Question

1. Describe the quaternary structure of hemoglobin. How does its quaternary structure enable it to fulfill its biological function? How would its function be affected if a mutation caused its subunits to dissociate?

2. Biomolecules are often symmetrical. What symmetry operations (that is, rotation, inversion, translation, etc.) are associated with the following:

a) dyad symmetry
b) n-fold symmetry c) mirror symmetry d) dihedral symmetry e) helical symmetry

3. a) Describe the A, B, and Z conformations of DNA
b) Besides Z-DNA, name and describe two other conformations that are favored by negative supercoiling of closed-circular DNA.

4. Starting from the definitions for enthalpy (∆H = q) and entropy (∆S = q/T), derive the expression for Gibbs free energy.

Answer 1

The quaternary structure of haemoglobin consists of four peptide subunits. Two of the subunits are identical and are called the alpha subunits. The remaining two subunits, called the beta subunits, are identical to each other but different from the alpha subunits. The subunits are interlocked in a compact globular structure held together by ionic and hydrogen bonds between the amino acid side chains of the polypeptide subunits. A heme prosthetic group containing an iron(Il) ion capable of carrying one oxygen molecule is associated with each of the four protein subunits of Hb. Globin is the protein of the Hb molecule. The Hb complex with oxygen is oxyhemoglobin; and that without oxygen is deoxyhemoglobin. Oxyhemoglobin, the major Hb of arterial blood, is bright red; deoxyhemoglobin of venous blood is purplish.

The advantage of having 4 proteins together is that once one of them is bound to oxygen, it makes it easier for the other forms to bind, creating Oxyhaemoglobin. The T-state is the deoxy form of hemoglobin (meaning that it lacks an oxygen species) and is also known as "deoxyhemoglobin”. The R-state is the fully oxygenated form: "oxyhemoglobin."

If there is a mutation in hemoglobin that separates its 4 polypeptides then an individual polypeptide of hemoglobin will act as the myoglobin molecules which does not show positive cooperativity i.e binding of a ligand molecule here oxygen increases the receptor's apparent affinity. Thus oxygen carrying capacity of hemoglobin would be decreased altogether.