Explain the similarities and differences between myoglobin and adult haemoglobin (60% of marks) and their physiologic relevance (40% of marks). 

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College Answer

Both are globular proteins that bind and deliver O2. Due to myoglobin containing a single globin 
chain its dissociation curve is hyperbolic in shape. Haemoglobin contains 4 globin chains and is a 
quaternary structure which exhibits cooperatively resulting in a sigmoid shaped dissociation 
curve. The differing dissociation curves mean that when the PO2 is high, as in the lungs, both 
myoglobin and haemoglobin are saturated with oxygen. However, at the lower levels of PO2 in 
the tissues, haemoglobin cannot bind oxygen as well as myoglobin. Myoglobin can bind the O2
released by haemoglobin, which it stores to meet the demands of muscle contraction. This 
means haemoglobin (with its higher p50) can offload O2 to myoglobin. Comments on the 
synthesis and degradation gained additional marks but were a common omission.
The physiological relevance was poorly explained. The similarities and differences mean that 
haemoglobin is the primary means of O2 transport from the lungs to the tissues and myoglobin is 
the primary O2 carrying pigment of skeletal muscle and acts as local O2 reserve for times of 
intense muscle activity.

Discussion

This answer lends itself well to a tabulated format:

Domain Myoglobin Haemoglobin
Chemistry Globin haemoprotein Globin haemoprotein
Quarternary structure Monomer Heterotetramer
Oxygen carriage capacity Single molecule Four molecules
Contribution to body oxygen stores
(on 21% FiO2)
200ml 850ml
Location Skeletal muscle and myocardium Erythrocytes
Oxygen dissociation curve Hyperbolic Sigmoid, due to positive cooperativity
Oxygen affinity Very high: p50 = 2.7 mmHg p50 = 26.7 mmHg
Role  Oxygen storage in skeletal muscle, 
for release during periods of anoxia
Oxygen transport, for release in the tissues; as well as buffering 
Physiological interaction
  • In the tissues, PO2 is low
  • Haemoglobin has low affinity for oxygen at this PO2, and it releases bound oxygen into the tissue fluids
  • Myoglobin has a high oxygen affinity at this PO2, and it collects the released oxygen
  • In this fashion, oxygen is transferred between haemoglobin and myoglobin
Synthesis Locally, in muscle Bone marrow
Degradation Locally, in muscle Reticuloendothelial system

References

References

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