Hemoglobin Flashcards
1
Q
Overview of heme synthesis
A
- 85% occurs in BM for RBCs
- 15% occurs in liver (mostly for CYP450)
- 8 rxns in forming heme
- First one and last 3 are in mito, intermediate 4 are in cytoplasm
- There is only feedback inhibition (heme inhibits ALA synthetase) in the liver (no inhibition in BM)
- We are responsible for steps 1, 2, 3, and 8
2
Q
First reaction in heme synthesis
A
- Formation of d-ALA (aminolevulinic acid), by condensation of succinyl CoA and glycine
- Catalyzed by ALA synthetase, requires pyridoxal phosphate (vit B6)
- Is the rate limiting step of heme synthesis in LIVER
- Will be inhibited by heme (negative feedback) in LIVER
- Occurs in the mito (where succinyl CoA is)
3
Q
Second reaction in heme synthesis
A
- Formation of porphobilinogen (PBG) via condensation of 2 d-ALA molecules
- Catalyzed by ALA dehydratase
- Occurs in cytoplasm
4
Q
Third reaction in heme synthesis
A
- Four porphobilinogen are linked head-tail to form a linear tetrapyrrole, hydroxymethylbilane
- One NH4+ is released for each methylene bridge formed
- Occurs in cytoplasm via nz porphobilinogen deaminase
- The product hydroxymethylbilane undergoes ring inversion during cyclization in the fourth reaction (side chains switch from acetyl/proprionyl to proprionyl/acetyl in the lower left quadrant only)
- The rest of the molecule retains the A/P pattern of side chains (clockwise)
5
Q
Reactions 4-7 of heme synthesis
A
- Hydroxymethylbilane is cyclized by uroporphyrinogen III cosynthetase and ring inversion occurs (see 3rd rxn)
- Subsequent reactions alter side chains and degree of saturation of the porphyrin ring
- The ring re-enters the mito btwn rxns 5 and 6
6
Q
Final reaction (8) of heme synthesis
A
- An atom of ferrous (2+) iron is incorporated into protoporphyrin IX to form heme
- Occurs in mito and is catalyzed by ferrochelatase (heme synthetase)
- Heme is prosthetic group for Hb, myoglobin (Mb), catalase, and Cyt C
- Heme will negatively feedback ALA synthesis, only in liver
7
Q
Regulation of heme synthesis
A
- In liver (non-erythroid tissue), heme directly feeds back to inhibit synthesis of ALA (step 1)
- In BM (erythroid tissue), depressed levels of Fe lead to decreased heme synthesis, and high levels of Fe enhance heme synthesis
- This is due to the ALA-S2 gene (found only in erythroid tissue)
8
Q
ALA-S2 gene
A
- The gene that encodes BM ALA synthetase nz contains an IRE (iron response element), which isn’t present in the ALA-S1 gene (liver isoform)
- When there is excess iron, Fe-S clusters (ISCs) bind to IRP (Iron regulatory proteins), preventing the IRPs to bind to IRE
- When the IRE is not bound, the synthesis of ALA-S2 gene is initiated, leading to higher amounts of ALA synthetase and increasing heme synthesis
- When Fe levels are low, ISCs are not created and IRP are free to bind to IRE, preventing synthesis of ALA synthetase and inhibiting heme generation
- Overview: high Fe-> ISCs+IRP-> ALA-S2 synthesized-> more heme production
- Overview: low Fe-> IRP+IRE-> no ALA-S2-> less heme production
9
Q
Transferrin
A
- Plasma transport protein for Fe (transported in ferric, or 3+ state, while in plasma)
- Upon binding to the receptor, transferrin is internalized
- Within the endosome the pH is lowered (to 5.5) and Fe dissociates from transferrin
- Transferrin is recycled to the surface in an empty state, ready to be used again
- Iron is converted to ferrous form (2+, the form used within cells)
- Fe2+ is used for heme synthesis or is stored in ferritin proteins, which condense to form hemosiderin structures
- Empty form of ferritin is apoferritin
10
Q
Inherited disease states
A
- Porphyrias: disease characterized by deficiencies of an nz in the heme synthesis pathway
- Manifested by cutaneous photosensitivity (from excess tissue porphyrins), and defects of the nervous system
- Subdivided into erythropoietc or hepatic based on where the excess porphyrins occurs
11
Q
Congenital erythropoietic porphyria
A
- Deficiency of uroporphyrinogen III cosynthetase (cyclizes hydroxymethylbilane, step 4)
- There is a buildup of uroporphyrinogen (which is normally not made, but in this case is made non-enzymatically) and other porphyrins in RBC, marrow, plasma, urine, and feces
- RBCs are prematurely destroyed and urine of patients is red due to excretion of porphyrins
- Skin is photosensitive and teeth fluoresce (buildup of porphyrins) and patients are anemic
- Rx is mostly IV hematin (form of heme), can also blood transfusion and BM transplant
12
Q
Acute intermittent porphyria
A
- Porphobilinogen deaminase is depressed and there is a compensatory increase in ALA synthetase
- In the liver and urine there are large amounts of ALA and porphobilinogen
- Manifests as intermittent abdominal pain and neurologic disturbances
- Rx by IV hematin
13
Q
Properties of hemoglobin (Hb)
A
- Normally heme contains ferrous (Fe2+) iron
- When Fe is oxidized to ferric (Fe3+), Hb is called methemoglobin (MetHb), which does not bind O2
- In deoxyHb, Fe2+ is bound to 4 nitrogens from the heme and a nitrogen from the histidine residue of the globin protein
- The 6th Fe coordination site is unoccupied (where O2 binds)
- Upon O2 binding to the 6th position of Fe, Hb becomes oxyHb and the structure of hemoglobin changes (from taught to relaxed)
- The relaxed state of Hb (oxyHb) gives the other subunits of Hb tetramer a higher affinity for O2 (cooperativity)
14
Q
Myoglobin (Mb) vs Hb
A
- Mb is a single chain (unlike Hb which is 4 chains)
- Mb does not demonstrate cooperativity, b/c of its single chain
- Mb has a rectangular hyperbolic curve of O2 binding (due to lack of cooperativity
- Hb has sigmoidal curve of O2 binding, enabling Hb to release more O2 at tissues (result of cooperativity)
15
Q
Hb evolution and mutation
A
- Normal adult Hb composition: 4 chains consisting of 2 alpha and 2 beta chains (A2B2)
- Adults also have small amounts of A2D2 (A2 Hb) and A2G2 (Fetal, F, Hb)
- Abnormal Hb chains: sickle cell Hb (HbS) is A2B(s)2, HbBarts (G4) and a-thalassemia Hb (HbH) is B4
- S Hb (sickle cell) has Glu changed to Val @ position 6 on the B chain
- Thalassemia: decreased synthesis of A or B chain
- A, B, D, G genes on 2 different chroms