8. Myoglobin and Hemoglobin

Heme Prosthetic Group

• Present in both Hemoglobin and Myoglobin

• Ferrous Fe2+ in center of protoporphyrin ring

Myoglobin

• globular proteins

• binds oxygen

• 8 alpha helix

Hemoglobin

• globular protein

• hetero-tetramer (two units of each alpha and beta)

• can bind 4 oxygens to each heme group (4 heme groups, one in each subunit)

Hemoglobin Deoxy form and Oxy form:

Deoxy: histidine is away from heme group and allows for delivery of oxygen

Oxy: histidine is more in contact to heme group and can hold onto oxygen better

Functional differences of myoglobin and hemoglobin

Myoglobin stores oxygen and is found in tissues

Hemoglobin delivers oxygen and is found in lungs

Oxygen binding of Myoglobin vs. hemoglobin

Hemoglobin:

• Sigmoidal = cooperative because there are 4 subunits

• in low pressure it releases oxygen because it is not saturated with oxygen in every subunit

• binds better at higher pressures

Myoglobin:

• hyperbolic: not cooperative because only 1 subunit

• binds at any pressure

Hill Coefficient

Reflects cooperativity

• Nh = 1 → no cooperativity

• Nh >1 → cooperativity

• Nh <1 → negative cooperativity : can be created by 3PG, CO₂

2, 3- Bisphosphoglycerate

• from glycolysis

• binds deoxy/loose form of hemoglobin beta 1 and 2 subunits

• has 6 negative charges neutralized by 6 positive charges of hemoglobin

• changes conformation of hemoglobin to loose form that shifts the oxygen binding curve right and makes it release oxygen better to tissues

• can happen as adaptation to high altitudes

CADET face right (lower O2 affinity and decreases cooperativity)

• CO2 (Hadane effect)

• Acidity (Bohr effect)

• 2,3-DPG

• Exercise

• T

Fetal Hb vs Adult Hb

Fetal Hemoglobin is alpha 2 gamma 2 and binds more tightly to oxygen

Adult is alpha 2 beta 2

Methemoglobin

Myoglobin in the Fe3+ form, poorly binds oxygen