Lecture 7 & 8

Question 1

What is hemoglobin?

Answer 1

haemoglobin is the predominant macromolecule i side red blood cells and gives blood its characteristic red colour.

Question 2

Structure of Hemoglobin?

Answer 2

The hemoglobin molecule consists of two distinct parts; a protein known as globin and a non protein unit called heam.

Question 3

Structure of the Heam?

Answer 3

At the center is an atom of iron in the ferrous Fe3+ state.

Question 4

What is the role of the iron atom in the heam?

Answer 4

This iron atom is central to the reversible carriage of oxygen around the body.

Question 5

What are the different conformation in which hemoglobin can exist?

Answer 5

When oxygen is being carried, hemoglobin is in the R-stateand referred to as oxyhemoglobin. When hemoglobin is not carrying oxygen on it’s return to the lungs it is in the T-state referredq to as deoxyhemoglobin.

Question 6

What is Myoglobin?

Answer 6

Myoglobin is biochemically related to hemoglobin

Question 7

Structure of myoglobin?

Answer 7

It is also a globin with a heme attached but has a slightly different structure and thus a different function.

Question 8

What is the function of myoglobin?

Answer 8

Myoglobin does not circulate in blood but is found in muscle cells, where it provides a reservoir of oxygen to indirectly energise muscular contraction.

Question 9

Colour of myoglobin?

Answer 9

Myoglobin is also red and this, rather than retained blood, given isolated muscle it’s colour.

Question 10

What is the structural difference between hemoglobin and myoglobin?

Answer 10

Hemoglobin exists as a tetramer( four globins attached together in a quaternary association. Whereas myoglobin molecules exist as monomers. As a result myoglobin, bind oxygen much more tightly than hemoglobin does.

Question 11

what are the two different types of protein found in hemoglobin?

Answer 11

Alpha and Beta

Question 12

What is a globin fold?

Answer 12

• regions, that are a-helical, folded into a tertiary shape

Question 13

What is the main function of hemoglobin?

Answer 13

Transport oxygen from the lungs to the tissues, as well as assisting in transport of carbon dioxide from tissues to the lungs.

Question 14

What does a S shapes (sigmoidal) saturation curve indicate

Answer 14

Molecular phenomenon of cooperativity

Question 15

Why can myoglobin not function as a transporter of oxygen in the blood?

Answer 15

It would become 100% saturated with oxygen in the lungs and remain almost 100% saturated at the tissues. Almost no oxygen would be released, in highly active muscle however, it is useful for storing oxygen and then releasing it when supplies become desperately low.

Question 16

How is the saturation curve for Myoglobin different to that of hemoglobin?

Answer 16

The myoglobin curve shows no sign of cooperativity.

Question 17

What is the structure of a heam?

Answer 17

Comprised of four rings, called pyrrole rings. They are linked together by CH (methane bridges) The whole structure is called a porphyrin ring. Attached peripherally methyl, vinyl and propionyl side groups. There are 15 possible ways of arranging these groups bu the same arrangement is always present in human globins.

Question 18

What isomer is used for all human globins>

Answer 18

protoporphyrin

Question 19

What is the role of the Iron atom?

Answer 19

This iron carries a molecule of oxygen (O2) but does not become oxidised to Fe3+. Oxygen does not bind to ferric (Fe3+) haem. The haem Fe2+ is oxygenated rather than oxidised. We rationalise that the purpose of the haemoglobin combination is to obstruct O2 from oxidising Fe2+, allowing only sufficient access as to be attached reversibly.

Question 20

Why is hemoglobin able to demonstrate cooperativity?

Answer 20

Normal haemoglobin in the human adult has two α and two β globins arranged in a symmetrical structure. The globins are not merely stuck to each other statically, they slide a little and interact such that the entire tetramer can exist in two states, known as R and T.

Question 21

Describe hemoglobin the the R state?

Answer 21

he R state haemoglobin structure is such that it has a high affinity for oxygen

Question 22

Describe hemoglobin the the T state?

Answer 22

In the state haemoglobin configuration has low oxygen affinity

Question 23

What happens when switching between the R and T state?

Answer 23

witching between the R and T states triggers all four globins to be in the
same state, there is not a mixture

Question 24

How does Oxygen binding and release effect the state?

Answer 24

O2 binding triggers a switch to the R state, which happens to favour O2 binding, whereas O2 release triggers the switch to the T state that further assists O2 release.

Question 25

What does the oxygen binding to iron cause?

Answer 25

O2 binding triggers a switch to the R state, which happens to favour O2 binding, whereas O2 release triggers the switch to the T state that further assists O2 release.

Question 26

Describe co-operative rocesses?

Answer 26

in the lungs O2 uptake favours more O2 uptake, and in the tissues the release of O2 favours the release of more O2 (i.e. the process is co-operative)

Question 27

Structural differences between oxyhemoglobin and deoxyhemoglobin?

Answer 27

deoxyhemoglobin has a dished haem, while in oxyhaemoglobin, oxygen flattens the haem, and pulls histidine F8 and helix F toward the binding site. Anything that keeps helix F away from the binding site will weaken oxygen binding

Question 28

In red blood cells what assists the shift from R state to T state?

Answer 28

presence of 2,3-bisphosphoglycerate (BPG)This small molecule, which has 5 negative charges, binds to a positively charged site in the centre of the haemoglobin tetramer.

Question 29

Purpose of binding BPG?

Answer 29

Binding of BPG to T state haemoglobin stabilises the T state and is
a major player in achieving the sigmoidal curve of haemoglobin oxygen binding

Question 30

What is an allosteric regulator?

Answer 30

A molecule that binds at a site other than the active site or functional site of a protein and affects protein function

Question 31

What is the name of the site where an allosteric regulator binds?

Answer 31

Allosteric site

Question 32

What is an allosteric regulator of hemoglobin?

Answer 32

BPG is an allosteric regulator of haemoglobin function (i.e. it binds somewhere other than the haemoglobin functional site (haem Fe) and affects protein function).

Question 33

What prevents oxidation of the haem iron?

Answer 33

The amino acids making up the globin, especially those closest to and surrounding the haem group, work together to prevent oxidation of the haem Fe upon oxygen binding through their various interactions. This is important as oxidised haem Fe cannot bind oxygen and act as an oxygen carrier.

Question 34

What is a methaemoglobin?

Answer 34

A haemoglobin that has its haem Fe in the Fe(III) oxidation state is called methaemoglobin.

Question 35

hemoglobin in sickle cell anemia?

Answer 35

In sickle cell anemia, the abnormal haemoglobin (Haemoglobin S) sticks together when it is in its deoxygenated form.

Question 36

What causes Hemoglobin S?

Answer 36

a point mutation of the 6th amino acid in the β chain. The polar positively charged glutamate, located on the surface of the protein, is replaced by the non- polar uncharged valine. This change makes Haemoglobin less soluble and in the deoxygenated form the valine binds to a hydrophobic pocked of another deoxygenated haemoglobin S protein, forming a long polymer of haemoglobin S proteins. The polymers essentially stretch the red blood cell out, giving it its sickle shape.