2011B12 Outline the similarities and differences between myoglobin and adult haemoglobin,
explaining the physiological relevance of the differences.






·  Erythrocytes

·  Striated muscle
(skeletal, cardiac)


·  4 x haem (protoporphyrin + Fe2+)

·  4 x globin (polypeptide chain)

·  HbA1 97% α2β2

·  HbA2 3% α2δ2

·  Single haem

·  Single globin


·  O2 carriage

o Complex with iron (haem)

o ↑CaO2 from ~2mL/100mL to ~20mL/100mL arterial blood

·  CO2 carriage

o CO2 + NH2 <-> NHCOO- + H+ (carbamino)

o Also buffers H+ from carbonic acid

·  Buffer in “ECF”

o KHb + H+ <-> HHb + K+

o Imidazole groups of histidine residues

·  Unclear

·  At rest: ? ↓O2 toxicity

·  In peak exercise: ?↑gradient

O2 affinity

·  Moderate, p50 26mmHg

·  Very high, p50 2.8mmHg

Dissociation curve shape

·  Sigmoid (see below)

o Multiple subunits with co-operative binding

o Also law of mass action

·  Hyperbolic (see below)

o Single subunit

o No co-operative binding

o Law of mass action only

Co-operative binding

·  Binding of one haem to O2 increases affinity of the next haem for O2

·  Due to allosteric interactions

·  Tense (T) closed -> relaxed (R) open state

·  Single subunits, no co-operative binding

Bohr effect

·  R shift due to ↑T, ↑PaCO2, ↑[H+], ↑[2,3-DPG]

·  Due to allosteric interactions as above.

·  N/A

Haldane effect

·  Capacity to bind or buffer CO2 and H+ is greater in HHb than HbO2

·  70% due to 3.5x ↑ability to form carbaminoHb

·  30% due to ↑pKa imidazoles 6.6 to 8.2

·  N/A


·  Yes (released upon intravascular haemolysis)

·  Yes (released upon rhabdomyolysis)





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