Heme Synthesis & Degradation

Question 1

What is the first reaction in heme synthesis?

Answer 1

ALA Synthase Reaction

Question 2

What’s heme structure?

Answer 2

usually associated with protein like hemoglobin and cytochrome. Without protein it’s lipophilic pro-oxidant. Protoporphyin + Fe 2+ doesn’t fit into plane. Iron binds between the 4 pyrrole rings. Under normal conditions Fe2+ (ferrous) becomes oxygenated NOT oxidized.

Question 3

What do isoenzymes ALAS1 and ALAS2 do?

Answer 3

ALAS1 in the liver makes heme from protoporphyrin but has short half-life because heme negatively feeds back decreasing transcription, increasing mRNA degredation. Also inhibited by glucose, and increased by drugs and hormones. ALAS2 is in marrow and makes up most heme. This ALAS2 is regulated by Fe levels so has IREs in 5’ becasue if iron low IREs won’t let ALAS2 make heme.

Question 4

What mechanisms coordinate heme and globin synthesis in erythroblasts?

Answer 4

Heme depends on iron availability, ALAS2 has IRE in 5’ (so if no heme iron won’t be stored), transcription of ALAS2 has same TFs as globin synthesis, erythropoietin increases transcript of both heme and globin, and low heme activate HRI that phosphorylates EIF-2

Question 5

Bohr Effect

Answer 5

Hb converted from T to R causes H+ release. This pushes equilibrim to left to release O2. So when pH is low, the affinity for O2 is low. We also see when CO2 in blood forms bicarbonate via carbonic anhydrase. The HCO2 leaves RBC via Cl- channel. This is reverse in lungs where O2 binds to Hb and H+ is kicked off, CO2 exhaled.

Question 6

Where does iron bind on in heme?

Answer 6

Iron binds to the 4 nitrogens of the pyrrole rings or protoporphyrin, normally the Fe 2+ becomes oxygenated and not oxidized, while oxidized Hb becomes Fe 3+ which can’t bind O2

Question 7

what are the two histindines of heme?

Answer 7

Proximal histidine (F) is where Fe2+ binds, the distal = E stabilizes the interaction of O2 and Fe by preventing oxidation or CO binding.

Question 8

How’s heme synthesized?

Answer 8

1st reaction (rate limiting) in mitochondria is ALA synthetase which uses glycine + succinyl CoA + pyridoxal phos(cofactor) to make f-aminolevulinic acid. ALA maked Fe 2+ –> protoporphyrin.The last and firs rxns take place in mitochondria. Dehydratase + Ferrochetalase is inhibited by heavy metals while rxn 1 is inhibited by heme.

Question 9

Where is most heme produced?

Answer 9

Bone marrow & liver, but all cells produce some heme

Question 10

What is metheme?

Answer 10

Oxidized form of Hemoglobin (Fe3+) that cannot bind O2.

Question 11

What can cause jaundice or build up of bilirubin?

Answer 11

Too much hemolysis, genetic, or bile duct obstruction. CAn be measured directly = conjugated (Bogh rxn) or indirect = unconjugated, needs to be separated from albumin.

Question 12

How’s ALA synthase synthesis of heme regulated?

Answer 12

Negative feedback from heme, or inhibition by Fe via IRE 5’ regulation, in liver glucose inhibits, and isteroids increase.

Question 13

How’s heme made in RBC?

Answer 13

Increased expression of transferrin to bring in more iron, ALAS2 increased by iron (more ferritin), and translation of globins increased since these are regulated together. These transcribed globin genes are HbA, but if someone is deficient in that, embryonic HbF (high affinity gamma globins) can increase.

Question 14

Where is hepcidin produced?

Answer 14

The liver

Question 15

What are conditions that cause intrahepatic jaundice ?

Answer 15

Riggler Najar = bilirubin gluconyl transferase; autosomal recessive. Gilbert Syndrome = mutation in bilirubin glucuromyl transferase. And Dublin Johnson syndrome = defective transport of bilirubin out hepatocytes.

Question 16

What does saturation curve show about Hb?

Answer 16

That at low partial pressures like in the peripherals, O2 is dropped at lower saturation rate (middle of curve). Hemoglobin displays a sigmoidal curve which shows allosteric regulation.

Question 17

What are symptoms & treatment for porphyria?

Answer 17

Accumulation of first intermediates lead to CNS problems/abdominal pain, built up of PGBs leads to photosensitivity. Treat by inhibiting ALAS with hematin, carb diet, less drugs.

Question 18

What is T form of Hb?

