L8- Oxygen Transporters

Question 1

What prosthetic group do myoglobin and hemoglobin have in common?

Answer 1

Both proteins contain a heme prosthetic group.

Question 2

What is the difference between a prosthetic group and a coenzyme?

Answer 2

A prosthetic group is firmly attached to a protein and usually cannot be removed during protein purification. A coenzyme is an organic molecule that is less firmly attached.

Question 3

What is the oxidation number of the heme iron found in oxygenated myoglobin? What is the oxidation number of the heme iron found in deoxygenated myoglobin?

Answer 3

Iron is in its ferrous state (Fe2+) irrespective of oxygen binding in both myoglobin and hemoglobin.

Question 4

What amino acid residue in hemoglobin binds iron?

Answer 4

Histidine (F8). Movement of this residue helps the protein sense whether or not O2 is bound to the heme group.

Question 5

Carbon monoxide binds to isolated heme molecules much more tightly than oxygen. How is the CO binding affinity of hemoglobin attenuated?

Answer 5

The ferrous iron in isolated heme binds CO at a 180 degree angle. Histidine E7 in the active site of heme prevents optimal CO binding by forcing the binding geometry to be 120 degrees, which is the optimal binding orientation of oxygen.

Question 6

What triggers myoglobin to undergo a conformational change when it binds oxygen?

Answer 6

When it binds to oxygen, the oxygenated ferrous ion moves into the plane of the porphyrin ring, pulling along some of the protein with it. This is mediated by histidine F8.

Question 7

Describe the shape of the myoglobin oxygen dissociation curve. What does it suggest with regards to the protein’s kinetics?

Answer 7

Hyperbolic. This suggests that simple equilibrium processes are involved in binding and release.

Question 8

Describe the shape of the hemoglobin oxygen dissociation curve. What does it suggest with regards to the protein’s kinetics?

Answer 8

Sigmoidal. A binding curve with this shape indicates cooperativity among multiple binding sites.

Question 9

What is the role of myoglobin in muscle?

Answer 9

It delivers the oxygen required for metabolism in actively exercising muscle.

Question 10

What is the role of hemoglobin?

Answer 10

It transports oxygen between the lungs and tissues.

Question 11

How many oxygen molecules do myoglobin and hemoglobin bind to, respectively?

Answer 11

Myoglobin binds to one oxygen molecule whereas hemoglobin binds up to four oxygen molecules.

Question 12

Myoglobin vs. hemoglobin: which one is an allosteric protein?

Answer 12

Hemoglobin is an allosteric protein. Myoglobin, which binds only a single molecule of O2, is not.

Question 13

What is meant by the statement ‘the binding of oxygen to hemoglobin is cooperative’?

Answer 13

It means that binding of oxygen to one site influences the oxygen affinity of the remaining sites on Hb.

Question 14

What does the Hill coefficient measure? What are its possible values and what do these values indicate?

Answer 14

It measures the sigmoidal character of a protein’s binding curve and indicates the degree of the protein’s cooperativity. A Hill coefficient of 1 means no sigmoidal character and no cooperativity. A Hill coefficient greater than 1 means positive cooperativity. Hill coefficients can also be less than one. In this case, binding one ligand inhibits binding of a second.

Question 15

What is the Hill coefficient of myoglobin?

Answer 15

1 (one).

Question 16

The cooperativity of oxygen in hemoglobin suggests that the heme groups communicate with each other. However, they are too far apart to do so directly. How is it that one heme group may affect the oxygen binding affinity of another heme group?

Answer 16

Heme groups communicate with one another through conformational changes that are transmitted across the hemoglobin subunit interfaces.

Question 17

Oxyhemoglobin is said to be in which conformational state?

Answer 17

The relaxed state. (It has oxygen so it is relaxed.)

Question 18

Deoxyhemoglobin is said to be in which conformational state?

Answer 18

The taut state.

Question 19

Which hemoglobin conformational state has the greatest affinity for oxygen?

Answer 19

The relaxed state.

Question 20

How do high CO2 and rising H+ levels (low pH) affect hemoglobin’s affinity for oxygen? What is this effect called?

Answer 20

Both CO2 and low pH promote the release of oxygen by lowering the oxygen affinity of hemoglobin. This is known as the Bohr effect.

Question 21

Describe the physical movement that occurs between the subunits of hemoglobin when an oxygen molecule binds to a heme group.

