Lecture 14- Oxygen transport

Question 1

a) Outline the general structure of haem

b) Outline the structure of haem in myoglobin and haemoglobin

Answer 1

a) - protoporphyrin ring and an fe atom bound to 4N atoms of the ring- planar
- Fe2+ can make 2 additional bonds to oxygen, one on either side of the plane

b) - 1 o2 binds to the haem group
- Fe atom bound to protein (my or hb) via a histidine residue via covalent bonds on other side of plane

Question 2

What is haem/heme?

Answer 2

A prosthetic group: tightly bound on protein that is required for function of a protein

Question 3

Myoglobin:
A) Normal function
b) Structure

Answer 3

a) protein containing haem, which carries and stores oxygen in muscle cells
b) Structurally similar to one subunit of hB
- water soluble globular protein
- high % of alpha-helical secondary structure (75%)
- Short: 153 AA’s
- 8 right handed alpha helixes (A to H)
- monomeric heme protein with one heme group in hydrophobic cleft in centre
- Histidine residue in 8th alpha helix (F8) covalently bonded

Question 4

Haemoglobin:

a) Normal function
b) Structure

Answer 4

a) the protein molecule in red blood cells that carries oxygen from the lungs to the body’s tissues and returns carbon dioxide from the tissues back to the lungs for expulsion

b) - Tetrameric
- 2 polypeptide chains: Alpha and Beta
- each chain contains a haem prosthetic group and hence it can bind 4 molecules of oxygen

Question 5

Outline how oxygen binds to the haem in myoglobin

Answer 5

• In deoxymyoglobin, Fe is slightly below the plane of the ring
• oxygen binding causes FE to move into the plane of the ring
• movement causes movement of Histidine F8 and small change in overall protein conformation

Question 6

Outline how oxygen binds to haem in Haemoglobin

Answer 6

Has 2 states of affinity:

1. T (Taut) state: low affinity –> deoxyhaemoglobin
   - oxygen binds to a molecule of deoxyhb pulling FE atom into plane of heme and histidine with it also
   - heme group more exposed in other subunits
   - change in conformation (tertiary structure) in one subunit leads to changes in binding between adjacent subunits
   - increasing the affinity hb has for o2 in further o2 binding
2. R (relaxed) state: high affinity –> oxyhaemoglobin
   - more stable

Question 7

Compare the shape of oxygen dissociation curves between

a) Myoglobin
b) Haemoglobin

Answer 7

a) Myoglobin- hyperbolic (towards a finite)
- myoglobin has a high affinity of oxygen than Hb and becomes saturated at lower partial pressures
- no cooperative binding as only one polypeptide chain

b) Hb- sigmoidal (S) shape
- due to cooperative binding: as each o2 binds it alters the conformation of Hb making subsequent binding easier
- transition from low to high (t to r) affinity state

Question 8

Haemoglobin is said to exhibit ‘cooperative’ binding.
A) What does this mean?
b) What causes this cxtic?

Answer 8

a) The binding of one o2 molecule promotes the binding of subseqent molecules , it means that o2 can be efficiently carried from lungs to tissues
- If affinity state too high: o2 wont be given up in tissues
- if affinity state too low: o2 would be too readily given up

b) The presence of 2,3-Bisphosphoglycerate

Question 9

a) What regulates oxygen binding to Hb?
b) How?
c) When is the concentration of this molecule increased?

Answer 9

a) 2,3-Bisphosphoglycerate
b) Allosteric effector- acts as an inhibitor
- present in RBC @5mM normally
- negatively charged, will bind to positive AA in centre of beta subunit in the T form
- Lowers the oxygen affinity of Hb so that o2 more readily released
c) At high altitudes- at high altitudes there is less o2 present therfore tissues need o2 more readily, BPG conc increases so affinity is lower (shift to the right)

Question 10

a) Outline the BOHR effect
b) Outline how this would look on an o2 dissociation curve
c) Why doesnt Myoglobin exhibit the bohr effect?

