What is Hemoglobin?

Hemoglobin Compound & Structure 

Hemoglobin, an oxygen-carrying pigment that gives blood its red color, consists of a protein component and the iron complex of a porphyrin derivative:

hemoglobin = globin (protein) + hemochromogen (Fe (II) complex).

Hemoglobin has a quaternary structure. It consists of two pairs of different proteins, designated the α and β chains. There are 141 and 146 amino acids in the α and β chains of hemoglobin, respectively.

The haem group is composed of a central iron atom complexed to four nitrogen atoms. Oxygen is capable of reversibly binding to the heme unit in a process known as oxygenation. The interactions among the subunits in a hemoglobin molecule are known as cooperativity. 

The subunits of hemoglobin do not act independently. When one subunit binds O2, its conformation changes. When a change in conformation at one site of an oligomeric protein is caused by a change in a spatially separated site of the oligomer, the change is called an allosteric effect, and the protein is called an allosteric protein. Hemoglobin is an allosteric protein. When one heme group in hemoglobin binds oxygen, it is easier for successive oxygen molecules to bind at the remaining three sites. Thus, once oxygenation occurs at one heme, there is cooperation at all other sites in hemoglobin.

During the breathing process, oxygen attaches loosely to the iron ion, and the oxyhemoglobin thus formed surrenders its oxygen in the body and reverts back to hemoglobin. Outside the body the iron converts to the trivalent form. Hemoglobin binds carbon monoxide more strongly than oxygen, its color weakening to cherry red, hence the pink complexion of victims of carbon monoxide poisoning.

One degradation product of hemoglobin is the brown bile pigment bilirubin. Another is green biliverdin, the result of breakdown by oxidation. The first occurs in bile, the second in the liver.

Function of Hemoglobin

• Deficiency of reduced nicotinamide adenine dinucleotide (NADH)
• Diaphorase deficiency  
• Structurally abnormal Hb (HbM)
• Toxic substances

Haemoglobin, is a protein whose main function is to transport oxygen from the lungs to tissues and to transport carbon dioxide from tissues to the lung. The hemoglobin molecule contains four separate folded peptide chains, which form a hydrophobic or water ‘repelling’ pocket around a heme group.

There are well-described regulators of the affinity of hemoglobin for oxygen that provide a control mechanism. The S-shaped graph of this oxyhemoglobin relationship is known as the oxyhemoglobin dissociation curve and represents the relationship between the partial pressure of oxygen (Po2) in mm of mercury (Hg) and the oxygen content per 100 ml of blood.

Haem metabolism

Haem belongs to a family of compounds known as the porphyrins, which are characterized by the presence of a tetrapyrrole ring. Haem is an iron-containing derivative, the iron atom being located at the center of the tetrapyrrole ring of protoporphyrin IX. The haem group is responsible for the oxygen-binding properties of haemoglobin.

Prevention of haem oxidation

When haemoglobin is oxidized (Fe⁺² to Fe⁺³) it is known as methaemoglobin (metHb). Excess metHb is caused by

• Deficiency of reduced nicotinamide adenine dinucleotide (NADH)
• Diaphorase deficiency  
• Structurally abnormal Hb (HbM)
• Toxic substances