If the substrate competes for the active site, we have a competitive inhibitor. A great example is methanol and ethanol BOTH compete for alcohol dehydrogenase. Thus if you drank methanol.....we can OVERCOME the methanol by giving IV ethanol. The substrates resemble the active site...The Vmax will be UNCHANGED. In noncompetitive inhibition, a molecule binds to an enzyme somewhere other than the active site. This changes the enzyme's three-dimensional structure so that its active site can still bind substrate with the usual affinity, but is no longer in the optimal arrangement to stabilize the transition state and catalyze the reaction. On the macroscopic scale, noncompetitive inhibition lowers the Vmax. Thus, the enzyme simply cannot catalyze the reaction with the same efficiency as the uninhibited enzyme. Note that noncompetitive inhibition cannot be overcome by raising the substrate concentration like competitive inhibition can. This is a very interesting topic that you will enjoy when you do BioChemistry in Dental School.
Hope this helps.
Dr. Romano