Hemoglobin Molecule- Definition, Properties & Structure

Presentation Notes on Hemoglobin molecule (Hb) Formula, description, Normal Size & hb Synthesis for Medical Biology Students

Properties of Hb

1. Oxygen affinity. The affinity of Hb, for oxygen, is ideal, it is neither excessive nor too little. If the affinity were too little, the Hb within the RBC would have failed to combine with the O2 of the alveoli of the lung. Had it been excessive, in the peripheral tissues, where the oxyHb is supposed to release its O2, the Hb would have retained its O2, {Affinity for oxygen is expressed as P5o}.

Further, this type of affinity of Hb for O2 produces the sigmoid shape  of the 02 dissoci­ation curve. This affinity decreases in (a) hot and/or (b) acid environment. This is teleologically desirable. The working muscles are hot as well as their pH is low; in these conditions the affinity for 02 is low and the O2 is released vigorously. Another compound, 2,3 DPG (2,3 diphosphoglycerate), which accumulates during metabolism, also causes loss of affinity. This results in rapid and easy release of 02 in exercising muscles.

2. Haem-haem interaction. In the initial phase (of oxygena­tion), the combination of haem and O2 is a bit slow, but once a little of O2 has combined with haem, further combi­nations are facilitated. This is called 'haem-haem interaction'. This explains the steeper part of the sigmoid shaped graph of O2 dissociation curve.

Structure of Haem

In a haem molecule, there are four pyrrole structures The four pyrroles are linked up with one another by methane(= CH - ) bridges, to form, what is known as porphyrin.
There are various types of porphyrins. The particular type of porphyrin found in hemoglobin is called protopor­phyrin III and as it contains an iron in the central part of the molecule, it is called iron protoporphyrin. Haem is iron protoporphyrin.

Iron Protoporphyrin

Synthesis of Hb

Hb is synthesized by the cells of erythroid series in the RBM (red bone marrow). Hb first appears at the stage of intermediate normoblast . The protoporphyrin can be synthesised by the normoblasts from products of metabolism, and amino acids, like succinyl CoA, glycine etc. Iron has to be obtained from food or iron contained in the Hb of dead RBCs. Important factor for Hb synthesis which is clinically important is iron (food must contain iron). Copper and cobalt are also necessary but they are more of academic interest only.

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