And we'll talk more about the heme group when we discuss hemoglobin. Hemoglobin is basically a biomolecule or protein that binds oxygen and carries oxygen to the different tissues found in our body. Now an example of a loosely health group that we're going to discuss is our coenzyme A which is basically a molecule that is involved in cellular respiration. Now an enzyme that is without its cofactor is known as the APO enzyme while the enzyme that has the cofactor bound to that enzyme is known as the hollow enzyme. Now the question still remains how exactly does the substrate actually bind to the active side of our enzyme? So we have two different models that basically describe two different ways by which our substrate can bind to our enzyme.