Answer 10

a) Increase in CO2 and H+ concentration lowers affinity of Hb to O2
- acting as allosteric inhibitors
- H+ and CO2 produced in metabolically active tissues e.g. muscles
b) Curve shifted to the right
c) Because it only consists of a single polypeptide

Question 11

Outline the process of carbon monoxide poisoning

Answer 11

• CO is a poison as it combines with ferromyoglobin and ferrohaemoglobin and blocks oxygen transport
• it binds to hb 250x more readily than oxygen
• once it is bound o2 can never bind again
• fatal when COhb is more than 50 percent

Question 12

How are globin genes organised on chromosomes?

Answer 12

1. Alpha globin like genes found on chromosome 16
2. Beta globin like genes found on chromosome 11: Beta, Gamma and delta
   - Hemoglobins formed by combinations of alpha globin like chains and beta globin like chains

Question 13

What are the types of normal adult haemoglobins?

Answer 13

1. HbA–> α2β2 (90 percent)
2. HbF –> α2γ2 (gamma isntead of beta <2%)
3. HbA2 –> α2δ2 (delta instead of beta 2-5%)

Question 14

What does glycosylation of HbA form and why is this useful clinically?

Answer 14

HbA1c

• blood test can measure amount of this
• tests average BGL for 2-3 months prior
• can determine how well a persons diabetes is being controlled

Question 15

a) Why is foetal haemoglobin important?

b) What would it look like on an oxygen dissociation curve?

Answer 15

a) HbF is the major Hb in foetal blood
- has a higher affinity of o2 than maternal Hb (HbA)
- becomes saturated rapidly at lower partial pressures
- this allows o2 transfer from maternal hb to fetal hb through placenta
- this means the fetus can aerobically respire to release energy for protein synthesis and growth
b) The curve will be shifted to the left of maternal Hb

Question 16

Sickle cell anaemia

a) What type of mutation?
b) What does this mutation cause?

Answer 16

a) Missense mutation
- mutation of glutamate (HbA) to valine in Beta globin (HbS)
- glutamate is hydrophilic and interacts with water whereas Valine is hydrophobic and so it buries itself
- Sticky hydrophobic pocket formed by valine allows deoxygenated HbS to polymerise + form long chains
b) Sickled celled Hb–> more prone to lyse, leading to anaemia

Question 17

a) What are thalassaemias?
b) Beta thalassaemia? Causes?
c) Alpha thalassaemia? Causes?

Answer 17

a) A group of genetic disorders where there is an imbalance between the number of alpha and beta globin chains
b) - decreased or absent B-globin chain production
- Beta thalassemias are caused by mutations in the HBB gene on chromosome 11, inherited in an autosomal recessive fashion.
- Alpha chains unable to form stable tetramers
- B-thalassemia minor: loss 1 alelle
- b-Thalassemia major: lose both alelle

c)- decreased or absent Alpha-globin chain production
- several different levels of severity due to the multiple copies of the alpha chains present
- beta chains can form stable tetramers with increased affinity for oxygen
onset before birth

Question 18

What 3 things regulate the oxygen binding to Hb and how?

Answer 18

• Co2, h+ and BPG

- changing stabilising the T state

Question 19

a) What are allosteric effectors?
b) Outline the difference in allosteric inhibitors and Activators, what effect they have on the curve and give examples of each.

Answer 19

a) Allosteric regulation is the regulation of activities of an enzyme or a protein caused by the binding of regulators at the site other than the active site of the enzyme or protein.

b) allosteric inhibition occurs when the binding of one ligand decreases the affinity for substrate at other active sites. For example, when 2,3-BPG binds to an allosteric site on hemoglobin, the affinity for oxygen of all subunits decreases.
- H+ and CO2
- Shifts to the right

Allosteric activators: occurs when the binding of one ligand enhances the attraction between substrate molecules and other binding sites. An example is the binding of oxygen molecules to hemoglobin, where oxygen is effectively both the substrate and the effector. The allosteric, or “other”, site is the active site of an adjoining protein subunit.
- shifts to left