The lock-and-key model proposes that the enzyme’s active site complements the shape of the substrate molecule so both fit together like a lock and key. If their shapes are not complementary, they are unable to bind as no structural changes occur. 

Limitations of the lock and key model include: 

• Assumes that enzymes have a rigid structure and do not change shape on binding with an appropriate substrate
• Does not account for the intricate chemistry of catalytic reactions
• Does not offer an explanation of how the enzyme-substrate complex is stabilized in the transition state
• Does not offer an explanation for binding multiple substrates to the enzyme
• Does not take proton transfer mechanisms into consideration