Hemoglobin, an AllostericProtein

Hemoglobin vs Myoglobin Hemoglobin (Hb): - found in red blood cells - responsible for transport of O 2 from lungs to cellular tissue - transport of some CO 2 and H + back to lungs Myoglobin (Mb): - Located in Muscles - Aids O 2 diffusion to tissue - Acts as O 2 storage reservoir

Protein Heme Histidines Myoglobin Binds O 2 in muscles A complex monomeric protein with 1 o, 2 o and 3 o structure Bundle of 8  -helices In 3 o structure The interior is largely Hydrophobic with Val, Leu, Ile, Phe, Met Figure 9.3

Red blood cells (erythrocytes) Each cell contains approximately 300 million hemoglobin molecules.

Hemoglobin is tetrameric and carries oxygen in the blood A complex tetrameric protein with 1 o, 2 o, 3 o and 4 o structure Figure 9.6

Fe(II)-protoporphyrin IX heme  the 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 bound Deoxymyoglobin has five ligands bound Proximal His Distal His Same for oxy- and deoxyhemoglobin Myoglobin is monomeric and binds O 2 in muscles Heme Histidines

Oxygen binding curves of hemoglobin and myoglobin Y = Fractional oxygen saturation of myoglobin Mb = Concentration of myoglobin molecules without bound oxygen MbO 2 = Concentration of myoglobin molecules with bound oxygen Mb + MbO 2 = total concentration of myoglobin molecules

Equilibrium reversible binding of Oxygen Figure 9.1 O 2 binding to Mb has a hyperbolic curve O 2 binding to Hb has a sigmoidal curve The larger value of pO 2 at Y=0.5 means lower affinity for oxygen O 2 binding to Hb must be cooperative pO 2(0.5) = 2.8 torr (Mb) pO 2(0.5) = 26 torr (Hb)

Cooperatively helps to release O 2 at location of tissue cells Both Mb and Hb are saturated with O 2

Cooperatively helps to release O 2 at location of tissue cells At the tissue cells 93% of Mb is MbO 2, BUT only ~32% of Hb binding sites have O 2 bound What causes this binding cooperatively??

Oxygen binding induces protein conformational changes Figure 9.4 Blue = Hb Red = HbO 2 The Fe 2+ is pulled into the plane of the heme porphyrin Movement of the proximal His induces a change in the  subunit Causing a change in the  unit

Oxygen binding induces protein conformational changes Figure 9.7 Grey structure Red structure This in turn changes the whole quaternary structure

Oxygen binding induces protein conformational changes Figure 9.8 The binding of the other three subunits dramatically increases

Hemoglobin is an allosteric protein -The binding of O 2 can be affected by allosteric interactions or binding of a allosteric effector - The allosteric effector binds reversibly at a site that is different from the functional site (e.g. O 2 binding site).

Binding of 2,3-BPG alters Hb affinity for O 2 Less O 2 is bound to Hb near tissue cells with 2,3-BPG bound. 2,3-BPG stabilizes the deoxyHb.

[H + ] and CO 2 can bind to Hemoglobin and decrease oxygen affinity The Bohr Effect As the pH decreases hemoglobin has lower affinity for O 2. The buffering of blood and HbO 2 affinity are directly related.

The drop in pH favors the deoxyHb structure via non-covalent interactions Drop in pH caused increase in His 146 protonation. A new hydrogen bond is formed Figure 9.19

The presence of CO 2 also alters O 2 binding affinity. The Bohr effect and CO 2 combined are important in exertion or exercising.

Hemoglobin (Hb) & buffering of blood and O 2 transport At Tissue Tissue cellsPlasmaRed Blood Cell C 6 H 12 O 6 + O 2 H 2 O + CO 2 CO 2 diffusion CO 2 + H 2 O Carbonic anhydrase H 2 CO 3 HCO H + HCO 3 - (buffer) Cl - + HbO 2 H + HbO 2 (low affinity) O 2 + H + Hb O2O2 (Delivery to cells) Figure 9.17 and 9.21

Hemoglobin (Hb) & buffering of blood and O 2 transport At Lungs LungsPlasmaRed Blood Cell CO 2 diffusion Carbonic anhydrase H 2 CO 3 + HbO 2 (high affinity) HCO 3 - Cl - (low affinity) O2O2 O2O2 O 2 + H + Hb H + HbO 2 H 2 O + CO 2 exhale out Acidosis: pH Alkalosis: pH Figure 9.17 and 9.21

How is Hemoglobin effected by genetics and environment? Read about sickle cell anemia and malaria and geese living near Mt. Everest: Clinical Ins. Pg 147 Fetal RBC have greater O 2 affinity than Maternal RBC….why? Clinical Ins. Pg 146

Assignment Read Chapter 9 Read Chapter 10