14B18: Exam Report

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

23% of candidates passed this question.

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.

F8i / 14B18: Explain the similarities & differences between myoglobin & adult haemoglobin & their physiological relevance



O2 store for active muscle


O2 carriage from lung → tissue

CO2 carriage

Acid-base buffer



Skeletal & cardiac m

Only in blood in pathological states (muscle injury)

Large amounts of myoglobin cause renal failure (rhabdo → ARF)



200 – 300 million Hb molecules per RBC




Both globular proteins with heme moiety that binds O­2


17,000 Da

Single globular chain ∴1 heme moiety

∴binds 1 x O2

NO +ve cooperativity


65,000 Da

4 polypeptide chains ∴4 heme moieties

∴binds up to 4 x O­2 WITH +ve cooperativity

O2 Carriage


Hyperbolic ODC

Higher affinity for O2 at low PO2

Operating range 1 – 5mmHg

P50 2.75mmHg

Well match to intracellular PO2 1 – 5mmHg

Myoglobin is highly concentrated in muscles which contract +++

Blood supply during contraction is often cut off ∴myoglobin provides ongoing O2

High O2 affinity can load O2 from Hb

NOT pH sensitive



Sigmoid ODC

Low O2 affinity

Operating range 20 – 100mmHg

P50 26.6mmHg

pH sensitive

∴BOHR EFFECT → offloads O2 at ↑H+/↑CO2