L3- Haemoglobin and Myoglobin

Question 1

What is the physiological role of Haemoglobin?

Answer 1

Oxygen is not a polar molecule and therefore does not dissolve well in blood. Haemoglobin carries oxygen from the lungs to respiring tissues.

Question 2

What is the physiological role of myoglobin?

Answer 2

Oxygen is not a polar molecules and therefore does not dissolve well in aqueous solutions. Myoglobin accepts oxygen from Haemoglobin and transports it to the tissues and muscles where it is mainly present.

Question 3

Why is Haemoglobin more suitable for its role as an oxygen transporter?

Answer 3

Co-operative binding - more sensitive to smaller changes in oxygen concentration, therefore can pick up in lungs and drop off at tissues.
Cooperative binding allows it to carry twice the amount of oxygen (thanks to increases afinity) than it could without this cooperation.
Can carry 4 O2 vs Myoglobin’s 1

Question 4

How is myoglobin more suitable for its role as an oxygen carrier in tissues?

Answer 4

It has a higher afinity for oxygen than myoglobin and therefore can take oxygen off of Haemoglobin at the tissues.

Question 5

How many oxygens can each haeme bind to in Haemoglobin and myoglobin?

Answer 5

One

Question 6

What is the maximum amount of oxygen that Haemoglobin can carry?

Answer 6

Four

Question 7

How is haem bound to the protein part of Haemoglobin?

Answer 7

Via a covalently bound histidine residue

Question 8

How does the oxygen dissociation curves of Haemoglobin and myoglobin differ?

Answer 8

Myoglobin’ curve is hyperbolic - as the concentration of oxygen increases so does the percentage saturation of myoglobin until it levels off at 100%.
Haemoglobin’s curve is sigmoidal and it has lower afinity than myoglobin (shifted further to the right). It is sigmoidal because its showing a the percentage saturation of a mixed population of Haemoglobin in its T-state or R-state at each ppO2, therefore is a mix of the low-affinity state curves and the high-affinity state curves.

Question 9

Describe the structure of haemoglobin

Answer 9

Tetramer: 2 alpha and 2 beta chains in the adult form
Each chain contains a haeme prosthetic group
Conformation of each polypeptide chain is very similar to that of myoglobin

Question 10

What effect does oxygen binding have on Haemoglobin’s conformation?

Answer 10

Oxygen binding promotes a change in Hb from its T-state to its higher oxygen-afinity R-state. This promotes the binding of subsequent oxygen molecules = COOPERATIVE BINDING

Question 11

Define cooperative binding of oxygen to haemoglobin

Answer 11

The binding of one oxygen molecule promotes the binding of oxygen to the next subunit.

Question 12

What are the advantages to Haemoglobin’s sigmoidal dissociation curve?

Answer 12

It means that Haemoglobin is more sensitive to small differences in O2 concentrations and therefore is more efficiently carried from the lungs to the tissues. Thanks to cooperative binding Haemoglobin can carry double the amount of oxygen.

Question 13

What is the affect of 2,3-BPG on Haemoglobin?

Answer 13

It is a bi-product of glycolysis that acts like an allosteric inhibitor to Haemoglobin.

It lowers Haemoglobin’s affinity for oxygen and therefore increases the amount of oxygen that is released at respiring tissues

Question 14

How many 2,3-BPG bind per Hb tetramer? How does it bind?

Answer 14

One. It contains negatively charged phosphate groups which bind to positively charged Haemoglobin residues.

Question 15

What affect does BPG have on the oxygen dissociation curve?

Answer 15

Shifts it to the right

Question 16

What is the Bohr Effect?

Answer 16

The Bohr effect is the name given to the effect that pH and CO2 had on the binding and release of oxygen by Haemoglobin. It refers specifically to the release of oxygen from Haemoglobin when the pH falls (increase in the concentraion of H+) or the [CO2].
Metabolically active tissues produce large amounts of H+ and CO2 (both from H2CO3 ions), therefore the Bohr effect ensures that delivery of oxygen is coupled to demand.

Question 17

What affect do H+ and CO2 have on the oxygen dissociation curve?

Answer 17

Both shift the curve to the right.

Question 18

What is the affect of CO on oxygen transport?

Answer 18

CO combines with ferromyoglobin and ferrohaemoglobin and blocks oxygen transport.
It binds 250X more readily to Hb than oxygen.
It also increases the affinity for oxygen for unaffected subunits, so Hb can take up oxygen in the lungs but cannot release it at the tissues.
COHb> 50% is fatal
If all 4 subunits of Hb are blocked with CO the molecule is effectively dead.

