Hemoglobin the Oxygen Carrier

Iron porphyrin, proteins and enzymes are collectively known as heme proteins or hemoproteins .these proteins exhibit distinctly different functions (/) reversibly oxygen (O,) binding for its transport as in hemoglobins or 0'/) storage of oxygen (0-,)as in myoglobins oxidation by 02 with reduction of От to H,0 as in the case of cytochrome c oxidase, decomposition of HtOt as it catalyses and peroxidases. Hemoglobin is the red colour of erythrocytes that account for the 95%of the dry weight of the red blood cell. Its main function is carrier of oxygen but it is also related with the transport of CO-, and acid -base balance of the body. Hemoglobin also absorbs a large volume of oxygen in the lungs and carries it to the tissues.

Hb + 02 ^ Hb—02 Hemoglobin Oxy hemoglobin

Hemoglobin has an oxygen absorbing capability e.g. lg hemoglobin when fully oxygenated contains 1.39 ml of oxygen. Besides, hemoglobin contains 50 mg of Fe for 100 ml of blood.

Transport of Oxygen

Oxygen transport is very important for respiration. Its production in photosynthesis provides examples of biologically important redox reaction which includes electron transfer, transfer of atom and photosyntheses processes.

Oxygen Carriers

The distribution of oxygen to the various parts of the body is carried by cell which contain iron where as carriers contain copper which is present in extra cellular fluids. Cell-interior is a reducing medium that contains Fe as Fe (II) besides this,cell gives a porphyrin ring which acts as a ligand to form noil- labile Fe(II) complex.

lron(ll)-protoporphyrin-IX (Heme-B); (Fe—-N^=Fe

Fig. 11.4: lron(ll)-protoporphyrin-IX (Heme-B); (Fe—-N^=Fe<-N)

The porphyrin ring is susceptible to oxidation and protein the reductases in the cell. On the other hand both Cu(I) and Cu(II) are able to form strong complexes with imidazole side chains of poly peptide residue. Therefore Cu-leased carrier can survive unaltered in the blood iron and copper carrier are able to play distinct role in the same organism.

Metal Ions Strong and Transportation in the Human System

Iron(II)from the stomach is oxidised .bound and transported in the blood stream as a complex of Fe(III) with a protein called transferrin. It can remain in the system (stored)only as Ferritin which is Fe (Ill)-protein complex with 15-20% iron. Feritin helps to store iron in the form of Fe (III) complex with proteins. It can be understood as a protein shell surrounding a core which is ferric hydroxy phosphate of composition -Fe0(0H)8(Fe0.H-,P04) containing 57% iron. Ferritin is present in the organs of all mammals specially in liver, spleen and bone marrow. The iron stored in such a way is released as and when needed in the synthesis of iron—containing hemoglobin, myoglobin and the cytochromes.

As a generalization it should be understood that the metal ions in the human body can be bound, stored and transported in the from of metal ion complexes. 70%of Fe metal is present as hemoglobin and myoglobin while the remaining is stored as ferritin.

Transferrin is a protein with a higher molecular weight .this can bind two iron atoms per molecule. The two binding sites are similar but not identical in their binding strength. The iron is bound so firmly that no other protein can take away iron. Except those at the site of hemoglobin synthesis. In transferrin iron (III)is bound to at least five groups in the protein.

Fe(lll) Transferring Complex

Fig. 11.5: Fe(lll) Transferring Complex

Copper in trace amount is essential for the synthesis of hemoglobin. It occurs in the form of metallo protein, hemo cuprein in the blood cells (0.34% copper).

Role of Some Metals in the Human Body

Selenium

Selenium is toxic when its concentration is above 10 ppm but it is also essential for life when the concentration is below 10 ppm. Average man requires < 0.2 mg per day. The deficiency of selenium is caused when the concentration becomes less than 0.1 ppm. the deficiency of Se result in liver necorsis and muscle dystrophy.

A good member of seleno-organic compounds are derived from bacteria fungi and green plants. Some naturally occurring organo-selenium compounds are listed below

Some naturally occurring organo-selenium compounds

Name

Formula

Sources

Dimethyl diselenide

Fungi; Astragalus

Se-methylselenocysteine

Se accumulators, e.g. Astragalus, Stanleya

Selenocysteine seleninic acid

Lollum, Trifolium

Selenomethionine

Bacteria; fungi; Lemna, Lolium, Trifolium

Selenocystine

Selenocystathionine

Se accumulators, e.g. Astragalus, Morinda

Selenohomocystine

One Astragalus spp.

Selenomethionine selenoxide

Lolium, Trifolium

Se-propenylselenocysteine selenoxide

Allium

Most of the above mentioned compounds are amino acids. Amino cystine seems to be largely distributed. Other seleno-compounds include peptides, adenosine phosphoselenate obtained from bacteria and yeasts. Still other seleno-compounds may be storage compounds of this highly toxic selenium.

Boron

At present ,the only naturally occurring boron is boromycin having the formula C45H74015NB obtained from streptomycin which is easily hydroysed to boric acid. It seems boron has role to play in RNA or carbohydrate transport. Boron deficiency in plants inhibits ATPase activity in the cell membrane, and uptake of P04“ ions.

Molybdenum

Molybdenum (Mo) is quite abundant in the earth's crust. In natural water concentration of Mo is higher than those of Fe, Cu and Zn. Aluminum is abundant but not very essential to the humans although an adult contains 60 mg of A1 in the lungs for inhalation. A1 is reported to give strength to enzyme activity but K+ and Zn2+ are required for the full enzyme activity. Silicon is also abundant on the earth’s crust but its ultra trace amount is essential for the human system. Ti and Zn are abundantly available but its trace amount are required to get extremely insoluble oxides at the relevant Ph values. They do not form stable complexes with the complexing agents present in the biological system. The essential element may be harmful at high level of concentration. In reality, many of the essential elements belong to the r/-block metals which have the ability to form coordination complexes and chelote compounds in the most characteristic aspect of chemistry.

 
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