Myoglobin & Haemoglobin

Question 1

Why must oxygen be bound to proteins?

Answer 1

Very poorly soluble in the bloodstream

Question 2

Structure of Myoglobin

Answer 2

1. Predominantly helical
2. Compact single polypeptide (153)
3. Haem group contains porphyrin 9 with bound iron in ferrous state

Question 3

What are the 6 co-ordination sites on Iron bound to?

Answer 3

4 to flat porphyrin ring system, 1 to proximal histidine on F-helix and other to oxygen

Question 4

In deoxymyoglobin, the interaction of the iron with proximal histidine induces

Answer 4

A puckering of the porphyrin ring system

Question 5

Binding site for oxygen on myoglobin

Answer 5

Deeply buried

Question 6

How does oxygen enter the binding site on myoglobin?

Answer 6

Flexibility of molecule -> local breathing allows oxygen to move through local cavities in the protein

Question 7

Mechanism for binding of oxygen on myoglobin

Answer 7

1. Binds to 6th co-ordinating bond of haem bound iron
2. Results in partial transfer of electron from ferrous ion to oxygen -> superoxide ion which isn’t stable
3. Location of distal histidine on helix-E creates a H-bond to the bound oxygen + stabilizes it
4. This prevents release of unreactive species
5. H-bond pulls oxygen into plane of porphyrin ring
6. Whole structure changes shape via movement of H-helix to tighten around bound oxygen molecule

Question 8

Curve of Myoglobin

Answer 8

Hyperbolic

Question 9

Curve of Haemoglobin

Answer 9

Sigmoidal

Question 10

Myoglobin has a ____ affinity for oxygen at ___ pp

Answer 10

High, low

Question 11

Structure of Haemoglobin

Answer 11

1. Quaternary
2. 2 alpha, 2 beta
3. Bound haem containing iron in ferrous form
4. Each subunit is capable of binding a molecule of oxygen
5. Subunits sequentially dissimilar but structurally similar to myoglobin
6. Key residues highly conserved and identical to myoglobin

Question 12

Max Perutz

Answer 12

60’s + 70’s -> showed there are 2 major conformations of haemoglobin

Question 13

Oxygen can bind to haemoglobin in both

Answer 13

T state or R state

Question 14

Oxygen has a higher affinity in the

Answer 14

R state, stabilizing R state

Question 15

In the T state

Answer 15

Predominant conformation of deoxyhaemoglobin. -> Binding of oxygen in T state triggers conformational change to R state, causing sub-units to change position relative to one another

Question 16

Similarities between oxygen binding in myoglobin and haemoglobin

Answer 16

Binding of oxygen pulls iron into plane of porphyrin ring which changes position of helix F in sub-unit

Question 17

Differences between oxygen binding in myoglobin and haemoglobin

Answer 17

Change in position of helix F in sub-unit in myoglobin acts only to stabilize structure but in haemoglobin this and other small changes causes sub-unit to rotate relative to other sub-units in structure so overall T to R conformation

Question 18

Why is it important that haemoglobin binds to oxygen cooperatively?

Answer 18

Transition from low affinity T state to high affinity R state allows oxygen to be released effectively at tissues i.e. low partial pressures of oxygen

Question 19

How is oxygen binding on haemoglobin co-operative?

Answer 19

1. Oxygen binding on individual sub-units on haemoglobin can alter affinity for oxygen in adjacent sub-units
2. First O2 that interacts with deoxyhaemoglobin binds weakly since sub-unit in T state
3. Causes T-R transition and T-R transition occurs more readily in second sub-unit
4. Oxygen binds readily to haem in final sub-unit b/c already in high affinity R state

Question 20

What type of protein is haemoglobin?

Answer 20

Allosteric

Question 21

Allosteric proteins

Answer 21

Proteins which have conformational changes that are induced by modulators making them more or less active forms