Question 19

How would you treat carbon monoxide poisoning?

Answer 19

Could use a hyperbaric chamber to increase the ppO2 in the lungs or in serious cases do a blood transfusion.

Question 20

What are the subunits that make up normal adult HbA?

Answer 20

2x alpha subunits

2x beta subunits

Question 21

What are the subunits that make up foetal Haemoglobin HbF?

Answer 21

2x alpha subunits

2x gamma subunits

Question 22

What are the subunits that make up HbA2?

Answer 22

2x alpha subunits

2x delta subunits

Question 23

Glycosylation of Hb produces which marker for long-term diabetes control?

Answer 23

HbA1C

Question 24

What property of HbF is advantageous to the foetus?

Answer 24

HbF has a greater afinity for oxygen than HbA which allows transfer of oxygen to the foetal blood supply from the mother.

Question 25

What amino acid substitution occurs to Haemoglobin causing sickle cell anaemia?

Answer 25

Glutamate to Valine (“glutval’!)

Question 26

How does the amino acid substitution in Sickle Cell anaemia affect the Haemoglobin?

Answer 26

1. It changes a hydrophilic amino acid for a hydrophobic amino acid
2. HbS then cluster together to prevent this hydrophobic AA interacting with water.
3. These clumps tend to bind the the RBC membrane, making the membrane more rigid.
4. This means that the RBC is not as flexible and can get block microvasculature and is more prone to lyse.

Question 27

What causes Thalassaemias?

Answer 27

Thalassaemias are a group of genetic disorders where there is an imbalance between the number of alpha- and beta- globin chains.

Question 28

What happens in Beta-Thalassaemias?

Answer 28

There is a decreased or absent beta-globin chain production.
Alpha-chains are UNABLE to form stable tetramers and can precipitate out of solution.
Symptoms appear AFTER birth.

Question 29

What happens in Alpha-Thalassaemias?

Answer 29

There is a decreased or absent alpha-globin chain production.
Beta-chains CAN form stable tetramers (HbH) with increased afinity for oxygen (don’t release)
Onset BEFORE birth

Question 30

Name three things that decrease Haemoglobin’s afinity for oxygen

Answer 30

BPG
CO2
H+

Question 31

Why does BPG have a greater afinity for deoxygenated Hb than oxygenated?

Answer 31

It is due to spatial changes 2,3-BPG (with an estimated size of 9 angstroms) fits in the deoxygenated Haemoglobin configuration (11 angstroms), but not as well in the oxygenated (5 angstroms).

Question 32

Why can acidosis precipitate a Sickle Cell crisis?

Answer 32

At high concentration of H+, the afinity of Haemoglobin for oxygen will be decreased (the Bohr effect), therefore more Haemoglobin will be in their deoxygenated T-state. In this state the mutant hydrophobic residue in Sickle Cell individuals is more exposed and Hb polymerises, causing sickling of cells. Therefore the more T-state Haemoglobin, the more Sickled cells, which can cause a sickle cell crisis.

Question 33

The binding curve of Haemoglobin is sigmoidal because:

Answer 33

The binding of oxygen to one haem group increases the oxygen affinities of the other haem groups.

Question 34

In Haemoglobin the transition form T state to R state (low to high afinity) is triggered by:

Answer 34

Oxygen binding

Question 35

Why is the Bohr effect physiologically important?

Answer 35

It allows a link between oxygen demand of the tissues to be coupled with oxygen delivery by Haemoglobin. Metabolically active tissues produce CO2 and other acidic compounds (e.g. Lactate). This causes a localised lowering of the blood pH, causing a decrease in the affinity of Haemoglobin for oxygen. Hence, O2 is released at the sites of most need.

Question 36

How can a deficiency of niacin in the diet cause a reduction in the activity of NAD and NADP dependent enzymes?

Answer 36

Niacin is needed in the body for the formation of Nicotinamide, a component of the coenzymes Nicotinamide adenine dinucleotide (NAD) and NADP. A deficiency in niacin affects all NAD(P) dependent dehydrogenases and can cause the disease pellagra.
Niacin (nicotinic acid) is a vitamin that is synthesised from the amino acid tryptophan. However humans cannot synthesise sufficient quantities of this to meet its requirements and most niacin must be derived from dietary sources.

Question 37

How does a competitive inhibitor differ from a non-competitive inhibitor?

Answer 37

A competitive inhibitor affect the Km but not the Vmax

A non-competitive inhibitor affects the Vmax but not the Km.