Difference Between Hemoglobin and Myoglobin

Hemoglobin vs Myoglobin

Myoglobin and hemoglobin are hemoproteins which have the ability to bind molecular oxygen. These are the first proteins to have its three- dimensional structure solved by X-ray crystallography. Proteins are the polymers of amino acids, joined via peptide bonds. Amino acids are the building blocks of proteins. Based on their overall shape, these proteins are categorized under globular proteins. Globular proteins have somewhat spherical or ellipsoidal shapes. The different properties of these unique globular proteins help organism to shift oxygen molecules between them. The active site of these special proteins consists of an iron (II) protoporphyrin IX encapsulated in a water resistant pocket.

Myoglobin

Myoglobin occurs as a monomeric protein in which the globin surrounding a heme. It acts as a secondary carrier of oxygen in the muscle tissue. When the muscle cells are in action, they need a large amount of oxygen. Muscle cells use these proteins to accelerate oxygen diffusion and take oxygen for times of intense respiration. The tertiary structure of myoglobin is similar to a typical water soluble globule protein structure.

Polypeptide chain of myoglobin has 8 separate right handed α-helices. Each protein molecule contains one heme prothetic group and each heme residue contains one central coordinately bound iron atom. Oxygen is bound directly to the iron atom of the heme prosthetic group.

Hemoglobin

Hemoglobin occurs as a tetrameric protein in which each subunit consists of a globin surrounding a heme. It is the system-wide carrier of oxygen. It binds oxygen molecules and then it is transported through the blood by red blood cells.

In vertebrates, oxygen is diffused through the lung tissue into the red blood cells. Since hemoglobin is a tetramer, it can bind four oxygen molecules at once. Bound oxygen is then distributed through the entire body and offloaded from red blood cells to respiring cells. Hemoglobin then takes the carbon dioxide and return them back to the lungs. Therefore, hemoglobin serves to deliver oxygen needed for cellular metabolism and removes the resulting waste product, carbon dioxide.

Hemoglobin consists of several polypeptide chains. Human hemoglobin is composed of two α (alpha) and two β (beta) subunits. Each α-subunit has 144 residues, and each β-subunit has 146 residues. Structural characteristics of both α (alpha) and β (beta) subunits are similar to myoglobin.

Hemoglobin vs Myoglobin

• Hemoglobin transports oxygen in blood while myoglobin transports or stores oxygen in muscles.

• Myoglobin consists of a single polypeptide chain and hemoglobin consists of several polypeptide chains.

• Unlike the myoglobin, concentration of hemoglobin in red blood cell is very high.

• At the beginning, myoglobin binds oxygen molecules very easily and lately become saturated. This binding process is very rapid in myoglobin than in hemoglobin. Hemoglobin initially binds oxygen with difficulty.

• Myoglobin occurs as a monomeric protein while hemoglobin occurs as a tetrameric protein.

• Two types of polypeptide chains (two α-chains and two β- chains) are present in hemoglobin.

• Myoglobin can bind one oxygen molecule so called monomer, while hemoglobin can bind four oxygen molecules, so called tetramer.

• Myoglobin binds oxygen more tightly than does hemoglobin.

• Hemoglobin can bind and offload both oxygen and carbon dioxide, unlike the myoglobin.