Answer 18

The deoxy state where Fe is puckered out, and the plane is dented. When O2 binds it pulls down the plane. This becomes the R form which has fewer bonds so higher affinity.

Question 19

Acquired Hemoglobinopathies

Answer 19

1. Carbon monoxide poisoning = CO competes with O2 to bind with Hb
2. Methemoglobinemia = oxidation exceeds capacity of reduction; causes chocolate blood, due to certain drugs, hbM mutation, or NADH reductase deficiency

Question 20

What’s function of 2,3-bisphosphate?

Answer 20

It binds to deoxy Hb to stabilizes the T form and decrease O2 affinity in tissues. Levels increased in cases of hypoxia (chronic anemia, COPD)

Question 21

Thalassemias

Answer 21

Partial or complete absence of one or more globin chains.
1. Barts Syndrome = all 4 alpha are deleted; leads to build up interstitial fluid and baby death
2. Hemoglobin H disease = 3 alpha deleted
3. Beta Minor = heterozygotes usually asymptomatic except mild anemia
4. Beta Major = homozygotes, need transfusions for severe anemia
5. Sickle cell = missense causes mutation of Beta globin which decreases its solubility of deoxy form, so RBCs sickle and cause vaso-occlusion (heterozygotes usually are ok)
6.HbC = glutamate to lysine causes mild hemolytic anemia

Question 22

What substrates are required for heme synthesis?

Answer 22

Glycine & succinyl CoA

Question 23

How’s heme synthesis coordinated with globin?

Answer 23

ALAS2 has IRE, heme synthesis depends on Fe levels, transcription of ALAS2 is controlled by globin TFs, erythroporem increases TS of ALAS2 and alpha/beta globin, heme inccreases TS of globins and low levels of heme actuate HRI lemase that phosphorylates/inhibits E1F2.

Question 24

How does old heme become bilirubin?

Answer 24

When phagocytosed by macrophages where oxygenase cleaves pyrolle ring it makes CO2 and bilirubin. Bilirubin reductase makes it into bilirubin. This bilirubin becomes conjugated by bilirubin glucuronyl transferase, so it can be transported by MRP2 into bile –> feces + urine.

Question 25

Where does iron come from, and how is it absorbed?

Answer 25

Fe comes from food either free Fe 3+ or Heme iron and absorbed via heme transporter, or Fe 3+ is reduced by cytochrome B (ferrireductase) then enters DMTI. Heme stored in mucousal ferritin or transported to blood via ferroportin. In blood, hephaestin (ferroxidase) oxidizes Fe 2+ to Fe 3+ to be transported throughout the body by transferrin.

Question 26

This enzyme inhibits release of heme from the mucosal cells in response to high iron concentrations and inflammatory cytokines.

Answer 26

Hepcidin

Question 27

How is iron absorbed into the cell?

Answer 27

In the heme form via a heme transporter or in non-heme form as a ferrous ion via DMT1

Question 28

How is protein synthesis regulated in response to iron levels?

Answer 28

IREs bind in 5’ on ferritin when intracellular levels of iron are low, decreasing translation. IREs bind in 3’ on transferritin (DMTI) and increase translation when iron low so cell brings it in.

Question 29

What do scavengers do?

Answer 29

Pick up old heme fragments. Heptaglobin binds alpha/beta dimers and brings to macrophages. Hemopexin binds metheme and brings to liver. Both make complexes that are excreted.

Question 30

What happens in lead poisoning?

Answer 30

Lead inhibits ALA dehydrogenase (2nd rxn) and ferrochetalase (last rxn). Leads to increased ALA in blood, causes anemia neurotoxicity. Lead competes with Ca 2+ so there can be build up in Ca 2+ rich regions of body.

Question 31

Describe the general structure of heme.

Answer 31

Ferrous ion bound to 4 nitrogens in the pyrrole rings of protoporphyrin IX. The ion also binds histidine and oxygen to form a total of 6 bonds. Conjugated double bonds give heme its color.

Question 32

What decreases/increases absorption?

Answer 32

Increased by absorbic acid and citric acid. Decreased by tannates, carbonates, phosphates. Inflammatory lytoleins cause increase hepicidin which prevents Fe 2+ from exiting the intestine (anemia of chronic disease). In the capillaries, Fe binds apotransferrin to be carried to tissue.

Question 33

What is hemoglobin structure?

Answer 33

4 globins, 2 alpha beta dimers. Heme binds to the crevices between globins. The expression of these globin chains is only 5-7 days during development.

Question 34

Where is iron absorbed into the body?

Answer 34

Small intestine

Question 35

What two components make up the structure of heme?

Answer 35

Protoporphyrin IX + ferrous iron (Fe2+)