Protein Function - Oxygen Transport

Question 1

What part of the haemoglobin does oxygen bind to?

Answer 1

An Fe2+ iron ion in the middle of a haem group.

Question 2

What does haem groups consist of?

Answer 2

A protoporphyrin ring and an Fe2+ that is bound to 4 nitrogen atoms in the ring.

Question 3

How is the Fe2+ bound to the haemgroup/protein?

Answer 3

Via a histidine residue (proximal histidine) on the other side of the ring.

Question 4

What are the features of a myoglobin structure?

Answer 4

It is a globular tertiary structure. It is 153 aa long and compact. It is 75% alpha helical, no beta-pleated sheets and His 93 in the 8th alpha helix is covalently linked to Fe.
It is not a quaternary structure.

Question 5

What is deoxymyoglobin?

Answer 5

A myoglobin molecules not bound to oxygen.

Question 6

Where does the Fe sit in the deoxymyoglobin in relation to the plane of the protoporhyrin ring?

Answer 6

Slightly below the plane.

Question 7

How does the Fe move as oxygen binds to it?

Answer 7

There is a small shift upwards so Fe sits in the middle of the plane of the ring. This is a small change in the overall protein conformation.

Question 8

Describe the curve of how haemoglobin bind to oxygen.

Answer 8

It is sigmoidal.

Question 9

How does the curve of myoglobin and haemoglobin differ and explain what this means.

Answer 9

Myoglobin is up-side-down L-shaped. Haemoglobin is sigmoidal. This means that myoglobin and haemoglobin binds oxygen differently. It also means that myoglobin has a higher affinity for oxygen than haemoglobin.

Question 10

Outline the features of a haemoglobin molecule.

Answer 10

4 polypeptide chains. Two alpha polypeptides and two beta polypeptides in a a2b2 tetramer. These alpha and beta structure are called subunits and are globular proteins bound together non-covalently
Each chain contains an essential haem prosthetic group.
It has a quaternary structure and each subunit is fairly similar to a myoglobin molecule. This also means that there are 4 haem groups and 4 oxygen molecules can bind to one haemoglobin.

Question 11

What are the two states of haemoglobin? How do they differ?

Answer 11

T state: a low affinity where the subunits are closer together as well as the polypeptide chains internally.
R state: a high affinity where the structure is a little bit more open so oxygen can bind more easily.

Question 12

Which state does oxygen binding promote stabilisation for?

Answer 12

The R state with higher affinity.

Question 13

Why is the oxygen binding curve for haemoglobin sigmoidal?

Answer 13

Oxygen makes a conformational change to the haemoglobin. This means that the more oxygen that bings to the Hb subunits the higher affinity for oxygen. This means that there is a promotion in the R state.
To clarify this means that binding the first oxygen molecule to the first subunits is hard as it has low affinity.
However the last oxygen molecule to the 4th subunit is easy because of its high affinity at that point.

Question 14

What does cooperative binding of oxygen to haemoglobin mean?

Answer 14

That the more oxygen that binds the higher affinity for subsequent molecules.

Question 15

Why is the sigmoidal curve of oxygen binding to haemoglobin important?

Answer 15

Because it means that oxygen can be efficiently carried from the lungs to the tissues.
It also mean that it is more sensitive to a small difference in oxygen concentration.

Question 16

What is the formula for the sigmoidal curve of oxygen binding to haemoglobin? Explain what it means.

Answer 16

Y = pO2 / (pO2 + p50)
Y = percentage saturation with oxygen as a fraction
pO2 = partial pressure of oxygen (concentration)
p50 = partial pressure giving 50% of the saturation

Question 17

What other measurement in MCBG is p50 similar to?

Answer 17

Mikaelis Constant (Km)

Question 18

What does an increase in p50 mean?

Answer 18

That there is a shift of the curve to the right. This means that the partial pressure of oxygen has to be higher in order to maintain 50% of oxygen saturation. This means that there is a lower affinity for oxygen to bind. This also means the haemoglobin is more prone to release oxygen.

Question 19

What does a decrease in p50 mean?

