Q. A 55-year-old woman with a history of chronic obstructive pulmonary disease (COPD) is seen in the clinic for a follow-up visit. Given her difficulty with oxygen saturation, you begin to explain to her the importance of hemoglobin, the oxygen-carrying protein in red blood cells. You tell her that when one oxygen molecule binds to hemoglobin, it causes a conformational change that increases its affinity for more oxygen molecules. What term is used to describe this behavior of hemoglobin?