What fundamental concept does the sigmoidal shape of the hemoglobin binding curve illustrate?

The sigmoidal shape of the hemoglobin binding curve illustrates the concept of cooperativity between multiple binding sites. Hemoglobin is a tetrameric protein, meaning it consists of four polypeptide chains, each capable of binding oxygen. As one oxygen molecule binds to one of the heme groups in hemoglobin, it induces a conformational change that increases the affinity of the remaining heme groups for oxygen. This positive cooperativity is reflected in the binding curve, which shows a gradual increase in oxygen binding at low pressures of oxygen that steepens at higher pressures.

This cooperative interaction allows hemoglobin to be highly efficient in oxygen uptake and release: it can pick up oxygen effectively in the oxygen-rich environment of the lungs and offload it in the oxygen-poor tissues. The sigmoidal curve indicates that hemoglobin does not merely bind oxygen in a simple, linear fashion; rather, the binding of each successive oxygen molecule becomes easier due to the structural changes in the protein, demonstrating how multiple binding sites influence each other's behavior.