Hemoglobin Physiology

Hemoglobin

red pigment protein in red blood cells that binds and carries oxygen

Why do we need hemoglobin?

Oxygen has very low solubility in water and does not dissolve well in plasma (only 1.5% of oxygen is freely dissolved) , therefore, we need hemoglobin to carry the other 98.5% of oxygen effectively throughout the body

What is the structure of hemoglobin?

HEmoglobin is made up of 4 protein subunits:

• 2-alpha chains

• 2 beta-chains

  • Each subunit has a heme group

What is a heme group?

active site that binds to oxygen and requires an iron atom (Fe2+) to function

• oxygen binds directly to the iron atom

How many oxygen molecules can each hemoglobin bind?

Each heme binds to 1 oxygen molecule, there are 4 heme groups per hemoglobin, so each hemoglobin protein binds 4 oxygen molecules total

Oxyhemoglobin( HbO2) =

bright red

Deoxyhemoglobin (HHb)

Dull red

The color of oxygenated and deoxygenated depends on how

heme reflects light when O2 is bound or unbound

Cooperative binding

means each O2 molecules binding makes it easier for the next one to bind

• 1st O2 makes 2nd easier

• 2nd makes 3rd easier

• 3rd makes 4th easiest

Cooperative unbinding

means that once one O2 is released, the next ones are released more easily

What does the Hemoglobin-Oxygen Dissociation Curve show?

Graph showing how much oxygen is bound to hemoglobin at different PO2 levels

• reflects how oxygen is loaded and unloaded depending on local conditions

What is the shape of the Hemoglobin-Oxygen Dissociation curve?

S-shaped (sigmoidal) due to cooperative binding

Why is the S-shaped curve or hemoglobin helpful?

• Allows hemoglobin saturation to remain high even if external PO2 drops slightly

• Hemoglobin “hangs on” to oxygen at high PO2

• At lower PO2, it rapidly unloads large amounts of oxygen, ensuring oxygen is delivered efficiently where needed most