Hemoglobin forms a tetramer (4 subunits) and is responsible for binding and transporting both oxygen and carbon dioxide through the bloodstream.  It does not possess catalytic activity.  However, when the first molecule of oxygen binds, it causes a change in the shape of hemoglobin that makes it easier for the second molecule of oxygen to bind, which then improves the ability of the third molecule of oxygen to bind, and that in turn promotes binding of the fourth and final molecule of oxygen.  Each oxygen can be viewed as a substrate in this regard, and causes slight changes in the shape of the hemoglobin tetramer.  This illustrates which concept of enzyme fit?