Proteins - Myoglobin

Question 1

What is the structure of Myoglobin?

Answer 1

Weighs 17 kDA, composed of 155 amino acids, compact globuler structure, 75% alpha-helicies, being eight forming a helical bundle.

Question 2

What is the general tertiary structure of Myoglobin?

Answer 2

Central heme group containing Fe2+ and a porphyrin ring connected to four nitrogen and two histidne residues surrounded by the alpa helicies.

Question 3

Prophyrin Ring

Answer 3

This is a ring-like molecule of 4 pyrroles being smaller rings composed of 4 carbon and 1 hydrogen.

Question 4

Heme Group

Answer 4

This is a prosthetic group involved in O2 transport and storage.

Question 5

Proline

Answer 5

This is a major amino acid of the molecular formula of C5H9NO2

Question 6

What terminates the helical segments?

Answer 6

Proline residues

Question 7

How is proline unique?

Answer 7

Its side group is cyclical forming a pyrrolidine ring being rigid and conformationally restircited.

Question 8

Why is the restricted conformation of proline important?

Answer 8

Breaks the arranged H-bonding structure of the alpha helicies allowing loop formation.

Question 9

What structure does proline allow for?

Answer 9

Loops between example helicies B and C due to pro45, creating a flexibile region

Question 10

What is heme group surrounded by?

Answer 10

Non-polar residues except for two histidines.

Question 11

What histidines does heme group bind to?

Answer 11

His93 in Helix F and His64 of the flexible loop region

Question 12

Why is enclosue of Heme group of Fe important?

Answer 12

O2 has only limited solubility in water.
O2 diffuses slowly in tissues
O2 is irreversible when bound to free heme

Question 13

Why does irreversibility of O2 bidning occur?

Answer 13

Oxidation state of Fe having ferrous(II) or ferric(III) forms

Question 14

What is the molecular structure of ferrous iron?

Answer 14

Unpaired electrons in the d-orbital interacting with O2

Question 15

Why is binding irreversible for ferric form?

Answer 15

Does not have electron pair, no covalent bond formation meaning Fe3-O2 bond is weak and breaks and O2 diffuses away.

Question 16

How does myoglobin facilitate conversion of ferric iron to ferrous?

Answer 16

Reducing agents like ascorbate and dithionite with electron transfer

Question 17

How is binding of iron to histidine residues important?

Answer 17

Imidazole group forming co-ordination bonds.

Question 18

How does the porphyrin ring act as a chromophore?

Answer 18

Binding of O2 to Fe changes electronc structure due to co-oridnate bonds shifting absorption spectrum.

Question 19

When is heme red?

Answer 19

When it is oxygenated, absorbing blue-green wavelenghts.

Question 20

What are the two histidine bonds called?

Answer 20

E7(Distal) and F8(Proximal)

Question 21

Why is the co-ordination histidine bond important?

Answer 21

Stabilisation as histidine residue has no net charge.

Question 22

What does histidine-iron binding cause?

Answer 22

The iron to be 0.3 Angrstrom out of plane., not directly bound to E7 however.

Question 23

What are the two functions of E7 distal bond?

Answer 23

Reduces affinity of CO binding to site
Promotes steric blocking of formation of haem-O2-haem bonding.

Question 24

How does E7 distal bond prevent CO binding?

Answer 24

CO forms co-ordination bonds with iron without hydrogen bond by E7 whilst O2 hydrogen bonds.

Question 25

How does steric hinderance prevent CO binding?

Answer 25

Negative charge of E7 repels negative charge O2 tilting its orientation relative to the iron allowing hydrogen bond to form.

Question 26

Le Chateliers Principle

Answer 26

This states that if a dynamic equilibrium is disturbed by changing conditions, the position of equilibrium shifts to counteract the change to resore equilibrium.

Question 27

What are exampels of Le Chateliers Principle?

Answer 27

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.

Question 28

What is the Le Chateliers equation?

Answer 28

A + B <–> AB

Question 29

Why is Le Chaterliers Princiole important?

Answer 29

Molecule absorb oxygen where it is abundant(lungs) and move it to less abundant place, releasing O2.

Question 30

What is the basis of oxygen transport?

Answer 30

Lungs have much higher partial pressure of oxygen than the tissues.