21 - Myoglobin, Haemoglobin And Allostery

Question 1

How oxygen carried

Answer 1

• 5% in blood - low solubility
  …

Question 2

When would organisms require a respiratory and circulatory system

Question 3

What curve myoglobin has

Answer 3

Hyperbolic

Question 4

What curve haemoglobin has

Answer 4

Sigmoidal (s shaped curve)

Question 5

Myoglobin info

Answer 5

• hyperbolic curve
• has simple binding to ligand - oxygen molecule
• well adapted to ensure good supply of oxygen to muscles
• Mb + O2 —> MbO2
• usually only found in the muscles

Question 6

Can we use simple binding of oxygen in myoglobin for transport of oxygen

Answer 6

No, as according to the hyperbolic curve, if lungs were more saturated than the tissue with O2, then the myoglobin wouldn’t release oxygen to the tissues

Question 7

Haemoglobin info

Answer 7

• sigmoidal binding curve
• does not use simple binding of O2
• higher affinity of O2 in myoglobin ensures efficient transfer of oxygen from haemoglobin
• steeper curve than myoglobin (simple binding)

Question 8

Structure of myoglobin

Answer 8

• Contains 8 alpha-helices
• contains heme group - prosthetic group with Fe atom in centre of protoporphyrin ring

Question 9

Structure of heme group

Answer 9

Protoporphyrin ring with central Fe prosthetic group

Question 10

How heme binds oxygen with high affinity

Question 11

Structure of haemoglobin

Answer 11

• tetramer - 2 alpha helices and 2 beta-pleated sheets - so has 4 binding sites for O2
• has quaternary structure
• alpha and beta - make up same secondary and tertiary structure as myoglobin
• primary structure in alpha and beta chains are different from myoglobin
• so we conclude that 3D structures are more highly conserved than a sequence

Question 12

Cooperativity

Answer 12

• sigmoidal curve is a sign of cooperativity
• the 4 binding sites of haemoglobin cooperate to bind and release oxygen (not independent)
• each subunit has two conformations:
• T state - tense - low affinity for oxygen
• R state - relaxed - high affinity for oxygen
• the T and R stages can interchange

Question 13

Two models of cooperativity

Answer 13

Concerted model
Sequential model

Question 14

Concerted model of cooperativity

Question 15

Sequential model of cooperativity

Question 16

Structure change of heme group when oxygen is bound

Answer 16

• Fe central in heme and helix come closer to heme
• changes structure throughout heme, including change to interfaces with other subunits