HEMOGLOBIN AND MYOGLOBIN
Hemoglobin (Hb) is the red blood pigment, exclusively found in erythrocytes (Greek:
erythrose—red; kytos—a hollow vessel). The normal concentration of Hb in blood in males
is 14–16 g/dl, and in females 13–15 g/dl.Hemoglobin performs two important biological functions concerned with respiration
1. Delivery of O2 from the lungs to the tissues.
2. Transport of CO2 and protons from tissuesto lungs for excretion.
Structure of hemoglobin
Hemoglobin (mol. wt. 64,450) is a conjugated protein, containing globin—the apoprotein part—and the heme—the non-protein part(prosthetic group). Hemoglobin is a tetrameric allosteric protein .
Structure of globin : Globin consists of four polypeptide chains of two different primary
structures (monomeric units). The common form of adult hemoglobin (HbA1) is made up of two
D-chains and two E-chains (D2E2). Some authors consider hemoglobin consisting of two identical dimers—(DE)1 and (DE)2. Each D-chain contains 141 amino acids while E-chain contains 146 amino acids. Thus HbA1 has a total of 574 amino acid residues. The four subunits of hemoglobin are held together by non-covalent interactions primarily hydrophobic, ionic and hydrogen bonds. Each subunit contains a heme group.
Structure of heme : The characteristic red colour of hemoglobin (ultimately blood) is due
to heme. Heme contains a porphyrin molecule namely protoporphyrin IX, with iron at its
center. Protoporphyrin IX consists of four pyrrole rings to which four methyl, two propionyl and two vinyl groups are attached .Heme is common prosthetic group present in cytochromes, in certain enzymes such as catalase, tryptophan pyrolase, and chlorophyll (Mg2+). In case of cytochromes, oxidation and reduction of iron (Fe2+ Fe3+) is essential for electron transport chain.
Myoglobin
Myoglobin (Mb) is monomeric oxygen binding hemoprotein found in heart and skeletal
muscle. It has a single polypeptide (153 amino acids) chain with heme moiety. Myoglobin (mol. wt. 17,000) structurally resembles the individual subunits of hemoglobin molecule. For this
reason, the more complex properties of hemoglobin have been conveniently elucidated through the study of myoglobin.Myoglobin functions as a reservoir for oxygen. It further serves as oxygen carrier that promotes the transport of oxygen to the rapidly respiring muscle cells.
Functions of hemoglobin
Hemoglobin is largely responsible for the transport of O2 from lungs to tissues. It also helps to transport CO2 from the tissues to the lungs.
Binding of O2 to hemoglobin
One molecule of hemoglobin (with four hemes) can bind with four molecules of O2. This is in contrast to myoglobin (with one heme) which can bind with only one molecule ofoxygen. In other words, each heme moiety can bind with one O2.
Other forms of hemoglobin
Besides the adult hemoglobin (HbA1)
described above, other minor hemoglobins are also found in humans (Table 10.1). In adults a small fraction (< 5%) of hemoglobin, known as HbA2 is present. HbA2 is composed of two D and two G (delta) chains. Fetal hemoglobin (HbF) is synthesized during the fetal development and a little of it may be present even in adults.Hereditary persistence of fetal hemoglobin (HPFH) is a condition in which fetal hemoglobin synthesis is not terminated at birth but continues into adulthood. Glycosylated hemoglobin (HbA1c), formed by covalent binding of glucose is also found in low concentration. It is increased in diabetes mellitus which is successfully utilized for the prognosis of these patients .