08- lecture 8 haemolglobin and myoglobin as conjugated protein

7/22/2019 08- Lecture 8 Haemolglobin and Myoglobin as Conjugated Protein

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Hemoglobin:Portrait of a

Protein in

Action

Copyright 2007 by W. H. Freeman and Company


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Erythrocytes

(Red cells)


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Hemoglobin and Myoglobin

These are conjugated proteins. A simple

protein has only a polypeptide chain. A

conjugated protein has a non-protein part in

addition to a polypeptide component. Bothmyoglobin and hemoglobin contain heme.

Myoglobin - 17000 daltons (monomeric)

153 amino acids

Hemoglobin - 64500 daltons ( tetrameric)

-chain has 141 amino acidsb-chain has 146 amino acids


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Hemoglobin O2carrying capability

Erythrocytes/ml blood: 5 billion ( 5 x 109)

Hemoglobin/red cell: 280 million ( 2.8 x 108)

O2molecules/hemoglobin: 4

O2ml blood: (5 x 109)(2.8 x 108)(4) = (5.6 x 1018)

or (5.6 x 1020) molecules of O2/100 ml blood


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A single subunit of Hemoglobin,

an b tetramer


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Myoglobin, monomeric


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3ostructure overlap:

myoglobin, -globin and b-globin

-Globin (blue)

b-Globin (violet)Myoglobin (green)


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Aromatic Heme


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Iron in Hemoglobin binding O2


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Iron in Myoglobin binding O2


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Resonance in Iron binding O2


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Hemoglobin, b tetramer


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O2binding: Hemoglobin & Myoglobin

P50= 2 torr

P50= 26 torr


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O2transport capability, a comparison


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Resting state vs exercise


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O2Binding Changes 4oStructure


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Decreasing O2affinity

2,3-bisphospho-

glycerate

(2,3-BPG)

Lowers the

affinity of oxygen

for Hemoglobin


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2,3-bisphosphoglycerate (2,3-BPG)

The binding pocket for BPG contains 4 His and 2 Lys


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Binding of bisphosphoglycerate


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The Bohr Effect

Bohr Effect:

Lowering the

pH decreases

the affinity ofoxygen for Hb


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Loss of O2from Hemoglobin

Carbamate:

CO2combines

with NH2at theN-terminus of

globins


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Carbamate formation

Covalent binding at the N-terminus of each subunit


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Combined

Effects

CO2, BPG and pH

are all allosteric

effectors of

hemoglobin.


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CO2& Acid from Muscle


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CO2& Hemoglobin Blood Buffering

Metabolic oxidation in cells uses oxygen andproduces CO2 .

The pO2drops to ~20 torr and oxygen is released

from incoming HbO2-.

HbO2- Hb- + O2

Release is facilitated by CO2reacting with the N- terminus of each hemoglobin subunit, by

non-covalent binding of BPG and the Bohr

effect.


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Events at Cell sitesThe localized increase in CO2results in

formation of carbonic acid which ionizes

to give bicarbonate and H+.

CO2+ HOH H

2CO

3

carbonic anhydrase

H2CO3 HCO3-+ H+ pKa = 6.3

The increase in [H+] promotes protonation of Hb-.

HHb Hb- + H+ pKa = 8.2


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Events at Cell sitesThe predominant species in this equilibrium at

pH 7.2 is HHb.

So, O2 remains at the cell site, HHb carries aproton back to the lungs and bicarbonate

carries CO2 .

Charge stability of the erythrocyte is maintainedvia a chloride shift, Cl- HCO3

- .


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Events at Lung sitesBreathing air into the lungs increases the partial

pressure of O2to ~100 torr.

This results in O2 uptake by HHb to form HHbO2.

HHb + O2 HHbO2

Ionization of HHbO2 then occurs and HbO2-

carries O2away from the lungs.HHbO2 HbO2

- + H+ pKa = 6.6

So, the predominant species at pH (7.4) is HbO2-.


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Events at Lung sitesThe localized increase in [H

+

] from hemoglobinionization serves to protonate HCO3

-.

H2CO3 HCO3-+ H+ pKa = 6.3

H2CO3 CO2+ HOHcarbonic anhydrase

The resulting H2CO3decomposes in presence of

carbonic anhydrase and CO2is released in thelungs.

Charge stability of the erythrocyte is maintained

again via a chloride shift, HCO3-

Cl

-

.


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Sickle Cell due to Glu 6 Val 6


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08- lecture 8 haemolglobin and myoglobin as conjugated protein

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