oxygen-binding proteins Flashcards
How does aerobic respiration requires O2 ?
formula for respiration
30 ATP molecules per molecule of glucose
O2 dissolves very poorly in water
-approx 200 micromoles at 37°C,1 atm
How is O2 stored and transported by specialised proteins ?
Some AA can react with dioxygen
Proline CANT be used to store and transport O2
irreversible reaction
needs catalyst for reaction to proceed
What does nature needs fast easy reversible binding to store and move O2 ?
-Dioxygen reacts slowly with most organic molecules
lewis structure gives no clue by molecular orbit theory indicates that O2 is radical
O2 -magnetic
most compounds do not have unpaired elctrons and are thus dimagnetic
What do radicals react with ?
with other radicals
binding to O2 reversibly is best done with transition metal ions most commanly iron
Electronic configuration of Fe: [Ar]3d64s2
Electronic configuration of Fe2+: [Ar]3d6
What do vertebrates use to bind to O2 ?
Myoglobin and haemoglobin
-covalently to O2 to Fe in heme groups
-heme groups are protein cofactors
-Heme Fe2+ is bound by 4 nitrogens of pophyrin ring
dative,coordinate covalent N-Fe2+ bonds
-nitrogen atoms are called ligands
-sqaure planar arrangement
not predicted by VSEPR theory
Define Myoglobin ?
monomeric ,globular protein found in muscle tissue
-stores O2 for muscle activity
-18kDa
-0.5 mM in muscle
Define Haemoglobin ?
contains 4 polypeptide chain
-tetrameric globular proteins foundin RBC
transports O2 from lungs to muscle
α2β2 quaternary protein
each subunit similar to myoglobin
-4 hemes total
What us the relationship between myoglobin and haemoglobin ?
Myoglobin & heme both bind to O2 at iron but have diff bond strength .
Myo-permanently high affinity 4 O2
-good at storing O2
-only releases when all O2 used
Heme-lower & variable affinity
high affinity in lungs
low in muscles
Describe O2 binding to globins ?
there is eqm between oxy and deoxy myoglobin
Mb + O2 ⇌ MbO2
draw and explain oxygen bonding curve ?
on paper
Wht colour is arterial blood ?
bright red -oxyhaemoglobin
what colour is venous blood ?
dark red -deoxygenated haemoglobin
What is irons shrinkage linked to ?
electronic change
deoxyhaem is purple and paramagnetic(unpaired electrons)
When O2 bins to iron the metals d electron pair up and occupy less space
oxyheame is red and dimagnetic (no unpaired electrons)
Functional MRI
How does permanently oxidised heme work ?
long term exposure to air leads to irreversible oxidation of heme groups
known as “met-myoglobin” oe mert heamoglobin
metheme is brown (rust colour)
indicates presence of Fe 3+ ion
What do nitrate ions produce ?
metheme
Nitrate binds more strongly to Fe 3+ than Fe2+
induces autocatalytic oxidation of Hb in blood
nitrite israre in environment but nitrate is found in agricultural run -off
which can enter water-supplies in developing world
gut microbes can reduce nitrate to nitrite
How deos Carbon monoxide bind to haem iron
?
CO binds to haem Fe2+ more tightly
CO similar size & shape to O2 bu tpolar forming stronger bonds
CO is poisonous beacuse it binds in place of O2 in respiratory proteins
isolated heme groups bind CO thousands of times more strongly than O2
If thi were the case in Hb and Mb we would suffer from perpetual CO-poisoning
What do Globins use to stabilise oxyhaemoglobin ?
Use histidine residue
Histidine forms a stronger bond with O2 than with CO
CO binds only 250x stronger than O2
Only 2-3% hemes are bound to CO
What is the oxygen binding curve of haemoglobin ?
its sigmodial curve which indicates cooperative behaviour
-Protein subunits are interacting and communicating when they bind to O2
Hb delivers most of O2 to working muscles
Doe Hb change its shape when oxygenated ?
Yes
non-covalent bonds are broken at interphase between monomers
-hydrogen bonds and salt bridges
corresponds with sudden change in quaternary structure
11 dimer rotates with respect to 22 dimer
Relaxed form binds O2 more easily at its empty hemes
The R state has a higher O2-affinity than the T state
O2 binding to Hb
Cooperativity maximises O2 delivery from lungs to muscles
Switches between R state in lungs & T state in muscle
What makes haemoglobin switch between T & R state ?
Fe2+ ion shrinks when O2 binds
Movement transferred through proximal histidine to a1,b1 -a2b2 interphase
What else can bind to Hb and reduce O2
some other molecules
Hb is both cooperative protein and an allosteric (binds at another site) protein
allosteric effectors bind other sites on protein
negative allosteric effectors-because they stabilise T state and reduce Hbs affinity for oxygen
1st negative allosteric effector ?
2,3-BPG found in RBC
binds to central cavity of T state Hb
-stabilises T state so ↓ affintiy for Hb for O2
-without 2,3-BPG Hb is stuck in R state
2nd negative allosteric effector ?
Aerobically respiring muscle prodoces CO2
CO2 reacts with terminal amine groups at dimer interphase
-ve charge on carbamate forms salt-bridges and stabilises T state
-increases O2 delivery to muscle
this is direct Bohr effect
CO2 also forms carbonic acid in solution ,lowering pH
The 3rd negative allosteric efffector ?
H+
low pH some amine groups & His side chains are protonated
Positively charged groups form salt bridges -stabilise T- state
H+ effect on Hb is known as indirect Bohr effect
because of BE Hb delivers more O2 to working muscle (high pCO2,lowpH )carries CO2 back to lungs
Difference in Fetal Hb ?
The O2 must be transferred from mothers placenta, blood to feotus
-Fetal Hb binds to O2 strongly because it binds 2,3-BPG less well than adult Hb in central cavity
How does Fetal Hb bind to 2,3-BPH comapred to adult?
less tightly
because fetal is a2y2 tetramer
y subunit is similar to B but 39 diff amino acids change the 2,3-BPG binding store
H143S mutation removes 2 salt bridges
other mutations move H2 further away weakening 2 more
Give an example of a genetic disease of haemoglobin ?
First genetic disease to be charcterised
single mutation on the B -chain
E6V mutation
introduced hydropohobic group on protein surface in T state
deoxyhemoglobin aggregates and precipitates
