Assuming you started with a saturated hemoglobin molecule, how many molecules of oxygen would need to be released in order for the hemoglobin to be 75% saturated

only one oxygen molecule  

Explanation:

Hemoglobin (Hb) is an iron-containing oxygen-transport protein localized in vertebrate red blood cells (i.e., erythrocytes). Moreover, the heme group is a component of the Hb protein which is capable of binding or releasing molecular oxygen. Hb consists of four globular protein subunits, and each subunit contains a heme group. This group (heme) binds to an oxygen molecule in a manner that each Hb protein can bind to four oxygen molecules in order to transport them throughout the bloodstream.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.