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. We have the lock and key model as well as the induced fit model. Now the induced fit model is the model is the theory that basically is accepted nowadays while the lock and key model is the older model that is basically no longer really accepted. And we'll see why that's the case in just a moment.