what is the name given to the theory that explains how a substrate fits the active site of an enzyme?

The theory that explains how a substrate fits the active site of an enzyme is called the "Lock and Key Model." This model suggests that the enzyme's active site and the substrate have specific complementary shapes, fitting together like a key fits into a specific lock. Another related theory is the "Induced Fit Model," which explains that the enzyme's active site undergoes a conformational change to fit more closely with the substrate after initial binding. Both models describe specificity in enzyme-substrate interactions but differ in how the fit is achieved.