Final answer:
The effect in which the binding of one molecule of oxygen to hemoglobin enhances further binding is known as cooperativity. The increased affinity for additional oxygen molecules as more bind is characteristic of hemoglobin's function and is depicted in its S-shaped oxygen-binding curve.
Step-by-step explanation:
The idea that the binding of one molecule of oxygen to hemoglobin enhances further binding of oxygen is called cooperativity. This concept describes how each successive oxygen molecule that binds to hemoglobin increases the affinity, or likelihood, that additional oxygen molecules will bind. Essentially, the binding of oxygen to one of the hemoglobin's heme sites alters the protein's structure in a way that facilitates the binding at the remaining heme sites. This positively cooperative binding is reflected in the S-shaped oxygen-binding curve seen in studies of hemoglobin's oxygen affinity, illustrating that hemoglobin's affinity for oxygen increases as more oxygen binds. Conversely, as the percent saturation of hemoglobin decreases without all of the oxygen dissociating, the affinity for oxygen also declines.
Hemoglobin is not unique in its ability to bind to oxygen; it can also bind to other small molecules such as carbon monoxide (CO) and nitrogen dioxide (NO). However, the binding of oxygen and the cooperative effects seen in hemoglobin's oxygen affinity dynamics are largely due to the specific interactions at the molecular level, including the reversible transfer of electron density between iron in the heme and the oxygen molecule.