Lectures 7-8 Hb

Question 1

primary structure of haemoglobin

Answer 1

around 150 amino acid-long primary sequence

Question 2

whats the secondary hemoglobin structures

Answer 2

8 alpha helices, A-H.

Question 3

whats the tertiary structure of haemoglobin

Answer 3

a globin fold with a hydrophobic pocket

Question 4

what the quatenary structure of haemoglobin

Answer 4

4 monomeric units interacting to make the heteromeric quaternary structure.

Question 5

what does the Haem group bind to

Answer 5

His F8

Question 6

describe the structure of the haem group

Answer 6

a central Fe2+ ion bound to His F8 as well as 4 N pyrrole rings, leaving the one binding site for O2.

Question 7

what other amino acid is important in O2 binding to haem and explain

Answer 7

the His E7. His E7 distorts the binding of the O2 to the Fe2+, This reduces the binding affinity of oxygen to myoglobin, making it easier to
release oxygen to the muscle cell. reducing the binding affinity through steric hindrance.

Question 8

what is allosteric hindrance

Answer 8

hindrance of binding but not at the active site of the protein

Question 9

what is the shape of the myoglobin curve

Answer 9

it is hyperbolic

Question 9

what does the hyperbolic curve shape of myoglobin mean

Answer 9

Myoglobin is saturated with O2 at low
pO2 only releasing O2
to muscle cells
when the cellular pO2
is very low.
This is shown in a ‘hyperbolic’ curve.
This suits its function as a “back-up”
store of O2 in muscle cells.

Question 10

what binds oxygen more tightly, the myoglobin or haemoglobin

Answer 10

myoglobin

Question 11

how are the 4 globin subunits of haemoglobin joint together

Answer 11

via non covalent association bonds

Question 12

describe T state

Answer 12

tense state and has low affinity for oxygen

Question 13

describe R state

Answer 13

relaxed state and has high O2 affinity

Question 14

describe the concerted model for Haemoglobin

Answer 14

Subunits can be in a low-activity,
tense (T) or high-activity, relaxed
(R) conformation.
* All subunits must be in the same
state (either T or R).
Binding each successive substrate
(S) shifts equilibrium in favour of R.

Question 15

in the concerted model, what do inhibitors and activators do

Answer 15

inhibitors stabilise the T form, activators stabilise the R form

Question 16

whats the sequential model

Answer 16

One substrate binding induces a T → R conformational
change in only one subunit.
* This conformational change influences the neighbouring
subunits (i.e. cooperativity), changing their affinity for substrate. Many conformations are possible, like 2T and 2R at the same time

Question 17

what are the haem shapes in deoxy and oxy haemoglobin

Answer 17

in deoxyhaemoglobin the shape is dish shaped and oxyhaemoglobin is more flat

Question 18

what flattens the haem group

Answer 18

the binding of O2 pulls iron ion into the plane of the haem

Question 19

what happens when haem binds O2 in terms of shape

Answer 19

the haem group flattens, this pulls His F8 toward the haem binding site, which then causes a conformational change in helix F, which changes C. which then causes conformational changes and cooperativity in the other helices.

Question 20

T/F, T state means deoxthaemoglobin

Answer 20

F

Question 21

are the T state and R state and the binding molecules specific to haemoglobin

Answer 21

no, they apply to all multimeric proteins.

Question 22

describe the conformational changes of haemoglobin binding

Answer 22

The change of position is in helix F, this changes angle between E and F. in Deoxy its more open, in oxy is closed. The binding moves FG in a fair bit.
As the F helix moves the FG loop moves upwards relative to the C helix.

Question 23

BPG is a what? and what is its charge

Answer 23

its an allosteric inhibitor and it has -5 charge

Question 24

where does BPG bind

Answer 24

into an allosteric site in the centre between the 2 beta strands, that site has His 2 and 143, and Lys 82

Question 25

when is BPG produced and what does this mean

Answer 25

BPG is produced during
respiration in peripheral
tissues, so promotes
oxygen release where it
is needed.

Question 26

what does BPG do

Answer 26

BPG stabilises Hb in the
deoxy T-state, reducing
oxygen affinity.

Question 27

what do the allosteric inhibitors, H+, CO2 and BPG do

Answer 27

they stabilise the T state, helping haemoglobin undergo cooperativity and unbind O2 in tissues

Question 28

what shows more cooperativity, stripped Hb or whole blood

Answer 28

whole blood that has CO2 and H+ in it

Question 29

whats the bohr effect

Answer 29

when CO2 and H+ cause lower affintiy of Hb to O2.

Question 30

where do the CO2 and H+ come from, and where does it bind

Answer 30

Like BPG, elevated CO2 and lowpH (elevated H+) in metabolising tissues. the CO2 binds to terminal NH3 and H+ binds to polarisable Aa side chains

Question 31

what happens to O2 binding of Hb at altitude and why

Answer 31

The first substantial adaptation to high altitude is
an increase in BPG. this lowers Hb affinity for O2. which means the O2 that is picked up by Hb is held onto less and thus dumped more effectively in the tissues.
long term at altitude out Hb numbers increase

Question 32

whats the structural difference between fetal haemoglobin and normal

Answer 32

fetal has 2 alpha and two gamma strands. the gamma strands have Ser 143 instead of His 143. so fetal Hb has less affinity for BPG.

Question 33

what does lower BPG affinity for fetal Hb mean

Answer 33

This allows the foetus to capture oxygen in the placenta.

Question 34

what is methemoglobin

Answer 34

when the Fe2+ of the haem group is oxidised to Fe3+ changing one subunit to the R state

Question 35

what does methaemoglobin cause for the haemoglobin molecule

Answer 35

the shift to Fe3+ changes one of the haemoglobin subunits to the R state permanently. this change to Fe3+ means the haem doesn’t bind O2, despite the R state.

also the three other subunits are made into the R state which means they don’t release the oxygen in the periphery

Question 36

how is methaemoglobin fixed

Answer 36

The enzyme cytochrome b5 reductase regenerates haemoglobin by reducing methaemoglobin back to Fe2+ state using transfer of electrons from NADH.

Question 37

what is HbM, or boston haemoglobin

Answer 37

there is a mutation to His E7 changing it toTyr E7, this causes Fe2+ to be oxidised to Fe3+. Haem plane moves slightly, breaking the connection of Fe-His F8 .
HbM remains in ‘T’ state, with low affinity for oxygen

Question 38

what causes sickle cell anemia

Answer 38

Abnormal haemoglobin is caused by a point mutation of the 6th amino acid in the beta chain. The positive Glu on the surface of the protein is replaced by non polar valine.

Question 39

what does the mutation of sickle cell anemia do?

Answer 39

This change makes haemoglobin less soluble and in the Deoxy form the valine binds to a hydrophobic pocket of another abnormal haemoglobin, forming a sickle shaped cell.