Hemoglobin, anAllostericProtein

Hemoglobin vs Myoglobin

Hemoglobin (Hb):

- found in red blood cells

- responsible for transport of O2 from

lungs to cellular tissue

- transport of some CO2 and H+ back to lungs

Myoglobin (Mb):

- Located in Muscles

- Aids O2 diffusion to tissue

- Acts as O2 storage reservoir

Protein

HemeHistidines

MyoglobinBinds O2 in muscles

A complex monomeric proteinwith 1o, 2o and 3o structure

Bundle of 8 a-helicesIn 3o structure

The interior is largelyHydrophobic withVal, Leu, Ile, Phe, Met

Figure 9.3

Red blood cells (erythrocytes)

Each cell contains approximately 300 million hemoglobin molecules.

Hemoglobin is tetrameric andcarries oxygen in the blood

A complex tetrameric proteinwith 1o, 2o, 3o and 4o structure

Figure 9.6

Fe(II)-protoporphyrin IX hemethe pyrrole rings provide 4 of the 6 ligands that bind to the Fe(II).

Heme, a Prosthetic Group in myoglobin and hemoglobin

The reversible binding of oxygen to heme

Oxymyoglobin has six ligands boundDeoxymyoglobin has five ligands bound

Proximal His

Distal His

Same for oxy- and deoxyhemoglobin

Myoglobin is monomeric and binds O2 in muscles

Heme

Histidines

Oxygen binding curves of hemoglobin and myoglobin

Y = Fractional oxygen saturation of myoglobinMb = Concentration of myoglobin molecules without bound oxygenMbO2 = Concentration of myoglobin molecules with bound oxygenMb + MbO2 = total concentration of myoglobin molecules

Equilibrium reversible binding of OxygenFigure 9.1

Mb + O2 MbO2

Hb + 4 O2 Hb(O2)4

O2 binding to Mbhas a hyperbolic curve

O2 binding to Hbhas a sigmoidal curve

The larger value of pO2 at Y=0.5 means lower affinity for oxygenO2 binding to Hb

must be cooperative pO2(0.5) = 2.8 torr (Mb)pO2(0.5) = 26 torr (Hb)

Cooperatively helps to release O2 at location of tissue cells

Both Mb and Hbare saturated with O2

Cooperatively helps to release O2 at location of tissue cells

At the tissue cells 93% of Mb is MbO2, BUT only ~32%of Hb binding sites have O2 bound

What causes this binding cooperatively??

Oxygen binding induces protein conformational changesFigure 9.4

Blue = HbRed = HbO2

The Fe2+ is pulled into theplane of the heme porphyrin

Movement of the proximal His induces a change in the a subunitCausing a change in the b unit

Oxygen binding induces protein conformational changesFigure 9.7

Grey structure

Red structure

This in turn changes the whole quaternary structure

Oxygen binding induces protein conformational changesFigure 9.8

The binding of the other three subunitsdramatically increases

Hemoglobin is an allosteric protein

- The binding of O2 can be affected by allosteric interactions or binding of aallosteric effector

- The allosteric effector binds reversibly at a site that is different from thefunctional site (e.g. O2 binding site).

Binding of 2,3-BPG alters Hb affinity for O2

Less O2 is bound to Hb near tissue cellswith 2,3-BPG bound.

2,3-BPG stabilizes the deoxyHb.

[H+] and CO2 can bind to Hemoglobin and decrease oxygen affinityThe Bohr Effect

As the pH decreases hemoglobinhas lower affinity for O2.

The buffering of bloodand HbO2 affinity are directly related.

The drop in pH favors the deoxyHb structure via non-covalent interactions

Drop in pH caused increasein His 146 protonation.

A new hydrogen bond is formed

Figure 9.19

The presence of CO2 alsoalters O2 binding affinity.

The Bohr effect andCO2 combined areimportant in exertion or exercising.

Hemoglobin (Hb) & buffering of blood and O2 transportAt Tissue

Tissue cells Plasma Red Blood Cell

C6H12O6 + O2

H2O + CO2 CO2

diffusionCO2 + H2O

Carbonicanhydrase

H2CO3

HCO3- + H+HCO3

-

(buffer)

Cl- Cl-

+ HbO2

H+HbO2 (low affinity)O2 + H+HbO2

(Delivery to cells)

Figure 9.17and 9.21

Hemoglobin (Hb) & buffering of blood and O2 transportAt Lungs

Lungs Plasma Red Blood Cell

CO2 CO2

diffusion

Carbonicanhydrase

H2CO3 + HbO2 (high affinity)

HCO3- HCO3

-

Cl- Cl-(low affinity)

O2 O2 O2 + H+Hb

H+HbO2

H2O + CO2

exhale out

Acidosis: pH

Alkalosis: pH

Figure 9.17and 9.21

How is Hemoglobin effected by genetics and environment?

Read about sickle cell anemia and malaria and geese livingnear Mt. Everest: Clinical Ins. Pg 147

Fetal RBC have greater O2 affinity than Maternal RBC….why?Clinical Ins. Pg 146

Assignment

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