Hemoglobin-like molecules are found in several invertebrates

Hemoglobin

What is haemoglobin? Hemoglobin (Hb) is an oxygen-carrying metalloprotein that contains iron. In vertebrates, haemoglobin is present in the red blood cells, while it is found in the tissues of some invertebrates. The primary functionof haemoglobin is to transport oxygen from the lungs or gills to other body parts. Read on to know more about Hemoglobin.

the function of haemoglobin:

It carries oxygen from the lungs to the body tissues and carbon dioxide from the tissues to the lungs. About 70 per cent of your body's iron is found in the red blood cells of your blood called hemoglobin.

iron is a component of specific proteins, essential for respiration and energy metabolism so it is also a source of iron compound.

Hemoglobin interacts with nitric oxide (NO) to regulate the blood pressure. NO is made by the endothelial cells and by its action on the vascular smooth muscle leads to vasodilation.

outside the brain, hemoglobin has a non-oxygen-carrying function as an antioxidant and a regulator of iron metabolism in macrophages, alveolar cells, and mesangial ceels in the kidney. Hemoglobin is an iron-protein compound in red blood cells that gives blood its red color.Hb is a conjugated protein present in RBCs.

It carries oxygen from the lungs to the tissue cells, and carbondioxide- the gaseous waste -from the cells to the lungs.fight germsHemoglobin is the oxygen-binding protein found in all vertebrates (except fish, belonging to the family Channichthyidae) and certain invertebrates. However, haemoglobin-like molecules are found in several invertebrates, fungi, plants, and certain bacteria.

In vertebrates, it is mainly responsible for the transportation of oxygen from lungs to the body tissues and carbon dioxide from peripheral tissues to the lungs.

This respiratory protein was first observed in the crystalline form by Friedrich Ludwig Hünefeld, in 1840. Since then, several researchers explored this protein, and even determined its entire amino acid sequence. The structure and function of hemoglobin has been briefly explained below.

Each hemoglobin molecule comprises two types of globins organized into four subunits. The two sets of globin chains have minute differences in the sequence and types of amino acids comprising them. This amino acid sequence of a protein is called the primary structure. These amino acid chains fold through internal hydrogen bonding to form helices and sheets, which are collectively called secondary structure of a protein. These secondary structures combine to form the final three-dimensional structure called the tertiary structure of protein.

Globin:

The globin tertiary structure comprises a helical structures joined by non-helical segments. Four such globins are arranged together, giving rise to the spherical quaternary structure of hemoglobin as shown in the figure above.

Hemoglobins are classified into different types, depending on the combination of the two sets of globin units. Most of the hemoglobin in adult humans comprises 2 α globins and 2 β globins. Other globins present in the different different types of hemoglobin found in humans include γ, δ, ε, and ζ.

Heme:

The heme group bound to each globin is an organic macromolecule with an iron at the center. It serves as the prosthetic group of hemoglobin. The prosthetic group of a protein refers to any tightly-bound non-protein entity, that is essential for the structural and functional integrity of the protein.

The heme group comprises a structure called the porphyrin ring, which is formed by the combination of four heterocyclic rings called pyrroles. An iron ion (Fe2+) is present at the center of this structure, and is bound to the nitrogen atoms of the four pyrrole rings. This central iron provides the reversible binding to oxygen and carbon dioxide molecules.

When the heme is bound to an oxygen molecule or carbon dioxide molecule, it is termed oxyhemoglobin, or carbaminohemoglobin respectively. When the heme groups of a hemoglobin molecule are not bound by any molecule, it is called deoxyhemoglobin. It is oxyhemoglobin that imparts a bright red color to blood.

Function

Oxygen transport is the main function of hemoglobin, and more than 98% of the oxygen in blood is carried through hemoglobin. In addition, it also transports carbon dioxide released by peripheral tissues to the lung tissues. During this process, the hemoglobin macromolecule undergoes conformational changes due to the binding and unbinding of oxygen and carbon dioxide.

Oxygen Pickup:

In the alveolar tissue of lungs, oxygen diffuses across the alveolar membrane into the lung capillaries and reaches the red blood cells. Here, the oxygen molecule binds reversibly to the central iron atom of heme. Since each hemoglobin molecule has four heme groups, it has the capacity to carry four oxygen molecules. The loading of hemoglobin with oxygen molecules occurs in a cooperative manner. When one oxygen molecule binds to one of the heme groups, it induces a conformational change that increases the affinity for oxygen in the other three subunits. The conformation of hemoglobin molecule, when fully loaded with oxygen, is called the relaxed (R) state.

Oxygen Delivery:

As hemoglobin travels from lungs and reaches the capillaries of peripheral tissues, it encounters low pH due to the increased concentration of carbon dioxide in blood. This results in a loss of affinity for oxygen in hemoglobin, thus facilitating the release of oxygen molecules. This inverse relation between oxygen affinity of hemoglobin with acidity and carbon dioxide concentration is known as Bohr effect. Apart from the Bohr effect, high concentration of a chemical called 2,3-bisphosphoglyceric acid in the peripheral tissues also facilitates the release of oxygen.

The unloading of oxygen also occurs cooperatively, and the release of first oxygen molecule facilitates the release of oxygen molecules from other heme groups of the same molecule. The conformation of the hemoglobin molecule, when fully unloaded, is called tensed (T) state.

Carbon Dioxide Pickup:

Deoxyhemoglobin has a higher affinity for carbon dioxide molecules than its oxygenated counterpart. This is known as the Haldane effect, and it facilitates the uptake of carbon dioxide molecules in the tissues. However, only 20% of carbon dioxide is transported through hemoglobin, and the rest is transported as carbonic acid.

Carbon Dioxide Delivery:

In the lungs, Haldane effect promotes the dissociation of carbon dioxide molecules in the presence of oxygen, and the hemoglobin is free to load oxygen molecules.

Hemoglobin plays a vital role in ensuring the supply of oxygen to every cell of the body. Even slightest alterations in the amino acid sequence of the constituent globins can result in severe hemoglobin disorders.