5 - Protein function V: Haemoglobin and retinol binding protein

Question 1

Temperature effect on oxygen association with haemoglobin

Answer 1

The increased temperature causes the curve to shift to the right, increasing the release of O2 to Active muscle.

Question 2

Changes in haemoglobin structure upon oxygenation

Answer 2

distance between iron atoms in b chains decreases from 3.99 to 3.34 nm.

Question 3

How is the T state (deoxyhaemoglobin) stabilised?

Answer 3

• by hydrophobic bonds and 8 electrostatic bonds which involve the C-terminal basic AAs of each subunit.
• Each of the penultimate Tyr residues is held in a pocket by H-bonding to a Val = anchors the C-terminal residues in positions favourable for electrostatic bond formation.
• In addition, the binding of 2,3-BPG between beta-subunits further stabilises the T state.

Question 4

Explain what happens upon oxygen binding to iron?

Answer 4

In deoxyhaemoglobin the proximal His F8 is bonded to the iron but the FeII is above the plane of the porphyrin ring by 0.06nm (0.6Å).

The binding of one O2 molecule to iron pulls it 0.039nm closer into the porphyrin plane.

Question 5

Effect of oxygen binding to iron and moving it

Answer 5

drags the His F8, so that the helix F, the EF corner and the FG corner follow.

These movements are transmitted to the subunit interfaces and result in the breaking of the 8 electrostatic bonds.

Question 6

Describe the conformation in relaxed R state.

Answer 6

• The Relaxed R state the 8 electrostatic bonds are broken.

- β subunits are now closer and the site for 2,3-BPG binding is no longer available.

Question 7

Summary the main changes upon oxygen binding to iron

Answer 7

• Oxygen binds to FeII
• This pulls the proximal His in the F helix
• Changes are transmitted to the subunit interfaces
• 8 electrostatic bonds are broken
• The 2,3-BPG site is lost
• One ab subunit rotates by 15 degrees with respect to the other.

Question 8

Retinol

Answer 8

• one of several active forms of Vitamin A
• derived from b-carotenes synthesised by plants.
• cleaved to retinol by most animals
• can be stored in the liver as retinol palmitate.

Question 9

Roles of vitamin A (retinol)

Answer 9

Vitamin A is important as:-

• an anti-oxidant
• a steroid hormone
• a vital component of night vision

Question 10

RBP

Answer 10

RBP is an example of a β-domain protein. It is an up-and-down β-barrel with 8 strands in a simple topology.