Proteins - Lecture Eight Flashcards
Proteins in Action: Oxygen Transport and Storage by Haemoglobin and Myoglobin
Relaxed, R-State
Oxyhaemoglobin
Taunt, T-State
De-oxyhaemoglobin
Rigid conformation, doesn’t bind enzymes
R- and T-states
Are stabilised by steric interactions and polar interactions
Allosteric effector example for haemoglobin
2,3-Bisphosphoglycerate (BPG) which has a very negative charge
What does BPG bind to?
Deoxy-HB by electrostatic interaction
What does BPG stabilise in deoxy T-state
Hb, reducing oxygen affinity
When and where is BPG produced?
During respiration in peripheral tissues, promoting oxygen release where it is needed
BPG
Less good at binding oxygen when in the muscles
Hyperbolic binding curve
Monomeric myoglobin
Sigmoidal binding curve
Cooperative, tetrameric haemoglobin
YO2
Fraction of protein bound to O2
MWC, Concerted Model
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
KNF, Sequential Model
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
Cooperativity
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.
The first substantial adaptation to high altitude
Increase in BPG, this reduces haemoglobin’s oxygen binding
Rightward shift of the binding curve
Delivers more oxygen to the tissues
How does carbon dioxide reduce oxygen affinity?
Both directly and via lowered pH of blood
Elevated CO2 and low pH (elevated H+) in metabolising tissues
Reduce the affinity of haemoglobin for O2, this is known as the Bohr effect
What stabilises the T-state
BOG, CO2 AND H+, this unmasks co-operativity
Isoforms
Refers to genetically different forms of enzymes
Fetal isoforms
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
Methaemoglobin impairs function in two ways
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.
Oxidation of haem Fe2+ to Fe3+
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.
His E7 mutation
To Try E7 changes the environment causing Fe2+ to Fe3+
The haem plane moves slightly breaking the connection of Fe-His F8
HbM
Remains in T-state with low affinity for oxygen
Mutation of Hb Beta6 Glu to Val
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
Haemoglobin function
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.
Haemoglobin physiological effects
Oxygen affinity is tuned in pregnancy and at high altitude.
Mutations to haemoglobin impair oxygen transport.
Sickle-cell anaemia results from haemoglobin polymerisation.
