Enzymes & Catalysis

You’ll be able to

Explain how enzymes lower activation energy without being consumed
Relate active-site shape and the induced-fit model to substrate specificity
Predict how temperature, pH, and inhibitors change reaction rate

The energy hill every reaction must climb

Even a reaction that releases energy overall must first reach an unstable, high-energy transition state. The energy needed to get there is the activation energy (Eₐ) — a hill the reactants must climb before they can roll downhill to products. At body temperature most biological reactions have activation energies far too high to happen on a useful timescale. Cells cannot simply add heat, so they use catalysts instead: molecules that provide an easier path over the hill.

Enzymes: biological catalysts

An enzyme is a biological catalyst, almost always a protein whose folded shape is its function. Enzymes lower the activation energy of a specific reaction, so it proceeds far faster at cellular temperatures. Crucially, an enzyme is not consumed — it emerges unchanged and can catalyze the same reaction again and again. It also does not change whether a reaction is energetically favorable or alter the overall energy released; it only lowers the barrier, speeding up the path to the same destination.

Enzyme catalysis

enzyme + substrate ⇌ enzyme·substrate → enzyme + product

The enzyme binds substrate, forms an enzyme–substrate complex, releases product, and is regenerated — never used up.

Active site and induced fit

Each enzyme has an active site — a pocket whose shape and chemistry fit a specific substrate, giving enzymes their specificity. The older "lock and key" picture is refined by the induced-fit model: when the substrate enters, the active site changes shape slightly to grip it more snugly, straining bonds and stabilizing the transition state. Because the active site is built from precisely folded amino acids, anything that changes the protein’s shape changes how well substrate fits — the key link between structure and catalytic function.

What changes the rate

Reaction rate responds to conditions. Temperature: rate rises with warmth (more collisions) up to an optimum, then falls sharply as heat denatures the enzyme and destroys the active site. pH: each enzyme has an optimal pH; extremes disrupt the bonds holding its shape and denature it (pepsin favors acidic stomach pH ~2, while most cellular enzymes prefer ~7). Substrate concentration: rate climbs until all active sites are busy (saturation), then plateaus. Inhibitors: a competitive inhibitor resembles the substrate and blocks the active site, while a noncompetitive inhibitor binds elsewhere (an allosteric site) and reshapes the enzyme so the active site no longer works.

Worked example

An enzyme catalyzes a reaction at a steady rate. A researcher adds a molecule that binds the active site directly, and the rate drops — but adding a large excess of substrate restores the original rate. What kind of inhibitor is this, and why does extra substrate reverse it?

1.The inhibitor binds the active site itself, so it must resemble the substrate and compete with it for that pocket — this is a competitive inhibitor.
2.Competitive inhibition is a numbers game: inhibitor and substrate contend for the same site, and whichever is more abundant wins more of the time.
3.Flooding the enzyme with excess substrate tips the odds back toward substrate binding, so active sites fill with substrate and the rate recovers.

Answer: It is a competitive inhibitor: it occupies the active site, but adding excess substrate outcompetes it and restores the rate. (A noncompetitive inhibitor binds elsewhere and could not be reversed this way.)

Checkpoint

How does an enzyme speed up a chemical reaction?

Watch out

Enzymes do not make an unfavorable reaction favorable, and they do not change the net energy released. They only lower the activation barrier so the reaction reaches equilibrium faster. Saying an enzyme "provides energy" for a reaction is a common exam error.

Checkpoint

A human enzyme works best at 37°C. When heated to 65°C its activity collapses and does not return upon cooling. What is the best explanation?

Checkpoint

A noncompetitive inhibitor differs from a competitive inhibitor because it:

On the exam

The AP throughline for enzymes is "shape → function." Any factor that changes the enzyme’s shape — temperature, pH, or a noncompetitive inhibitor — changes the active site and thus the rate. When you explain a rate change, name the effect on shape and the active site explicitly.

Answer the 3 checkpoints as you read.