One measurement of ligand binding is called the "saturation fraction" or the "association fraction." 

• Ya simply equals the fraction of all receptor ligand binding sites occupied by that ligand under a given set of conditions. In some cases, a protein will only bind one ligand of given kind whereas other proteins, especially multi-subunited proteins, may bind several ligand molecules of the same type.  In fact, many proteins bind multiple different kinds of ligands, each being characterized by its own saturation fraction under specified conditions.
  
• By convention, Ya varies in value from 0 to 1 (or 0% to 100%). For example, if a protein with two binding sites for the one type of ligand is "50% saturated," only 1 out of the 2 ligand binding sites -- on average -- will be occupied by ligand in the entire protein population.  Specifically, different protein molecules in the population will have 0, 1 or 2 ligand molecules bound, but only half of all of the potential ligand binding sites will be occupied if Ya = 0.5, or 50%. 

• In order to determine the amount saturated, simply multiply Ya by the total capacity, i.e., "amount "= Ya * "capacity." 
  
  
• For example:
  
  The amount of water held by a sponge equals the product of the saturation fraction with the total liquid capacity of the sponge.
  
  The amount of water held by a cup equals the product of the saturation fraction with the total liquid capacity of the cup.
  
  The amount of acid saturated with protons (in moles per liter, or moles) equals the product of the saturation fraction with the total concentration of acid (in moles per liter, or moles).  
  
  The amount of oxygen bound to hemoglobin (in moles per liter, or moles) equals the product of the saturation fraction with the total concentration of hemoglobin (in moles per liter, or moles) and the valence or number of oxygen-binding sites, which equals 4 for each hemoglobin molecule.

1. A protein's activity must be quantitatively related to the saturation binding fraction of a given ligand, which can be a substrate and/or regulatory molecule.
2. The biological range of the ligand concentration variations in the cell or in the organism must be known.
3. The temporal fluctuations of the ligand concentration must also be known.

1. Monovalent ligand binding systems
2. Bivalent, non-interactive ligand binding systems
3. Trivalent, semi-interactive ligand binding systems
4. Bivalent, interactive ligand binding systems
5. Multivalent, interactive ligand binding systems
6. Michaelis-Menten enzyme kinetics
7. Ligand-regulated enzyme kinetics