The Transport and Storage of O₂ Flashcards
Oxygen is used as a electron …….. molecule within the electron transport chain?
Electron accepting
But before oxygen can be metabolised, it must be absorbed and transported by the organism
What is the oxidation state and coordination of the metal in Haemoglobin?
What is the oxidation state and coordination of the metal in Haemocyanin?
What is myoglobin (Mb)
This is the protein which takes oxygen off haemoglobin
Its function is to store oxygen in the muscles tissues
Draw out a haem group
(this is a simplified version which needs to be known for exams)
Draw out a Histidine N
This is a simplified version which will be used within exams
Also known as proximal histidine (close to iron)
Draw a simplifed version of haemoglobin with the proximal and distal histidine attached
X-ray crystallography shows that the coordination about the Fe centre changes when O₂ is bound
Explain
- In oxy-hemoglobin, four of the coordinated sites of iron are occupied by nitrogens of porphyrin ring
- The 5th site is occupied by the proximal histidine residue of globin
- The 6th position is occupied by dioxygen bonded to the distal histidine via hydrogen bonds
What happens to the geometry of haemogobin when oxygen bind
- In deoxy-haemoglobin the porphyring is domed shaped. The Fe(II) is paramagnetic and is positioned above the plane of the ring
- In oxy-haemoglobin the size of the iron ion is reduced and can fit into the cavity of planar porphyrin ring which triggers the conformational changes in other globin subunits and thus be opening other haem sies
How might scientists research the binding of oxygen on haemoglobin?
What is the benefit of this approach?
Using a parallel approach: prepare complexes that mimic the coordination environment of the metal centre (small molecule analogues or model compounds). These complexes can be:
* Simple or sophisticated
* Spectroscopically accessible
* Easily modified
* Functionally relevant
Why would we not use simple Fe-porphyrin complexes as an analogue for the Haem centre of Myoglobin
- B = pyridine or imidazole (cf His)
- Oxygen will bind onto the face of the iron (LHS)
- BUT then a second porphyrin ring attack the other oxygen, resulting in a dimer
- Which then drops down to an Fe(III) - O - Fe(III) bridge (effectively rust)
How can we overcome the dimerisation issue for simple Fe-porphyrin complexes as an analogue for Haem centre of Myoglobin
We can use ‘picket-fence’ porphyrins to increase steric hindrance (Fe unit cannot reach the oxygen), which stops dimerisation
How can we use Resonance Raman to tell the difference between the oxy and deoxy-forms of our analogues Haem?
- Resonance Raman will produce a v(O-O) at 1159cm⁻¹ and is silent on EPR for the oxy form
- Deoxy form is EPR active
- This is due to having an Fe(III) superoxide (O₂⁻) species in the oxy form and an Fe(II) in the deoxy form
Describe the Molecular orbital bonding diagram for the following molecule of myoglobin
- Is Fe(II) and has 5 x coordinate
- Hence is an axially elongated octahedron
- It is a d⁶ centre and is a 5-coordinate haem
- so is high spin
- Total spin = 4(½) = 2
- Hence is EPR active because it’s got unpaired electrons
Explaing the molecular orbital diagram for a molecule of myoglobin
- An electron from the dz² on the metal form a sigma bond with a π star electron on the oxygen
- For a bonding combination we also have an antibonding combination
- The other electron from the π star electron on the oxygen is non-bonding
- The 5 electron left from the d-orbitals which need to be placed in bonding orbitals (Fe has 6 ligands and hence is low spin)
what are the differences which occur to iron when oxygen bonds forming oxy-myoglobin
- There are 6e on deoxy-Mb and 5e on oxy-Mb
- Fe(II) is high spin in deoxy-Mb, whereas Fe(III) is low spin on oxy-Mb
What are the difference which occur to oxygen when oxygen bonds forming oxy-myoglobin?
- O₂ has 2 electron whereas the superoxide formed in oxy-myoglobin has 3 electrons
- (Matches EPR and Raman spectroscopy)
Is oxy-myoglobin EPR active or inactive?
- Inactive
- The single electrons in the dxy and π star orbitals have opposite spins and hence cancel out
- There is a weak pairing interaction between these electrons too - antiferro magnetic coupling
- Meaning there is no free-radicals within oxygen in the oxy-Mb form
Haemoglobin is made up of how many myoglobin like units and how do they adhere together
What does this form
4
quaternary structure through electrostatic interactions - salt bridges
This allows for stabilisation of the deoxy form
Why does haemoglobin and myoglobin have different binding profiles for oxygen?
- Haemoglobin and myoglobin have similar affinities for oxygen in the lungs, however the sigmoidal shape of the haemoglobin curve means its affinity for oxygen is lower in the muscles (oxygen moves from Hb to Mb)
Haemoglobin has cooperative binding of oxygen, how?
(this is why a sigmoidal curve is produce)
- The four subunits of Hb are connected by electrostatic interactions between amino acids (‘salt bridges’) that stabilise deoxy-Hb
- In regions of high oxygen partial pressure, oxygen binds causing the Fe ion move into the porphyrin plane. The proximal His (connected to the amino acid chain) also moves
- Once one or two oxygen molecules bind, the tug from the proximal His is sufficient to break the salt bridges thereby destabilising deoxy-Hb. As a consequence more oxygen binds
- When oxy-Hb reaches the muscles, high pp of CO₂ and low pH mean salt bridges reform, destabilsing oxy-Hb with the release of oxygen
Is Haemocyanin EPR active?
- Weakly bonding interaction between the electrons on Cu and electron on O
- Creates anti ferromagnetic coupling
- Meaning there is no unpaired electrons
- Hence is EPR silent (diamagnetic)
Each copper (I) has how many Histidine side chains?
3
(there is a gap between the two coppers where the oxygen binds)
What forms do copper and oxygen exist in, within Haemocyanin?
- 2Cu(I) → 2Cu(II) + 2e
- Cu(II) in haemocyanin
O₂ + 2e⁻ → O₂²⁻ - Superoxide in haemocyanin
