Question

A hyperbolic binding curve differs from a sigmoidal binding curve in that the hyperbolic curve
a) binds up to four molecules of oxygen
b) binds more oxygen after the initial proteins first bind oxygen
c) has a single equilibrium constant for oxygen binding
d) shows cooperativity
e) all of the above

the main property of myoglobin and hemoglobin that makes them an efficient system for oxygen delivery from lungs to muscles is
a) movement of protein shapes
b) cooperativity
c) different binding affinities for oxygen
d) hydrophobicity
e) all of the above

which statement is correct concerning the T (deoxy) and R (oxy) states of tetrameric hemoglobin?
a) in the T to R transition, the central cavity of the tetramer decreases in size, preventing the binding of the allosteric modulator, 2,3-bisphosphoglycerate
b) in the bohr effect, lowering the pH preferentially stabilizes the R form and enhances oxygen binding
c) the R form involves interchain salt links that are broken in the transition to the T form
d) in the T to R transition, the positions of several helices change, but the position of the iron atom in the heme remains fixed.

Answer 1

Ans. #1. Correct option. A. The hyperbolic curve (Michaelis-Menten plot) is a simple kinetics plot of Vo vs [S]. One enzyme binds to one substrate at a time. This kinetics gives a single equilibrium constant for O₂ binding.

In sigmoid model, the Hb molecule binds the next O₂ more rapidly than the previous O₂ binding, bind 4 O₂ molecules and exhibit cooperativity.

#2. Correct option. C. different binding affinities for O₂. Due to tetrameric structure, Hb shown cooperativity and has higher O₂ affinity than Myb. Difference in O₂ affinity enables Hb and Myb transporting O₂ to different specific tissues.

# Option A. Incorrect. Conformational change occurs during O₂ binding to Hb and Myb. However, “change in shape” alone does not specify O₂-transport ability of a protein. Almost all enzymes undergo conformational change but none of them transports oxygen.

So, O₂-binding affinity is the most appropriate option.

Option B. Incorrect. Myb is a monomeric protein and does not exhibit cooperativity.

Option D. Incorrect. The hydrophobicity of a protein does not specifies its ability to transport O₂.

#3. Correct option. A. While T- to R-transition, the R-state Hb has lower affinity for 2,3-BPG.

# Option B. Incorrect. Lowering pH i.e. increasing [H⁺] or making the solution more acidic favors T-state and reduces O₂-binding affinity.

Option C. Incorrect. The R- to T-transition involves numerous non-covalent modifications leading to the 3D conformational change. Inter chain salt-bridges are not specific for the transitions.   

Option D. Incorrect. The transition from T (deoxyHb) to R (OxyHb) must also be accompanied by exposure of the Fe-atom of heme towards the surface so that the next O₂ molecule is bound much rapidly than the previous one.