Question

When using the Sequencing and Structure Alignment tool, the degree of the structural similarity is shown by the superimposition of the backbones. Accordingly, which of the following statement best describes the comparison of deoxyhemoglobin and oxyhemoglobin?

	A.	Only slight differences in the backbone traces occur throughout the aligned proteins.
	B.	The only differences in the backbone traces occur near the oxygen-binding sites.
	C.	The backbone traces of the proteins are drastically different.
	D.	The backbone traces of the proteins are identical.

Answer 1

Red blood cells of the vertebrates contains haemoglobin which is an iron-containing metalloprotein responsible for transporting oxygen. Red colour of the blood is due to presence of haemoglobin present in red blood cells. The haemoglobin has a quaternary structure, which consists of four subunits, the subunits have a heme group which is surrounded by a coiled protein called goblin. The non-protein heme group contains an iron ion held in a heterocyclic porphyrin ring. The iron center of the heme group has six coordination sites.

The iron center coordinates with the four nitrogen atoms of the porphyrin ring (in the plane of the molecule). Further, the iron center is covalently bound to the histidine residue of the globular protein (below the plane of molecule) and at the sixth coordination site, the iron ion reversibly binds to an oxygen molecule by a coordinate covalent bond (above the plane of molecule).

During respiration, when the oxygen molecule is bound to the sixth coordination site of the heme group, oxyhemoglobin is formed. Whereas, in a deoxyhemoglobin molecule, the sixth coordination site of the heme group is free and not bound to an oxygen molecule.

This binding affects the electronic structure and the magnetic properties of the molecule.

The backbone representation is a simplified 3D representation that allows the polymeric chain structure of a protein molecule to be seen clearly.

The backbone trace is a line that connects all the alpha-carbon atoms in a protein molecule, thereby simplifying the complex structure of a protein. The backbone trace line does not overlap or follow the bonds in the structure of the protein.

Therefore, when the backbone traces of the oxygenated and deoxygenated haemoglobin molecule were observed, the only difference was near the oxygen-binding site.

Therefore, the correct option is: B. The only differences in the backbone traces occur near the oxygen-binding sites.