Chapter 6- Globular Proteins: Haemoproteins

Haemoproteins

- Proteins that contain a haem prosthetic group
- Haem group binds oxygen in haemo/myoglobin

Haem composition

- Haem made of protoporphrin and ferrous iron (Fe2+)

How many bonds can iron make?

6- 4 with nitrogen of protoporphrin

Where is myoglobin found?

cardiac and skeletal muscle

Function of myoglobin

stores and carries oxygen in muscle cells

How many molecules of oxygen does each myoglobin molecule bind?

1

Myoglobin structure

- Made of 1 polypeptide chain, identical to haemoglobin peptide
- 140-150 AA long with 8 helices and one Haem group

What does the oxygen bonding site involve in myoglobin?

2 histidines
- Proximal His binds Fe
- Distal his stabilises O2 binding to Fe

Where is haemoglobin found?

red blood cells

Function of haemoglobin

transport oxygen

Haemoglobin structure

Tetramer consisting of two alpha and two beta chains,each globulin contains haem molecule and each haem carries one oxygen (four oxygen for each Hb molecule)

How many O2 molecules can hemoglobin carry?

four

How does haemoglobin structure change upon binding oxygen?

T form taut
R form relaxed

T form

- Very compact
- Deoxyhaemoglobin
- 4 haem molecules to each chain

Interactions in R form

- Some ionic and hydrogen bonds between aB dimers are broken in oxygenated state

Interactions in T form

- Weak ionic and hydrogen bonds between dimer pairs
- Strong hydrophobic interactions between a and B chains forming stable dimers

R form

- Relaxed
- Oxyhaemoglobin

Conversion of T form to R form

- In deoxygenated form, His pushes Fe2+ out of plane of poryphyrine ring
- When O2 binds, it pulls the Fe2+ back in the plane which pulls the His with it resulting in conformational changes
- Conformation now relaxed

Oxygen transport in Hb

binds oxygen cooperatively ie binding of the first O2 enhances binding of the following ones by opening its structure

Myoglobin binding curve

-hyperbolic: has same affinity for O2 regardless of bound O2

Haemoglobin binding curve

- sigmoidal binding curve due to cooperative binding of oxygen molecules
- lower pH and higher temperature = reduced O2 affinity - exercise causes pH to drop and temperature to rise in skeletal muscle - causes a righwards shift making it easier to release O2 into the tissue

Two ways to favour unloading of O2

1. Decrease pH (when CO2 and H+ increase)
2. Increase of 2, 3 biphosphoglycerate (normal metabolic product in RBCs)
- Both favour Deoxyhaemoglobin conformation by stabilising it

Stored blood 2, 3 biphosphoglycerate concentration

Low

Transport of CO2

- On the amino terminus of haemoglobin as a carbamate group- Carbaminohaemoglobin

Effect of CO2 on deoxyhaemoglobin

- Stabilises the deoxy form of haemoglobin and decreases its affinity for O2
- O2 disassociation curve shifts to the right

Carbon monoxide

- Binds to haem sites like oxygen
- Shifts the haemoglobin conformation to a relaxed state, so the oxygen binds with high efficiency
- O2 disassociation curve becomes hyperbolic and shifts to the left
- O2 cannot be released in tissues anymore
- O2 not delivered to tissue- toxicity of carbon monoxide

Haemoglobinopathies

Diseases caused by DNA mutations that occur in alpha and beta Globin protein genes

Normal beta globin

HbA sequence: Glutamate at position 6 (charged)

Sickle cell anaemia

HbS sequence: Valine at position 6 (hydrophobic)

HbC sequence- less severe condition

Haemoglobin C sequence: Lysine at position 6

Sickle cell anaemia

- Sickle shaped cells
- Affects red blood cells
- Point mutation DNA
- Valine at positon 6
- Hydrophobic amino acid
- Fits in hydrophobic pocket in haemoglobin
- Long rope like rigid fibres form and distort erythrocyte shape
- Occlude normal blood flow in capillaries
- Possible advantage for heterozygous carriers as have less susceptibility to malaria