MB - Haemoglobin I

Question 1

What are the key differences between myoglobin and hemoglobin?

Answer 1

Myoglobin binds O2 in muscle and acts as a store

Hemoglobin binds O2 and acts as a carrier in blood

Question 2

Describe the structure of myoglobin. (3)

Answer 2

• Globin polypeptide (153 AA, 8 α-helices)
• Heme prosthetic group (Fe2+ in protoporphyrin IX ring)
• Fe2+ coordinated by 4 N atoms and His93 at position 5, O2 binds at 6

Question 3

How does O2 bind to myoglobin?

Answer 3

O2 binds at an angle as distal histidine interferes and stabilizes binding to prevent superoxide radical formation

Question 4

What happens when Fe2+ is oxidized in myoglobin?

Answer 4

Fe3+ cannot bind O2, forming metmyoglobin

Question 5

What does the myoglobin saturation curve indicate?

Answer 5

Most myoglobin remains O2 bound at tissue pO2. Intense exercise/diving is required to lose O2

Question 6

Describe the hemoglobin saturation curve.

Answer 6

Sigmoidal/cooperative curve -

• saturated at lung pO2 (high affinity)
• releases O2 at tissue pO2 (low affinity)

Question 7

How does hemoglobin increase O2 carrying capacity?

Answer 7

300 million Hb per RBC, allowing blood to carry 100x more O2 than plasma alone

Question 8

What are the subunits of hemoglobin?

Answer 8

Tetramer of 2 x alpha-beta dimers

• 2 α (141 AA) globin chains
• 2 β (146 AA) globin chains

Question 9

What conformational changes occur when O2 binds hemoglobin? (2)

Answer 9

• Small scale: Pull on helix F alters dimer-dimer interface
• Large scale: 15° rotation of αβ dimers (quaternary shift)

Question 10

What are the steps involved during O2 binding? (3)

Answer 10

1. Electron rearrangements shrink Fe, allowing it to move further into the ring
2. Proximal His is pulled along with it
3. Proximal His pulls on helix F

Question 11

What causes low affinity in deoxy (tense) hemoglobin?

Answer 11

Inter-subunit salt bridges resist movement between

• α1-β2
• α2-β1
• α1-α2

(not β-β)

Question 12

What causes high affinity in oxy (relaxed) hemoglobin?

Answer 12

Salt bridges broken

Rotation of dimers and changed quaternary structure

Question 13

What models describe hemoglobin cooperativity? (2)

Answer 13

Concerted/MWC: All T ↔ All R

• Each O2 that binds to the T form increases the chance of it flipping to R form

Sequential/KNF: Progressive T → R transitions

• Each O2 that binds to a T flips it to R and makes nearby subunits T’

The real model has features of both

Question 14

Why are parts of both models true?

Answer 14

α can bind O2 in T and R, but β only binds in R state.

α and β subunits are not identical