Hemoglobin (Hb) is a tetrameric protein in red blood cells crucial for oxygen transport, unlike myoglobin’s monomeric oxygen storage role in muscles. Two statements about Hb are incorrect based on its structure and cooperative binding properties.​

Correct Answer

(A) II

Only statement II is incorrect, as detailed below.​

Statement I Evaluation

Hb demonstrates higher oxygen carrying capacity compared to myoglobin. This holds true because Hb has four heme groups, each binding one O₂ molecule, for a total capacity of four, while myoglobin binds only one. Hb’s sigmoidal oxygen dissociation curve enables efficient loading in lungs and unloading in tissues, outperforming myoglobin’s hyperbolic curve and higher O₂ affinity.​

Statement II Evaluation

There is covalent bonding between the four subunits of Hb. This is incorrect. Hb subunits (two α and two β chains) associate via non-covalent interactions like hydrogen bonds, salt bridges, and hydrophobic forces, allowing conformational shifts between tense (T, deoxy) and relaxed (R, oxy) states. No covalent bonds link the subunits.​

Statement III Evaluation

During deoxygenation the loss of the first oxygen molecule from oxygenated Hb promotes the dissociation of oxygen from the other subunits. This is correct. Hb exhibits negative cooperativity in oxygen release: initial O₂ dissociation reduces affinity in remaining subunits, shifting from R to T state and facilitating further unloading in low-O₂ tissues.​

Why Hb Excels in Oxygen Delivery

Hb’s cooperative binding contrasts myoglobin’s tight grip, ensuring 60-70% O₂ release in tissues versus myoglobin’s minimal unloading. This suits Hb for transport and myoglobin for storage.​