Proteins - Lecture Eight

Question 1

Relaxed, R-State

Answer 1

Oxyhaemoglobin

Question 2

Taunt, T-State

Answer 2

De-oxyhaemoglobin

Rigid conformation, doesn’t bind enzymes

Question 3

R- and T-states

Answer 3

Are stabilised by steric interactions and polar interactions

Question 4

Allosteric effector example for haemoglobin

Answer 4

2,3-Bisphosphoglycerate (BPG) which has a very negative charge

Question 5

What does BPG bind to?

Answer 5

Deoxy-HB by electrostatic interaction

Question 6

What does BPG stabilise in deoxy T-state

Answer 6

Hb, reducing oxygen affinity

Question 7

When and where is BPG produced?

Answer 7

During respiration in peripheral tissues, promoting oxygen release where it is needed

Question 8

BPG

Answer 8

Less good at binding oxygen when in the muscles

Question 9

Hyperbolic binding curve

Answer 9

Monomeric myoglobin

Question 10

Sigmoidal binding curve

Answer 10

Cooperative, tetrameric haemoglobin

Question 11

YO2

Answer 11

Fraction of protein bound to O2

Question 12

MWC, Concerted Model

Answer 12

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

Question 13

KNF, Sequential Model

Answer 13

One substrate binding induces a T to R conformational change in one subunit, this influences the neighbouring subunits
Explains negative co-operativity, instead of accelerating they decelerate when more substrates are bound
All four subunits don’t need to be identical

Question 14

Cooperativity

Answer 14

Refers to oxygen binding to one subunit influence how it’s going to bind to another subunit- binding to the first subunit makes it easier to bind to the second subunit.
When in the T state, they want no bonded oxygens and in the R state, they want all bonded oxygens.

Question 15

The first substantial adaptation to high altitude

Answer 15

Increase in BPG, this reduces haemoglobin’s oxygen binding

Question 16

Rightward shift of the binding curve

Answer 16

Delivers more oxygen to the tissues

Question 17

How does carbon dioxide reduce oxygen affinity?

Answer 17

Both directly and via lowered pH of blood

Question 18

Elevated CO2 and low pH (elevated H+) in metabolising tissues

Answer 18

Reduce the affinity of haemoglobin for O2, this is known as the Bohr effect

Question 19

What stabilises the T-state

Answer 19

BOG, CO2 AND H+, this unmasks co-operativity

Question 20

Isoforms

Answer 20

Refers to genetically different forms of enzymes

Question 21

Fetal isoforms

Answer 21

Better at binding oxygen than adult isoforms so they can pick up oxygen across the placenta from the parental haemoglobin and they’re better at binding because they’re lacking one of the positively charged amino acids in the BPB binding sides so it doesn’t bind BPG as well and remains more in the R-State rather than the T-state

Question 22

Methaemoglobin impairs function in two ways

Answer 22

The other subunits of the tetramer are shifted to the R-state, so do not release oxygen in the tissues as they should.
The enzyme cytochrome b5 reductase regenerates haemoglobin by reducing methaemoglobin back to Fe2+ state with transfer of electrons from NADH.

Question 23

Oxidation of haem Fe2+ to Fe3+

Answer 23

Shifts one subunit to the R-state conformation, without oxygen bound. - binding oxygen and releasing oxygen but this is not a redox reaction, the iron should remain in the 2+ state, if it does shift to a 3+ state it transfer that tetramer of haemoglobin into methaemoglobin which is far less functional.

Question 24

His E7 mutation

Answer 24

To Try E7 changes the environment causing Fe2+ to Fe3+

The haem plane moves slightly breaking the connection of Fe-His F8

Question 25

HbM

Answer 25

Remains in T-state with low affinity for oxygen

Question 26

Mutation of Hb Beta6 Glu to Val

Answer 26

Enables an abnormal hydrophobic interaction between Hb molecules, particularly when in the deoxy form, causing polymerisation of Hb into chains that distort the red blood cells

Question 27

Haemoglobin function

Answer 27

Oxygen binding is weakened allosterically by BPG, CO2 and low pH, this is described as shifting the tetramer to the T-state.
When shifted, haemoglobin displays co-operative binding of oxygen, evident in a sigmoidal binding curve.
The R- and T- states differ in how helix F interacts with the haem and with the helix C, and spacing between H helices.

Question 28

Haemoglobin physiological effects

Answer 28

Oxygen affinity is tuned in pregnancy and at high altitude.
Mutations to haemoglobin impair oxygen transport.
Sickle-cell anaemia results from haemoglobin polymerisation.