Hemoglobin 2

Question 1

Describe the globin protein structure

Answer 1

• The globin protein monomer is mostly composed of a-helix
• A few bends or turns are present
• Globin is an unusual structure protein in that it is void of B-sheet

Question 2

How is iron ion (ferrous 2+) is protected ?

Answer 2

Iron ion (ferrous 2+) is protected by porphyrin ring and buried deep in the globin protein

Question 3

Ferrous iron,2+ is protected by porphoryn ring, why is this important?

Answer 3

• It sets up a system where the oxygen molecule can enter/leave the hemoglobin in a defined path which is good since it reactive( might be released as superoxide without said system)
• Superoxide- O2+e-( superoxide is a free radical)

Question 4

What is myoglobin designed to do?

Answer 4

Store oxygen in the cell, and deliver to the mitochondria when needed

Question 5

What is the shape of the myoglobin dissociation curve?

Answer 5

The shape of the curve is hyperbolic

Question 6

What is the importance of conservation of key amino acids?

Answer 6

This ensures correct shape, and function

Question 7

Give examples of “Conservations of important amino acids ensures correct shape, and similar function”

Answer 7

Sperm whale myoglobin and human B-globin have a very similar structure despite primary sequence is remarkably different

Question 8

Give the structure of human hemoglobin

Answer 8

• is a tetramer of 4 polypeptide chains
• Hemoglobin A, is a pair of identical aB divers (a2B2 heterodimers)
• Heme groups are widely spaced

Question 9

What type of interactions are hemoglobin capable of?

Answer 9

Extensive interactions between the subunits
-Hydrophobic

  - Ionic
  - hydrogen bonds

Question 10

How much oxygen can hemoglobin hold?

Answer 10

4 globin monomers so each carries a molecule of oxuygen

So 4 O2

Question 11

Why is hemoglobin a heterotetramer?

Answer 11

it is a a2B2 tetramer, meaning it has a a1, B1, a2 and B2 polypeptides

Question 12

Describe the associations between the aB peptides

Answer 12

The aB peptides that form a dimer held together very tightly
-they don’t change, they stay tightly linked together

The two dimers associate with each other, but may shift slightly
-this structural change explains how O2 affinity may be regulated

Question 13

How is affinity for O2 regulated?

Answer 13

Between both aB dimer 1 and aB dimer 2 there are hydrogen-bonds, ion-dipole bonds and ionic interactions -in the form of deoxyhemoglobin it is in a tautomeric state or “T”

As O2 binds to hemoglobin,/oxyhemoglobin forms, which is a much more relaxed form, it breaks the hydrogen and ionic salt bridges

Question 14

Describe the interactions between each a and B polypeptide in hemoglobin

Answer 14

Many hydrophobic interactions exist between aB peptides to form a strong dimer, very strong LH attached to each other

Of course, some hydrogen bonds and ionic salt bridges also

Question 15

What amino acids can stabilize the hydrophobic interactions of the a-globin B chains to form the aB dimers?

Answer 15

The branched chain amino acids- valine, leucine and isoleucine

Amino acids with aromatic side chains-phenylalanine, tyrosine and tryptophan

Aldo methionine, proline, glycine and alanine

Question 16

Why is histidine so important to hemoglobin?

Answer 16

Histidine is the only common amino acid that has an ionizable side chain whose pKa is close to physiological pH and can be modulated

Allows Hb to do two very important things:

1. Help to buffer the pH of the blood by absorbing protons.
2. Regulate O2 affinity in response to pH

Question 17

How is the pKa of histidine adapted for human buffering?

Answer 17

The pKa of histidine is around 6 but within the hemoglobin structure, the pKa is pushed to around 7. This is a more physiologically useful buffer

Question 18

Describe the difference in shape in the porphyrin ring in an oxygenated vs. deoxygenated form

Answer 18

Deoxygenated- when Fe or heme is deoxygenated, porphyrin ring is puckered, heme is domed(nonplanar)

Oxygenated - When Fe or heme is oxygenated, porphyrin ring is planar, heme is planar and Ferrous ion comes into the plane

Question 19

How does O2 binding change the shape of hemoglobin?

Answer 19

• Upon O2 binding, the heme ring assumes the more planar shape which tugs on the proximal hemoglobin which then tugs on the alpha helix which then changes the shape of the globin monomer
• in hemoglobin, this “tugging” can be communicated to the other subunits to change the shape of the tetrameric protein

Question 20

Does binding of oxygen affect the shape the same way as in hemoglobin?

Answer 20

No, binding of oxygen doesn’t have this effect as myoglobin has only one polypeptide, so it has nothing to communicate to

Question 21

Why is the oxygen -dissociation chrve a sigmoidal curve?

Answer 21

Cooperative ligand binding creates a sigmoidal curve because with every binding of an oxygen molecule, the affinity for binding of the next globin increases

Question 22

What is the heme prosthetic group?

Answer 22

Porphyrin ring with a ferrous iron in the center

Question 23

What is a P50?

Answer 23

The pressure at which 50% of O2 binding sites are saturated with oxygen

Question 24

What is the p50 of myoglobin?

Answer 24

The p50 of myoglobin is about 1 mmHg, this does not change

Question 25

What is the p50 of Hb at pH 7.4?

Answer 25

At pH 7.4, p50 of Hb is about 26 mmHg, this changes with pH

Question 26

Differentiate 2,3 BPG level between a normal person and a person adapted to high altitudes. Why the trend?

Answer 26

Normal= 2,3 BPG-5mmol

Adapted to altitudes= 2,3 BPG-8 mol

Decrease in affinity for release of oxygen