Hello everyone,this article is about: Haemoglobin. In this article you will learn about _
- Haemoglobin structure
- Bohr effect
- Haldane effect
- Oxygen dissociation curve.
Porphyrins are organic pigments each consisting of a tetrapyrrole ring system made of four pyrrole rings linked by methene bridges. Tetrapyrrole carry a metal ion such as Fe2+, Fe3+ or Mg2+ bound to its pyrrole rings.different porphyrins carries different side chains such as methyl(M), vinyl(V), acetate(A) and propionate(P).
In type I porphyrin that is coproporphyrin-I and Uropprphyrin-I, the substituents are arranged symmetrically on all four pyrrole rings; but in type-III such as copropprphyrin-III and protoporphyrin-III the substituents are arranged asymmetrically.
Porphyrin solution emit red fluorescence in ultraviolet light due to their double bonds and show characteristics absorption bands in solutions.
Metalo porphyrin such as heme, bind to specific proteins to form hemoproteins for example hemoglobin and myoglobin.
Properties of porphyrin:
The properties of porphyrin are:
- They have both the -COOH and -NH2 groups.
- The isoelectric pH of porphyrin varies from 3 to 4.5.
- Porphyrin emits fluorescence due to alternating double bonds.
Human haemoglobin (molecular weight 64.5KD) is a chromoprotein made up of a apoprotein called globin and the prosthetic group of Fe2+ – protoporphyrin-III compound called heme.
Globin tetrameric basic protein of the histone class. It is made of paired peptide of two different types such as α, β, γ and δ.Each of four peptide chain remains bound to heme molecule.Heme bears methyl and vinyl side chain on pyrrole- II and methyl and propionate side chain on pyrrole-III and IV.
The Fe2+ ion of heme molecule is co-ordinately linked to the imidazole-N of two histinine residues of the peptide chain.
Normal human Globin is made of 2α and 2β chain. The α chain contain 141 amino acid and the β chain contain 146 amino acid residues.
The secondary structure of each peptide chain consists of 7 α-helical segments alternating with short nonhelical randomly coiled segment.
The tertiary structure of each peptide chain results from folding of its chain and non helical segments.Non polar amino acid residues are located deep inside the coil while polar amino acid residues mostly on the surface of the coil. The tertiary structure buries the heme in a hydrophobic pocket.
The quaternary structure of globin consist of a globular tetrameric molecule made of two pairs of peptide chains , each with its individual secondary- tertiary structure and it’s own heme group. The peptide bonds are held together and the quaternary structure is stabilized by many hydrophobic bonds,some hydrogen bond and electrostatic bond.
The range of haemoglobin in blood is measured in grams(gm) per deciliter (dL) . It depends on age and gender of the person. The ranges of haemoglobin in blood are as follows:
- Average haemoglobin content in blood: 14-16 gm/dL.
- In newborn: 17 to 22 gm/dL.
- In children: 11 to 13 gm/dL.
- In adult male: 14 to 18 gm/dL.
- In adult women: 12 to 16 gm/dL
Normally 3 types of haemoglobin are found in human. They are heamoglobin A(Hb A), haemoglobin A2(Hb A2) and haemoglobin F(Hb F).
- About 97% of haemoglobin that is found in normal adult is Hb A.
- It is consist of 2α and 2β chains.
- Hb A appears in foetal blood by the 20th week of foetal life and increases to 20% and 90% of total haemoglobin at birth and 4 months after respectively.
- About 2% of haemoglobin that is found in normal adults is Hb A2.
- It contains 2α and 2δ chains.
- Each δ chain possess the same total number of 146 amino acid residues as the β chain but differs from the later in nature of 10 amini acids.
- Foetal haemoglobin is found in growing foetus.
- It has 2α and 2γ chains.
- Each γ chain consists of 146 amino acid residues as the β chain but differs from later in 37 amino acids.
Shift of the the oxygen haemoglobin dissociation curve by changes in the blood CO2 and H+ ions is important to enhance oxygenation of blood in the lungs and also it enhance the release of oxygen from the blood in the tissues, this is called bohr effect. It can be explained as follows_
As the blood passes through the lungs ,CO2 diffuses from the blood into the alveoli. This reduces the blood PCO2 and also decreases the H+ ion concentration because of the resulting decrease in the blood H2CO3 ,both this effect shift the oxygen haemoglobin dissociation curve to the left and upward. Therefore the quantity of O2 that binds with the hemoglobin at any given PO2 becomes considerately increase. Thus allowing greater O2 transport to the tissues. Then,when the blood reaches the tissue capillaries exactly the opposite effect occurs. CO2 entering the blood from the tissues shifts the curve right word which displaces O2 from hemoglobin and therefore delivers O2 to the tissues at a higher PO2 than both otherwise occur.
Significance of Bohr effect:
The O2 dissociation curve shift to right when CO2 and H+ ion concentration is increased. This increases O2 dumping.
This mechanism allows for the body to adapt the problems of supply more O2 to tissue that need in the most.
CO2 transport in blood is influenced by PO2, binding of O2 with hemoglobin tends to displace CO2 from blood particularly in lung capillaries this is called haldane effect.
Oxyhaemoglobin is relatively more acidic and thus has a less affinity to CO2, instead it releases H+ from HHb, H+ converts HCO3– to H2CO3, H2CO3 on dissociation release CO2 from blood into alveoli.
Deoxyhaemoglobin is relatively alkaline and thus combine with CO2 in tissue capillaries.
What is Oxygen dissociation curve?
The oxygen dissociation curve is a graph that shows the percent saturation of haemoglobin at various partial pressure of oxygen. It is also known as oxyhaemoglobin dissociation curve.