Proteins - Myoglobin Flashcards
What is the structure of Myoglobin?
Weighs 17 kDA, composed of 155 amino acids, compact globuler structure, 75% alpha-helicies, being eight forming a helical bundle.
What is the general tertiary structure of Myoglobin?
Central heme group containing Fe2+ and a porphyrin ring connected to four nitrogen and two histidne residues surrounded by the alpa helicies.
Prophyrin Ring
This is a ring-like molecule of 4 pyrroles being smaller rings composed of 4 carbon and 1 hydrogen.
Heme Group
This is a prosthetic group involved in O2 transport and storage.
Proline
This is a major amino acid of the molecular formula of C5H9NO2
What terminates the helical segments?
Proline residues
How is proline unique?
Its side group is cyclical forming a pyrrolidine ring being rigid and conformationally restircited.
Why is the restricted conformation of proline important?
Breaks the arranged H-bonding structure of the alpha helicies allowing loop formation.
What structure does proline allow for?
Loops between example helicies B and C due to pro45, creating a flexibile region
What is heme group surrounded by?
Non-polar residues except for two histidines.
What histidines does heme group bind to?
His93 in Helix F and His64 of the flexible loop region
Why is enclosue of Heme group of Fe important?
O2 has only limited solubility in water.
O2 diffuses slowly in tissues
O2 is irreversible when bound to free heme
Why does irreversibility of O2 bidning occur?
Oxidation state of Fe having ferrous(II) or ferric(III) forms
What is the molecular structure of ferrous iron?
Unpaired electrons in the d-orbital interacting with O2
Why is binding irreversible for ferric form?
Does not have electron pair, no covalent bond formation meaning Fe3-O2 bond is weak and breaks and O2 diffuses away.
How does myoglobin facilitate conversion of ferric iron to ferrous?
Reducing agents like ascorbate and dithionite with electron transfer
How is binding of iron to histidine residues important?
Imidazole group forming co-ordination bonds.
How does the porphyrin ring act as a chromophore?
Binding of O2 to Fe changes electronc structure due to co-oridnate bonds shifting absorption spectrum.
When is heme red?
When it is oxygenated, absorbing blue-green wavelenghts.
What are the two histidine bonds called?
E7(Distal) and F8(Proximal)
Why is the co-ordination histidine bond important?
Stabilisation as histidine residue has no net charge.
What does histidine-iron binding cause?
The iron to be 0.3 Angrstrom out of plane., not directly bound to E7 however.
What are the two functions of E7 distal bond?
Reduces affinity of CO binding to site
Promotes steric blocking of formation of haem-O2-haem bonding.
How does E7 distal bond prevent CO binding?
CO forms co-ordination bonds with iron without hydrogen bond by E7 whilst O2 hydrogen bonds.
How does steric hinderance prevent CO binding?
Negative charge of E7 repels negative charge O2 tilting its orientation relative to the iron allowing hydrogen bond to form.
Le Chateliers Principle
This states that if a dynamic equilibrium is disturbed by changing conditions, the position of equilibrium shifts to counteract the change to resore equilibrium.
What are exampels of Le Chateliers Principle?
If heat was applied, the temperature would increase concetration of the side of which consumes the heat.
Increasing pressure would mean shift to equilibrim to whichever side reduces pressure.
What is the Le Chateliers equation?
A + B <–> AB
Why is Le Chaterliers Princiole important?
Molecule absorb oxygen where it is abundant(lungs) and move it to less abundant place, releasing O2.
What is the basis of oxygen transport?
Lungs have much higher partial pressure of oxygen than the tissues.
