Lock and Key Hypothesis
Substrate molecules has a complementary shape and charge to the active site of an enzyme. Hence, substrate molecules fits into the active site (like key into a lock) forming an enzyme-substrate complex.
At the active site:
Enzyme-substrate complex is stabilized by intermolecular ionic and weak H-bonds between the contact residues and substrate molecules.
Catalytic R-groups of amino acids at the active site are positioned to act on the bonds within the substrate molecules.
The H-bonds and ionic bonds between substrate molecule and catalytic R-groups weaken the bonds within the substrate, causing them to break.