Question

Myoglobin is a heme-containing protein composed of a single polypeptide chain. It is capable of binding oxygen to its heme group. Hemoglobin is a heme-containing protein composed of four polypeptide chains, each chain being capable of binding oxygen to its own heme group.
Structural analysis has revealed the structure of the myoglobin protein to be nearly identical to the structure of the individual polypeptide chains of hemoglobin. Scientists have compared the primary structure of both myoglobin and hemoglobin proteins from humans to those of other species as a way of tracing evolutionary relatedness. Examine the data table below.
Protein Sequence Difference Between Human and Other Species

	Myoglobin	Hemoglobin
Whales	25	17
Lampreys	64	125
Sharks	88	-
Horseshoe Crabs	-	-

Based on the data table and the information provided regarding the structures of the two molecules, which of the following statements is correct?
(a) The myoglobin gene arose from the partial duplication of the hemoglobin gene.
(b) The hemoglobin gene arose from the repeated duplication of the myoglobin gene.
(c) Myoglobin and hemoglobin are examples of convergent evolution.
(d) The nucleotide sequences for myoglobin and hemoglobin are nearly identical.

Answer 1

Hint: Both myoglobin and hemoglobin are heme compounds which means they are iron-containing compounds. This iron combines with the ligand oxygen to form a coordinate bond after which they transport that oxygen.

Complete Answer:
Hemoglobin is found all over the body but myoglobin is present only in the muscle tissues. The protein structure of hemoglobin has 4 polypeptide chains which include 2 alpha and 2 beta chains. Thus hemoglobin is known as a tetramer. But myoglobin is a monomer as it only has a single peptide chain in it. So, we can say that myoglobin is similar to hemoglobin except that it has only 1 polypeptide chain instead of 4. Thus, myoglobin was formed by the partial duplication of the hemoglobin gene, if it formed by the complete duplication then it too would have 4 polypeptide chains.

Additional information: Let us know more about hemoglobin and myoglobin.
Hemoglobin: It consists of a heme group and a globin group. The heme group has iron as the central metal which is attached to four pyrrole rings. The globin part consists of 4 proteins and has porphyrin in it. It is responsible for giving the red color to the blood.
Myoglobin: It consists of 8 right-handed alpha-helices. It also contains the heme group like hemoglobin. It exists in the secondary protein structure as the protein has a linear chain of amino acids.
So, the correct option is ‘(a) The myoglobin gene arose from the partial duplication of the hemoglobin gene.’

Note:
- Oxygen binds to the iron of hemoglobin in the +2 oxidation state.
- If due to some reason the oxidation state of iron becomes +3, it will no longer be able to bind to oxygen and transport it.
- Such a state of hemoglobin is referred to as the hemin state when the iron is in the +3 oxidation state.