Handout 6 - Oxygen Binding Curves

What is this graphical representation showing?

Ligand (L) Binding

• Generalized for binding that is saturable

• Shows hyperbolic plot; not all binding curves look like this, but most do!)

What is the y-axis representing on the ligand (L) binding graph?

Theta (θ), or fractional occupancy of ligand binding sites (minimum of 0, maximum of 1)

What is fractional occupancy?

The mole fraction of available binding sites that have a ligand bound.

What are the parameters for mole fractions?

The value varies from 0 (no ligand bound) to 1 (all available sites have ligand bound)

• When the mole fraction is 0.5, then 50% of available sites have ligand bound.

What is the value, the ligand concentration required to get to ½ maximal occupancy, called?

K_d, its a reflection of the binding affinity

The smaller the K_d value…

The tighter the binding affinity

How can we quantify the binding of O2 in myoglobin?

We can quantify the binding of O2 to the heme on the protein as a function of the partial pressure O2.

Units (kPa, tor, atm)

For myoglobin, what is being plotted on the graph?

Theta (θ) or fractional occupancy of O2 binding sites (sometimes designated as Y_O2) is plotted vs pO2 (partial pressure of oxygen)

What is the general shape for the Mb graph?

It is a rectangular hyperbola; we see saturation binding at low pO2.

What is P₅₀?

• The index of binding affinity (it is the K_d)

• The pO₂ required to get 50% of the available sites bound to oxygen.

The lower the P₅₀ …

the tighter the binding affinity

What is the P₅₀ needed for O₂ to tightly bind to Mb (in torr)?

3 torr

In this graph, what is venous pressure?

• The dotted line on the left

• Shows the p_O2 (about 30 torr) typically found in venous circulation.

  • Low end of the scale, in the tissues, far from the point in which oxygen enters the body.

In this graph, what are the three takeaways for O₂ binding to myoglobin?

1. Hyperbolic curve for oxygen binding to myoglobin

2. O₂ binding to Mb is saturated at very low p_o2

3. The smaller the P₅₀, the greater the fractional occupancy at a given pO₂