Answer 19

That there is a shift of the curve to the left. This means that the partial pressure of oxygen will be lower in order to maintain a 50% of oxygen saturation. This means a higher affinity for oxygen to bind.

Question 20

What part of the curve is found in tissues such as muscles and placenta?

Answer 20

The steep curve where the affinity is low.

Question 21

What part of the is found in lungs?

Answer 21

The plateau where the affinity is high.

Question 22

Give 4 factors that will regulate the p50.

Answer 22

pH (a low pH means less affinity) muscles pH is lower due to lactic acid e.g. The harder a tissue works the more acidic.
Temperature (high temperature means less affinity) also found in muscles.
pCO2 (an increase in pCO2 means less affinity for oxygen)
BPG (2,3 - Biphospoglycerate) Haemoglobin with 2,3 BPG has an increased p50 which results in lower affinity.

Question 23

Where can you find an increased p50?

Answer 23

Muscles and placenta

Question 24

Where can you find a decreased p50?

Answer 24

Lungs

Question 25

What is 2,3-BPG? Where can it be found in elevated concentration?

Answer 25

An important step (intermediate) in glycolysis.

In pregnant women where a low affinity for oxygen is needed in order to supply the foetus with oxygen.

Question 26

How does 2,3-BPG relate to altitude?

Answer 26

At high altitudes BPG concentration is increased. This promotes O2 release to tissues.

Question 27

How is carbon monoxide harmful to the body?

Answer 27

CO has a much higher affinity than oxygen (250x) so it renders the haemoglobin it binds to useless. This essentially reduces the amount of haemoglobin which can bind to oxygen and therefore carry oxygen to tissues.

Question 28

When is carbon monoxide poisoning fatal?

Answer 28

When COHb is over 50%.

Question 29

How do you treat a patient with carbon monoxide poisoning?

Answer 29

You increase the partial pressure of oxygen pO2. A hyperbaric oxygen chamber e.g.

Question 30

What are the forms of Haemoglobin found in a normal adult? What percentages?

Answer 30

HbA (90%) a2b2
HbF (<2%) a2y2
HbA2 (2-5%) a2d2

Question 31

How is different forms of Haemoglobin regulated during development?

Answer 31

Different alpha and beta-globing genes are regulated during development. Before birth the y-globin chains are highly active (HbF). The b-globin first becomes active after birth. The alpha-globing is active at around 1 month of pregnancy and plateaus around 3 months of pregnancy.

Question 32

Why is there more HbF than HbA in foetal blood and how does it relate to the maternal blood?

Answer 32

Because HbF has a higher affinity than HbA. So O2 flows from maternal blood to foetal blood more easily. Essential it makes it easier for the foetus to pick up oxygen from the mother.

Question 33

Outline what causes sickle cell anaemia and how the new haemoglobin molecule is structured.

Answer 33

A missense mutation of glutamate to valine in b-globin. Glutamate is polar and hydrophilic and Valine is non polar and hydrophobic. This new hydrophobic haemoglobin is called HbS and this allows deoxygenated HbS to polymerize so they stick together. Therefore there is less binding of oxygen and more likeliness of blockage of a vein.

Question 34

What are thalassaemias?

Answer 34

A group of genetic disorders where there is an imbalance between the number of alpha and beta globin chains. This means that oxygen can’t be transported as effectively.

Question 35

What are beta-thalassaemias?

Answer 35

A decreased or absent beta-globin chain production.
Alpha chains are unable to form stable tetramers
Symptoms appear after birth
Hb Bart’s in form of 4 y-globins
or
an alpha-chain precipitate

Question 36

What are alpha-thalassaemias?

Answer 36

decreased or absent alpha-globin chain production.
several different levels of severity
beta-chains can form stable tetramers but have an increased affinity for oxygen.
this shows an onset before birth.

Question 37

Why is the oxygen binding curve for myoglobin hyperbolic?

Answer 37

Because it is a single sub-unit as a globular protein and can only bind one oxygen molecule. Therefore there is no cooperative binding as no further subunits of the myoglobin exist and therefore no further binding of that molecule to oxygen occurs.