Final answer:
The sigmoidal curve of a protein in a graph of fractional saturation versus ligand concentration indicates cooperative binding. This means that after the first ligand molecule binds, the protein's affinity for subsequent ligands increases, then plateaus as it becomes saturated.
Step-by-step explanation:
If a newly-identified protein has a sigmoidal curve in a graph of fractional saturation versus ligand concentration, several important properties of this protein can be deduced. The sigmoidal or S-shaped curve suggests that the protein exhibits cooperative binding, similar to the binding of oxygen to hemoglobin. This means that the binding of one ligand molecule to the protein increases its affinity for subsequent ligand molecules. However, at some point, as the ligand concentration increases, it becomes more difficult to bind additional ligands, leading to a plateau in the curve. This property is also described by the dissociation constant (K value), which indicates the ligand concentration at which the binding site on a particular protein is half occupied.
The graph would show that it is easier for the second and third ligands to bind after the first, due to a change in the protein's conformation. Conversely, the binding of the fourth ligand might be more difficult. This cooperative aspect of ligand binding is critical to the functioning of many biomolecules, including enzymes and transport proteins such as hemoglobin.