Haemoglobin: Allostery and Cooperativity

Question 1

This has a dished haem shape.

Answer 1

Deoxyhaemoglobin

Question 2

When in the presence of oxygen, this flattens the haem and pulls histidine F8 and helix F towards binding site.

Answer 2

Oxyhaemoglobin

Question 3

These are shifts in the orientation of protein secondary elements.

Answer 3

Conformational changes

Question 4

This is an allosteric inhibitor of O2 binding to haemoglobin

Answer 4

BPG

Question 5

How is BPG produced?

Answer 5

BPG is produced during respiration in peripheral tissues, and promotes oxygen release where needed

Question 6

This allows efficient unloading of oxygen.

Answer 6

Cooperativity

Question 7

Sigmoidal binding is an indication of what in hemoglobin?

Answer 7

Cooperativity

Question 8

This model suggests that subunits must be in the same state.

Answer 8

MWC model

Question 9

In the MWC model, the _______ stabilise the T form and the ________ stabilise the R form.

Answer 9

Inhibitors, activators

Question 10

The R form is usually what form of hemoglobin?

Answer 10

The oxygenated form

Question 11

The T form is usually what form of hemoglobin?

Answer 11

Deoxygenated form

Question 12

What stabilises the T state of hemoglobin?

Answer 12

Inhibitors

Question 13

What does unmasking cooperativity mean?

Answer 13

Stabilising the T state - so making sure that O2 will be released

Question 14

Why do we need inhibitors?

Answer 14

To push the hemoglobin to a T state, that will release O2 and push the hemoglobin back to an R state

Question 15

What other 2 molecules reduces the affinity of hemoglobin for O2?

Answer 15

elevated CO2 and low pH (so elevated H+)

Question 16

This effect describes how elevated CO2 and low pH in metabolising tissues reduces the affinity of hemoglobin for O2.

Answer 16

Bohr Effect

Question 17

This type of hemoglobin includes alternate isoforms with higher affinities for O2.

Answer 17

Foetal hemoglobin.

Question 18

What type of hemoglobin enables the foetus to capture oxygen in the placenta?

Answer 18

Foetal hemoglobin

Question 19

What is on the B chain’s binding site for BPG in foetal hemoglobin? (this is what makes it structurally different to adult hemoglobin)

Answer 19

Has a serine instead of a histidine

Question 20

This is a mutation by which the oxidation of haem Fe2+ to Fe3+ occurs, resulting in a shift of one subunit to the R state conformation without oxygen bound.

Answer 20

Methaemoglobin

Question 21

How does the methaemoglobin mutation impair the function of hemoglobin? (there are 2 ways)

Answer 21

1) the methaemoglobin subunit doesn’t bind to oxygen despite being in R state (because of Fe3+)

2) 3 other subunits are shifted to the R state - so they don’t release oxygen in tissues

Question 22

This enzyme is responsible for regenerating hemoglobin by reducing methaemoglbin back to Fe2+ state using transfer of electrons from NADH.

Answer 22

Cytochrome b5 reductase

Question 23

This is the mutation of His E7 to Tyr E7 - it changes the environment which causes Fe2+ to oxidise to Fe3+

Answer 23

HbM / Boston haemoglobin

Question 24

This has a gain of function mutation - which enables an abnormal hydrophobic interaction between Hb molecules particularly when in deoxy form

Answer 24

HbS / sickle cell hemoglobin

Question 25

How does sickle cell hemoglobin distort the red blood cells?

Answer 25

Through polymerisation of Hb into chains

Question 26

Sickle cell anemia results from _______

Answer 26

haemoglobin polymerisation

Question 27

What interactions enables BPG to bind to deoxygenated-Hb?

Answer 27

electrostatic interactions

Remember: BPG is an inhibitor so it will stabilise the T state of haemoglobin (reduce O2 affinity) so it will always promote oxygen release where its needed.

Question 28

How do positively charged side chains like histidine and side chain and amino termini contribute to the binding of BPG?

Answer 28

They signal BPG where to bind (facilitate the binding of BPG)

• positively-charged side chains in B chains make positively charged allosteric sites for allosteric effectors like BPG

Question 29

What does stripped haemoglobin mean?

Answer 29

The haemoglobin is predominantly in the R state - shows little cooperativity because R-state means that oxygen affinity to haem is quite high.

Question 30

Why is cooperativity most prominent in the presence of allosteric inhibitors?

Answer 30

Because allosteric inhibitors unmask cooperativity and stabilise the T state.

Remember: stabilising the T state means that the binding affinity of O2 is reduced so haemoglobin will release O2 more efficiently (thus undergo conformational shape change more likely)

Question 31

What 3 things stabilise the T-state and unmask cooperativity?

Answer 31

Allosteric inhibitors like BPG, CO2, and H+

Question 32

What is the ideal pH where O2 can bind less tightly to Hb?

Answer 32

Low pH (this means elevated H+) (low pH reduced affinity of haemoglobin for O2)

Question 33

Why is there an increase in BPG in higher altitudes?

Answer 33

Being in higher altitudes means a decrease in the partial pressure of O2 which increases the rate of respiration - therefore increasing the amount of BPG made. this will deliver more oxygen to the tissues.

Question 34

Why does foetal haemoglobin bind O2 more tightly?

Answer 34

Because foetal haemoglobin is less sensitive to BPG.

The beta chain (which would usually contain positively charged side chains) is replaced by a gamma chain - which makes foetal haemoglobin less sensitive to BPG

Question 35

This is the type of haemoglobin where the His E7 is mutated to change the environment, which causes oxidation of Fe2+ into Fe3+.

Answer 35

HbM or Boston Haemoglobin

Question 36

How does BPG bind to Hb?

Answer 36

BPG binds to the positively charged active site in the Hb through ionic interactions.

BPG has five negative charges