Biochem Lecture 4

Question 1

What does Myoglobin do and where is it found?

Answer 1

It is present in heart and skeletal muscle where it binds and stores O2, releasing when needed.

Question 2

What does hemoglobin do and where is it found?

Answer 2

In erythrocytes where it transports O2 and CO2 to and from the tissues and lungs.

Question 3

What is the structure of Myoglobin?

Answer 3

It is a monomer that is globular and very compact. Interior has many nonpolar amino acids.

Question 4

How does myoglobin bind oxygen?

Answer 4

It has a globin fold domain in a crevice in which there is one heme group that binds one O2 molecule.

Question 5

What is the structure of hemoglobin?

Answer 5

It is a tetramer with two alpha globin and two beta globin polypeptide chains. It is a tetramer of two identical dimers.

Question 6

What is a prosthetic group?

Answer 6

It is a small molecule tightly attached to a protein and essential for its function. In Mb and Hb, the heme group is a prosthetic group that allows them to bind O2.

Question 7

Why is the heme group necessary for Hb and Mb to bind O2?

Answer 7

When O2 and iron bind alone, it is irreversible. The heme group around the iron atom allows the O2 binding to be reversible.

Question 8

What is the name for the shape of the Hb O2 dissociation curve?

Answer 8

Sigmoidal. It is kind of an S shape.

Question 9

Which molecule has a higher affinity for O2, Hb or Mb?

Answer 9

Myoglobin does. This allows it to be a more efficient storage molecule for O2.

Question 10

What are the two states of Hb conformations?

Answer 10

“T” for taught and “R” for relaxed.
In the T conformation, Hb has a low affinity for O2 and is called deoxyhemoglobin.
In the R conformation, Hb has a higher affinity for O2 and is called oxyhemoglobin.

Question 11

How does Hb affinity for O2 change as it goes between conformations?

Answer 11

As O2 molecules bind to Hb, the heme groups holding the Iron atoms flex and become planar. This pulls on the Histidine group attached to the iron atom and changes the tertiary and quaternary structure of the Hb molecule. The more O2 that binds, the more stable the new conformation becomes.

Question 12

Binding of O2 changes Hb from which conformation and into what new conformation?

Answer 12

Hb is in the T state when no O2 is bound and turns into the R state as O2 binds.

Question 13

What is the phenomenon known as cooperativity?

Answer 13

The binding of a ligand to one site affects the binding of the ligand to another site. O2 does this to Hb and is apparent by the sigmoidal shape of its binding curve.

Question 14

What is positive cooperativity?

Answer 14

Ligands that increase affinity for additional ligands is positive cooperativity.

Question 15

What is allosteric protein regulation?

Answer 15

It is the regulation of protein activity through the binding of a ligand called an effector. Their binding causes a conformational change that affects other parts of the protein. This can only happen in proteins with quaternary structure. Allosteric proteins have sigmoidal binding curves like Hb.

Question 16

What is the difference between homotropic and heterotropic effectors?

Answer 16

Homo means the actual ligand is the same as the effector. Hetero is different.

Question 17

What are negative effectors and what is their effect on the binding curve?

Answer 17

They have a negative effect on the action of the protein. They are inhibitors. They shift the binding curve to the right for the original ligand.

Question 18

What are positive effectors and their effect on the binding curve?

Answer 18

Positive effectors increase a protein’s ability to do its job. They shift a binding curve to the left.

Question 19

What are the negative and positive allosteric effectors for Hb?

Answer 19

Negative: 2,3-BPG, CO2, Temperature, H+ (pH Bohr Effect)
Positive: CO, O2

Question 20

How does 2,3 BPG affect Hb?

Answer 20

It binds to the two beta subunits and stabilizes Hb into the T state where it has a lower affinity for O2. This is important to allow Hb to release more O2 in the tissues. Fetal Hb doesn’t have beta units so this mechanism can’t work.

Question 21

How does temperature affect Hb binding to O2?

Answer 21

Increased temperature has a negative effect which helps the Hb to unload O2 in the tissues during exercise.

Question 22

How does CO2 affect Hb?

Answer 22

CO2 binds to the N terminus of the amino acid chain and has a negative effect on Hb. Some of the CO2 in the body is transported in this way to the lungs. The majority is transported as bicarbonate (HCO3-).

Question 23

How does pH affect Hb binding to O2?

Answer 23

Increasing [H+] stabilizes the deoxy form of Hb. It therefore helps unload O2 in the tissues.

Question 24

How does H+get produced in the tissues and how does Hb interact with this reaction?

Answer 24

Metabolically active tissues produce CO2 which reacts with water to form H2CO3 which converts easily into HCO3- and H+. Hb then acts as a buffer and accepts protons. This stabilizes Hb in the T state.

Question 25

What happens to protonated Hb when it gets to the lungs?

Answer 25

The pO2 is high enough in the lungs that Hb will become saturated and any bound CO2 or H+ will get kicked off. The H+ will react with HCO3- to form H2CO3 and then CO2 and H2O. The CO2 is then exhaled.

Question 26

What is the Bohr effect?

Answer 26

It is the reciprocal binding of H+ and O2 back and forth.

Question 27

How does CO affect Hb?

Answer 27

CO binds better to heme groups than O2 so CO takes up spots where O2 would usually bind. CO also increase Hb’s affinity O2 so it gets loaded up in the lungs, but drops very little of it in the tissues. This is shifting the binding curve to the left.

Question 28

What does the saturation curve indicate? What is it measuring?

Answer 28

It shows the percentage of molecules that have O2 bound at a given pO2.