Answer 21

When oxygen binds to a ferrous iron, one pair of alpha-beta subunits rotates by 15 degrees relative to the other pair of alpha-beta subunits in the hemoglobin heterotetramer.

Question 22

Where on hemoglobin does carbon dioxide bind?

Answer 22

Carbon dioxide binds to the N-terminal amino groups of the alpha subunits. Note that this is different from carbon monoxide, which binds to the oxygen binding site.

Question 23

Describe the mechanism by which the binding of carbon dioxide to hemoglobin reduces oxygen affinity.

Answer 23

Carbon dioxide reacts reversibly with an N-terminal amino group (in an alpha subunit) to form a carbamate. The carbamate groups form ionic bonds that stabilize the taut (deoxy) state of Hb, thereby lowering the oxygen affinity of hemoglobin.

Question 24

Because they reduce the oxygen affinity of hemoglobin without binding directly to the site of oxygen binding, both carbon dioxide and protons are called ______________ effectors.

Answer 24

Negative allosteric.

Question 25

Which conformational state of hemoglobin has a greater affinity for protons?

Answer 25

The taut (deoxy) state.

Question 26

What are the subunits of fetal hemoglobin? What are the subunits of adult hemoglobin?

Answer 26

Fetal hemoglobin is composed of two alpha and two gamma subunits. Adult hemoglobin is composed of two alpha and two beta subunits.

Question 27

What is the role of 2,3-bisphosphoglycerate (BPG) with regards to hemoglobin affinity for oxygen? How does high altitude affect levels of 2,3-BPG?

Answer 27

2,3-BPG reduces the oxygen affinity of hemoglobin. 2,3-BPG levels increase at high altitude where oxygen is scarce.

Question 28

How many 2,3-BPG binding sites are there in one molecule of hemoglobin and where are they found?

Answer 28

One. It is located in a central cavity formed between the beta subunits.

Question 29

Describe the nature of the bond that forms between 2,3-BPG and hemoglobin. Which state does 2,3-BPG stabilize?

Answer 29

2,3-BPG is negatively charged and binds to hemoglobin primarily through electrostatic interactions. 2,3-BPG reduces oxygen affinity and therefore stabilizes the taut (deoxy) form of hemoglobin.

Question 30

Why does fetal hemoglobin have a higher oxygen affinity than adult hemoglobin?

Answer 30

Fetal hemoglobin contains gamma subunits instead of beta subunits. The gamma subunits have fewer positively charged amino acid residues in the 2,3-BPG binding region. Therefore, 2,3-BPG binds to fetal hemoglobin less strongly than to adult hemoglobin. This confers fetal hemoglobin a higher oxygen affinity than adult hemoglobin.

Question 31

What is the molecular defect associated with sickle cell anemia?

Answer 31

It is an amino acid substitution of a valine for a glutamate on the exterior of hemoglobin. This results in an altered charge of the beta subunit.

Question 32

What causes sick cell hemoglobin (HbS) to polymerize?

Answer 32

The oxygenated form of HbS has normal solubility, but the deoxygenated form is relatively insoluble. In areas of low oxygen (such as high altitude), HbS polymerizes and distorts red cells, leading to clogging of capillaries and blood vessels.

Question 33

Why do sickle cell anemia symptoms not become apparent until 3 to 6 months after birth?

Answer 33

It is not until then that the beta subunits of adult hemoglobin are produced. The symptoms associated with sickle cell anemia correlate with decreased synthesis of gamma subunits and increased synthesis of beta subunits.

Question 34

Name two drugs that are used to treat sickle cell anemia and explain how they work.

Answer 34

Hydroxyurea (increases the production of gamma subunits of HbF) and erythropoetin (increases proliferation of red cells containing HbF).

Question 35

What is the oxidation number of the heme iron in methemoglobin?

Answer 35

Fe3+ (ferric). This is an abnormal state.

Question 36

List two causes of methemoglobinemia.

Answer 36

Ingestion of large amounts of oxidizing agents such as nitrates, or mutations near the heme group.

Question 37

How is methemoglobinemia treated?

Answer 37

By administration of reducing agents such as vitamin C or methylene blue.

Question 38

What is the defect in alpha-thalassemias and what does it cause?

Answer 38

Alpha-thalassemias result from a defect in the synthesis of the alpha-subunits of hemoglobin. This leads to an imbalance in the amount of alpha-globin vs. beta-globin.

Question 39

Myoglobin vs. hemoglobin: which one binds oxygen more tightly?

Answer 39

Myoglobin binds oxygen more tightly than hemoglobin.