Explain how enzymes (a type of protein) lower activation energy and carry out cellular processes, and how temperature, pH, and substrate fit affect enzyme activity (GSE SB1.c).

What this topic is asking

Standard SB1.c treats proteins as molecules that carry out cellular processes, and the most-tested protein on the Georgia Milestones Biology EOC is the enzyme. You must explain how an enzyme lowers activation energy, why its active site makes it specific to one substrate, and how temperature, pH, and substrate concentration change its activity, including what denaturation does. Reading an enzyme-activity graph is a recurring item.

What an enzyme does

Without enzymes, the reactions that keep a cell alive would run far too slowly at body temperature. The enzyme does not supply energy or change the products; it provides a faster route by lowering the energy barrier. The molecule an enzyme acts on is its substrate, and the enzyme binds the substrate at its active site.

Specificity: the active site

Each enzyme works on one substrate (or one type of reaction) because its active site has a specific shape that only the matching substrate fits, like a key in a lock (the lock-and-key model; a refined version is the induced-fit model, where the site adjusts slightly to grip the substrate). This is a direct example of structure determining function: the protein's folded shape is the active site, and that shape is what makes the enzyme specific. Lactase breaks down lactose; it will not break down sucrose, because sucrose does not fit lactase's active site.

What changes enzyme activity

Three factors are tested most:

• Temperature. As temperature rises, molecules move faster, so enzyme and substrate collide more often and the rate rises up to the enzyme's optimum (about 37 degrees Celsius for human enzymes). Above the optimum, heat breaks the bonds holding the enzyme's shape, the active site changes shape, and the rate falls sharply (denaturation).
• pH. Each enzyme has an optimum pH. Too acidic or too basic an environment disrupts the active site, lowering activity. Stomach enzymes (pepsin) work best in acid; most others near neutral pH.
• Substrate concentration. More substrate raises the rate until the enzymes are saturated (all active sites busy), after which adding more substrate makes no difference.

Reading an enzyme graph

EOC items often show rate versus temperature or rate versus pH as a curve that rises to a peak (the optimum) and then falls. To explain it: the rise is more frequent enzyme-substrate collisions; the peak is the optimum condition; the fall is the active site changing shape (denaturation) so the substrate no longer binds. For a rate versus substrate concentration graph, the curve rises and then levels off (plateaus) at saturation, when every active site is occupied.

Try this

Q1. State what an enzyme does to the activation energy of a reaction. [1 point]

• Cue. It lowers the activation energy, so the reaction proceeds faster.

Q2. Explain why raising the temperature far above an enzyme's optimum reduces its activity. [2 points]

• Cue. The heat breaks the bonds holding the enzyme's shape, so the active site changes shape (denatures) and can no longer bind the substrate.