Canonical Question
Blood constituents
Master answer
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.
| Haemoglobin | Myoglobin |
|---|---|
| Function | |
| Oxygen carriage → lungs to tissues, CO₂ carriage, Acid-base buffer | Oxygen Store → for exercising muscle |
| Binds CO2, CO, NO, O2, H+ | Binds O2, tightly and firmly |
| Location | |
| in large concentrations (≈15g/L) in red blood cells that circulate throughout the blood stream. | haem containing pigment protein found in skeletal and cardiac muscle. |
| Haemoglobin is only found in blood stream following intravascular haemolysis | Myoglobin is only found in the blood stream when it is released following muscle injury → abnormal finding (↑ in AMI or rhabdomyolysis) |
| Structure | |
| structurally related. Both are globular proteins and both contain a haem moiety which binds O₂. | |
| Haem is an protoporphryin ring derivative with a central Fe2+ molecule that binds O₂ | |
| MW ≈ 65000 daltons | MW ≈ 17,700 daltons |
| Hb contains 65-70 % of total body iron | Myoglobin contains 4-5 % of total body iron |
| Globular molecule made up of four subunits, each containing a haem moiety conjugated to a polypeptide. Polypeptides collectively = globin → two pairs 2α + 2β → 4 haem moieties (Tetramer) | Myoglobin is a single-chain globular protein containing a single haem moiety (Monomer) |
| Can bind a total of 4 O2 and also exhibits cooperative affinity (each subsequent O₂ binding takes less energy → sigmoid shaped OHDC | Unlike haemoglobin it does not exhibit cooperative affinity when binding oxygen since it does not exist in a tetramer formation → its dissociation curve is a rectangular hyperbola rather than a sigmoid curve. |
| Carriage of O2 (See figure below) | |
| Sigmoid shaped dissociation curve | Rectangular hyperbole dissociation curve |
| P50 26.6mmHg, operating range 100-20mmHg | P50 2.75mmHg, operating range 5-1mmHg |
Hb has to carry oxygen from the lungs (PaO2 100) down the ‘oxygen cascade’ to the tissues. P50 suits this operating range and enables appropriate loading and unloading of O2. | Myoglobin needs to have a P50 less than Hb so it can take up O₂ from it. Myoglobin needs to be able to load and unload O₂ in the range of pO2 values that occur within the cell → it’s p50 of 2.75mmHg is well matched to the intracellular operating range of pO2 (1↔5mmHg) The myoglobin content is greatest in muscles specialized for sustained contraction → the muscle blood supply is often compressed during such contractions and myoglobin may provide O₂ when blood flow is cut off. |
| Bohr and Haldane Effect | Nil |
| Synthesis | |
| Haeme is synthesised in mitochondria and cytosol of immature RBCs, Globin is synthesized by ribosomes in cytosol. Production continues till RBCs lose their RNA after entering vasculature | Expressed solely in cardiac myocytes and oxidative skeletal muscle fibres |
| Degradation | |
| RBCs at the end of their life cycle get phagocytosed by macrophages in liver or spleen, or get hemolysed in circulation. Haemoglobin is then broken up – Haeme gets degraded into bilirubin, and the iron gets recycled. | It is released from muscle tissue by cell destruction and alteratins in permeability of muscle cell membrane. It is filtered by glomerulus and rapidly excreted by kidney. |
| Nephrotoxicity | |
| Released upon intravascular haemolysis | Released upon rhabdomyolysis Myoglobin is toxic to renal tubular epithelium and large amounts of myoglobin (eg in rhabdomyolysis) can cause renal failure. |
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2014B Q18 | Explain the similarities and differences between myoglobin and adult haemoglobin (60% of marks) and their physiologic relevance (40% of marks). | historical_member | — |