lock-and-key versus induced fit
How does an enzyme manage to grab just the right molecule out of the crowded soup of the cell and hold it perfectly for chemistry? Two pictures have been proposed, and the contrast between them tells you a lot about how proteins really behave.
The older picture is lock-and-key, proposed by Emil Fischer in 1894. It says the enzyme's active site is a rigid shape, and the substrate is a key cut to fit it exactly; only the right key slots into that lock, which neatly explains specificity. The newer, more accurate picture is induced fit, proposed by Daniel Koshland in 1958. It says the active site is not rigid but somewhat flexible: when the right substrate approaches, the enzyme actually changes shape, closing around the substrate like a hand wrapping around a ball, or a glove molding to a hand as it slides in. The binding and the shape change happen together. This snug closing not only grips the substrate but helps line up the catalytic side chains and strain the substrate toward its transition state.
Why does the difference matter? Lock-and-key is a fine first sketch and still a useful shorthand for specificity, but induced fit is closer to reality and explains things lock-and-key cannot: why some enzymes only become active once their substrate arrives, and how binding at one spot can reshape another. Be honest about this in your own thinking: a rigid-key model is an approximation; real proteins are flexible, breathing machines, and that flexibility is part of how they work.
Hexokinase, the enzyme that tags glucose with a phosphate, visibly clamps shut around the glucose like a Pac-Man closing its mouth — a textbook image of induced fit that a rigid lock-and-key could never capture.
Real enzymes often close around their substrate — flexibility the rigid lock-and-key picture misses.
Lock-and-key is the simpler, older model; induced fit is the more accurate one. Lock-and-key is not 'wrong' so much as incomplete — it ignores the protein's real flexibility.