Myoglobin (a single-subunit protein) has a hyperbolic oxygen-binding curve, while hemoglobin (an α₂β₂ tetramer) has a sigmoidal curve. What property of hemoglobin's quaternary structure produces the sigmoidal shape?
AHemoglobin has four heme groups rather than one, so it can bind four oxygen molecules simultaneously
BCooperative conformational changes between subunits: oxygen binding to one subunit shifts the whole tetramer toward the R state, increasing affinity in the remaining subunits
CThe α and β subunits have different amino acid sequences, causing them to bind oxygen at different affinities
DHemoglobin's larger size means it has more surface area for oxygen to interact with
The sigmoidal curve arises from cooperativity — a property that requires multiple interacting subunits. When O₂ binds the first hemoglobin subunit, it triggers a conformational change that propagates across subunit interfaces, converting the tetramer from the low-affinity T (tense) state toward the high-affinity R (relaxed) state. This makes subsequent O₂ binding progressively easier. Myoglobin, lacking this inter-subunit communication, binds O₂ with constant affinity regardless of occupancy — hence a hyperbolic curve. Simply having four heme groups (option A) would not create cooperativity without the coupled conformational mechanism.
Question 2 Multiple Choice
In people adapted to high altitude, 2,3-BPG levels in red blood cells increase. 2,3-BPG stabilizes the T (tense, low-affinity) state of hemoglobin by binding in the central cavity. What physiological effect does elevated 2,3-BPG produce?
AIncreased O₂ affinity, helping hemoglobin load more oxygen in the low-O₂ atmosphere
BDecreased O₂ affinity, making hemoglobin release oxygen more readily to tissues
CNo effect on O₂ binding, because 2,3-BPG does not contact the heme groups directly
DConversion of the hemoglobin tetramer into two independent dimers with higher affinity
By stabilizing the T state, 2,3-BPG shifts hemoglobin's oxygen-binding curve rightward — lower affinity means hemoglobin releases O₂ more readily at the lower partial pressures found in peripheral tissues. At high altitude, where atmospheric O₂ is reduced, the primary problem is not loading in the lungs (that is limited by pO₂) but ensuring adequate delivery to tissues. Elevated 2,3-BPG ensures that what hemoglobin does load gets released where it is needed. This is allosteric regulation via quaternary structure — a molecule binding at a site distant from the heme groups modulates function through the subunit interface.
Question 3 True / False
The subunits of multi-subunit proteins like hemoglobin are primarily held together by disulfide bonds between cysteine residues across the subunit interface.
TTrue
FFalse
Answer: False
False. Most multi-subunit proteins are stabilized by non-covalent interactions at the subunit interface: hydrophobic contacts (hydrophobic patches on one subunit pack against complementary patches on another), hydrogen bonds, and ionic interactions (salt bridges). Disulfide bonds between chains do occur in some proteins (notably antibodies), but they are the exception rather than the rule. The non-covalent nature of quaternary interactions is actually important — it allows the conformational changes that transmit cooperative signals between subunits.
Question 4 True / False
A protein with quaternary structure can achieve cooperative binding and allosteric regulation — properties that are impossible for a single-subunit protein of the same overall size.
TTrue
FFalse
Answer: True
True. Cooperativity and allosteric regulation through quaternary structure require the transmission of conformational signals across subunit interfaces. A single polypeptide chain, regardless of its size, has no such interfaces — it can have allosteric sites, but cannot exhibit the same kind of inter-subunit communication that gives rise to sigmoidal binding curves and the physiological benefits of cooperative O₂ transport. Hemoglobin's functional properties (steep sigmoidal curve, 2,3-BPG sensitivity, Bohr effect) all emerge from its tetrameric organization.
Question 5 Short Answer
Explain why hemoglobin's sigmoidal oxygen-binding curve is physiologically advantageous compared to the hyperbolic curve of myoglobin, and what structural feature of hemoglobin produces this shape.
Think about your answer, then reveal below.
Model answer: The sigmoidal curve has a steep middle section: hemoglobin has low affinity at low pO₂ (releasing O₂ efficiently to oxygen-depleted tissues) and high affinity at high pO₂ (loading O₂ efficiently in the lungs where pO₂ is high). A hyperbolic curve, like myoglobin's, saturates rapidly and releases oxygen less readily over the physiological range. The sigmoidal shape arises from cooperativity: binding of the first O₂ molecule triggers a conformational shift from the T (low-affinity) to R (high-affinity) state that propagates across the subunit interfaces, making each subsequent O₂ easier to bind. This inter-subunit communication requires the tetrameric quaternary structure.
The steep sigmoidal curve essentially acts as an on/off switch across the physiological pO₂ range (roughly 20–100 mmHg), loading in the lungs and unloading in tissues far more efficiently than a hyperbolic binder would. This is precisely what cooperativity via quaternary structure enables — a single-subunit protein has no mechanism to 'remember' previous binding events and adjust its affinity accordingly.