Explain the similarities and differences between myoglobin and adult haemoglobin (60% of marks) and their physiologic relevance (40% of marks).
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.
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 |
|
|
| Synthesis | Locally, in muscle | Bone marrow |
| Degradation | Locally, in muscle | Reticuloendothelial system |
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