Answer: hemoglobin
Explanation: Hemoglobin is the component of the red blood cells that carry oxygen found in the blood. Hemoglobin is a tetrameric protein having four subunits made up of two alpha subunits and two beta subunits. Hemoglobin has an oxygen binding site in each of its subunit. The binding of one oxygen molecule to a subunit increases the affinity of hemoglobin to bind another. This phenomenon in which the binding of oxygen atom to one subunit of hemoglobin enhances the binding of another oxygen molecule is known as positive cooperativity.