Final answer:
The two major competing theories on the workings of enzymes are the lock-and-key model and the induced fit model.
Step-by-step explanation:
The two major competing theories on the workings of enzymes are the lock-and-key model and the induced fit model. The lock-and-key model suggests that the enzyme's active site and the substrate fit together perfectly, like a lock and key. On the other hand, the induced fit model proposes that the enzyme's active site is somewhat flexible and can change shape to accommodate the substrate.