Question

Which of the following statements is not correct? (a) in oxyhaemoglobin, \(\mathrm{Fe}^{2+}\) is paramagnetic. (b) during respiration, the size of \(\mathrm{Fe}^{2+}\) increases when it changes from diamagnetic to paramagnetic state. (c) four heme groups are present in haemoglobin. (d) heme is the prosthetic group and it is non protein part of haemoglobin.

Short answer

Statement (a) is not correct.

Step-by-step solution

Analyzing statement (a)

Statement (a) claims that in oxyhaemoglobin, \( \mathrm{Fe}^{2+} \) is paramagnetic. Oxyhaemoglobin is formed when oxygen binds to haemoglobin. In this state, \( \mathrm{Fe}^{2+} \) loses its unpaired electrons, making it diamagnetic, not paramagnetic. Thus, statement (a) is not correct.

Evaluating statement (b)

Statement (b) states that during respiration, the size of \( \mathrm{Fe}^{2+} \) increases when it changes from diamagnetic to paramagnetic state. When haemoglobin releases oxygen, \( \mathrm{Fe}^{2+} \) changes from a diamagnetic to a paramagnetic state, and the ionic size of \( \mathrm{Fe}^{2+} \) indeed increases. Therefore, statement (b) is correct.

Checking statement (c)

Statement (c) claims that four heme groups are present in haemoglobin. Haemoglobin is a complex protein with four subunits, and each subunit contains one heme group, making a total of four heme groups in haemoglobin. Thus, statement (c) is correct.

Verifying statement (d)

Statement (d) asserts that heme is the prosthetic group and it is the non-protein part of haemoglobin. This statement is correct as the heme is indeed the prosthetic group, consisting of an iron ion surrounded by a porphyrin ring, which is the non-protein component of haemoglobin.

Key concepts

Oxyhaemoglobin

Oxyhaemoglobin is a critical component in the process of oxygen transport within the bloodstream. When oxygen molecules bind to haemoglobin, specifically to the iron ions in the heme groups, oxyhaemoglobin is formed. This binding process occurs in the lungs, where oxygen concentration is high. Interestingly, when oxygen binds to the iron ions, it changes the magnetic properties of the molecule.
In its bound state, the iron ion ( Fe^{2+}) in oxyhaemoglobin becomes diamagnetic. This means that all the electrons in the iron ion are paired, and it does not create a magnetic field of its own. Thus, oxyhaemoglobin is not paramagnetic. This is an important distinction as paramagnetic substances are attracted to magnetic fields, unlike oxyhaemoglobin when oxygen is bound.
When haemoglobin releases oxygen, it reverts to its deoxygenated form, and the iron becomes paramagnetic again as some electrons become unpaired.

Paramagnetic

The term paramagnetic refers to a substance or element that contains unpaired electrons in its atomic or molecular structure. These unpaired electrons allow the material to be attracted to external magnetic fields. This property is important in understanding how molecules like haemoglobin behave in different states.
In haemoglobin, the iron ion ( Fe^{2+}) changes its magnetic properties when it binds to oxygen. In a deoxygenated state, such as when haemoglobin releases oxygen in the tissues, the iron ion exhibits paramagnetism. The presence of unpaired electrons in this state enables the ion to interact with magnetic fields.
When oxygen binds to the iron, these unpaired electrons find new partners, and the ion shifts to a diamagnetic state, as discussed earlier. This switch between paramagnetic and diamagnetic states explains not only how haemoglobin functions but also the role of iron in its magnetic behavior.

Heme Group

At the heart of haemoglobin's ability to bind and transport oxygen lies the heme group. Each haemoglobin molecule consists of four heme groups, one embedded in each of its four protein subunits. The heme group is responsible for the red color of blood due to its structure and function.
The heme group is composed of an iron ion ( Fe^{2+}) centrally located within a large, ring-shaped organic structure known as a porphyrin ring. This setup is critical because the iron ion is the site where oxygen binds, enabling the transport of oxygen from the lungs to tissues throughout the body.
Due to the iron's ability to change its oxidation state, the heme group facilitates crucial processes in oxygen transport and delivery, making it an indispensable component of haemoglobin's function.

Prosthetic Group

The term prosthetic group refers to a non-protein component that is tightly and permanently attached to a protein molecule, playing a vital role in the protein's function. The heme group in haemoglobin is an example of a prosthetic group.
The significance of the heme as a prosthetic group lies in its role in enabling haemoglobin to carry out its physiological function. Although haemoglobin is largely made of protein, it cannot transport oxygen without the heme groups. The heme provides the necessary chemical environment for iron to reversibly bind and release oxygen.
This relationship showcases the importance of prosthetic groups in biological molecules. They extend the function of proteins beyond their amino acid sequences by incorporating non-protein molecules essential for activity, like the porphyrin ring and iron ion in heme.