Metalloproteins - Iron

Question 1

How can carbon monoxide poison the porphyrin Heme?

Answer 1

Binding of CO increases the affinity of the surrounding Heme prosthetic groups for their oxygen. This means that they cannot off load their oxygen at low oxygenated tissues, leading to death of the organism.

Question 2

What is the general structure of porphyrins like Heme?

Answer 2

pg 2

Question 3

What is the hemoglobin concentration?

Answer 3

5 mM

Question 4

Myoglobin vs Hemoglobin
Number of subunits?
Cooperativity (curve in graph)?

Answer 4

Myoglobin Hemoglobin
1 4
No Yes (pg2)

Question 5

What is the starting amino acid and TCA-cycle intermediate at the beginning of the Heme biosynthetic pathway?

Answer 5

Glycine and succinyl-CoA

Question 6

What is the final enzyme in the biosynthetic heme pathway, which catalyzes the insertion of an Fe 2+ into protoporphyrin IX to produce the Heme molecule?

Answer 6

Ferrochelatase

Question 7

What two things can Heme carry?

Answer 7

electrons and O2

Question 8

What groups do electron transfer and Redox proteins use?

Answer 8

Heme groups and iron-sulfur clusters

Question 9

What is the pathway of electrons from cytochrome c

Answer 9

Cytochrome c > CuA > Haem a > (Haem a3 + CuB) > O2

pg 3

Question 10

Where does the sulfur in iron-sulfur cluster proteins come from?

Answer 10

Cysteine residues.

Question 11

Name proteins with a 4, 3, 2 and 1 Iron-sulfur cluster.

Answer 11

4 Fe Ferredoxin
Aconitase
2 Fe Ferredoxin
Rubredoxin

Question 12

What is the result of the desulfurization of cysteine in the biogenesis of iron-sulfur clusters?

Answer 12

2Fe 2+ + 2Cys (ISCU) > 2Fe2S + 2Ala (holo - ISCU)

Question 13

What proteins use ATP to bind holo-ISCU

Answer 13

HSC20 and HSC70 (Heat Shock Cognate)

Question 14

What happens after the heat shock cognate proteins bind Fe-S clusters? Does it require ATP

Answer 14

They use ATP to transfer Fe-S clusters onto recipient proteins. They may use an intermediate scaffold to do this.

Question 15

Ribonucleotide reductases (RNRs)
Function?
Use Haem or Fe-S clusters?

Answer 15

They catalyze the reduction of ribonucleotides to deoxyribonucleotides (RNA -> DNA)
Most RNRs contain a diiron cluster.

Question 16

Fe 3+ / Fe2+
Solubility?
Anaerobic or aerobic?
pH?
Stored?

Answer 16

Fe 3+/ Fe2+
Not very soluble / highly soluble
aerobic / anaerobic
high pH / low pH
Ferritin (liver) / ions (intracellular)

Question 17

How does the body prevent iron radicals through Fenton chemistry?

Answer 17

It keeps iron tightly bound in proteins.

Question 18

What cell(s) are DMT (divalent metal ion transporter) and Hephaestin found in? What are their purpose?

Answer 18

Enterocytes (gut cells)
They are involved in the intake of iron.

Question 19

What cell(s) contain ceruloplasmin?

Answer 19

Macrophages (re-uptake of iron)
Hepatocytes

Question 20

Transferrin
Function?

Answer 20

transports Fe 3+ ions through serum to bone marrow.
transports Fe 3+ from the gut to the liver for storage.

Question 21

Explain the process of cellular iron uptake via the Tf cycle

Answer 21

pg 5

Question 22

Ferritin
Function?
Location?
Subunits?

Answer 22

Stores iron as a mineral
Mostly found in the liver
24 identical subunits

Question 23

Hepcidin
Function?
Binds to?
Disease from too much Hepcidin?
Disease from too little Hepcidin?

Answer 23

It is synthesized by the liver in response to high iron concentrations and released into the blood stream where it prevents iron uptake in the gut enterocytes.
It binds to ferroportin (FPN)
Anemia
hereditary hemochromatosis

Question 24

What happens when ferroportin is bound by hepcidin?

Answer 24

Ferroportin becomes phosphorylated by JAK2 kinase, which marks it for internalization and degradation.

Question 25

During inflammation. What are some host responses to retain iron?

Answer 25

1) Hepatocytes secrete more hepcidin to prevent release of iron from cells
2) Neutrophils secrete lactoferrin which acts like transferrin only it binds iron even more strongly.
3)Neutrophils will also release siderophores which compete with the siderophores of pathogens. Ex: NGAL (siderocalin/ lipocalin)
4) Cells will store iron inside them in proteins like ferritin.

Question 26

How do bacteria steal iron from their host?

Answer 26

1) Secrete siderophores which bind iron very tightly and compete with lactoferrin and transferrin.
2) steal or borrow ferric siderophores made by other organisms like yeast (ferrichrome)
3) Lyse RBCs using cytotoxic peptides to access heme from hemoglobin.
4) have transferrin or lactoferrin receptors to extract iron directly from host proteins.

Question 27

How many membranes must iron pass through to enter Gram-negative bacteria?

Answer 27

Two. The outer membrane and inner membrane.

Question 28

What kinds of molecules does the porin OmpF allow through the outer membrane of Gram-negative bacteria?

Answer 28

Nothing larger than 600 Daltons.
includes: glucose, phosphates, salts and amino acids
excludes: heme groups and siderophores.

Question 29

How do iron containing siderophores enter bacteria?

Answer 29

Fep system.

Question 30

Enterobactin
Function?
Produced in what organisms?
binding strength?
coordination geometry bound iron?

Answer 30

It is a ferric iron binding catecholate siderophore
some bacteria, yeast and algae.
Kd = ~10^-40
Fe 3+ is bound in an octahedral geometry by the three catecholate groups (6 oxygens).

Question 31

How do the Fep/ TonB systems work?

Answer 31

pg 6

Question 32

Ferrichrome
Function?
Produced/ used by?
Coordination geometry?
Iron access?

Answer 32

it is a hydroxamate siderophore of iron.
It is only produced by yeasts but can be stolen by several bacteria.
It forms an octahedral geometry with three hydroxamate groups.
Iron is accessed inside the cell from by hydrolyzing the peptide bonds to break open the protein.

Question 33

HasA protein
Function?

Answer 33

It binds iron containing heme groups and keeps them soluble. HasA then binds to receptors on bacterial surface so they can endocytose and uptake the iron.

Question 34

Which of the 4 iron obtaining processes of bacteria are TbpB and TbpA involved in?

Answer 34

Extracting iron directly form Transferrin or lactoferrin via receptors on bacteria’s surface

Question 35

What is Desferal/ Desferrioxamine B/ DFO used to treat and how does it work?

Answer 35

DFO treats thalassemia patients who have chronic anemia due to a reduced production of hemoglobin. They require multiple blood transfusions to get enough iron but this can sometimes lead to too much iron in their blood. Desferal is an iron chleating agent which removes the excess ferric iron from the body.