life
increased levels of free protoporphryin IX in RBC (not excreted into urine - but excreted in stool, so may see higher levels here)
what is the deficiency, main clinical presentation, and age of onset for:
a. AIP
b. PCT
c. EPP
these are all porphyrias - deficiencies in enzymes for heme synthesis
a. AIP (acute intermittent porphyria): PBGD deficiency, 20s/30s, port-wine urine and acute attacks
b. PCT (porphyria cutanea tarda): UROD deficiency, 40s/50s, bullae on skin with blistering and coral-red urine
c. EPP (erythropoietic protoporphyria): mutation in ferrochelatase, early childhood, severe photosensitivity with blistering
