Construct an explanation of how enzymes lower activation energy to speed up reactions, and how temperature and pH affect enzyme activity (Tennessee Academic Standards for Science, Biology I, BIO1.LS1).

What this topic is asking

The Tennessee LS1 standards ask you to explain how enzymes make the reactions of life happen fast enough at body temperature, and how conditions change their activity. For the Biology I EOC that means knowing that enzymes are proteins that act as catalysts by lowering activation energy, that each enzyme fits a specific substrate (lock and key), and how temperature and pH change the rate, including denaturation. Items very often give a graph of rate against temperature or pH and ask you to interpret it.

What an enzyme does

Every chemical reaction needs an initial input of energy, the activation energy, to get going. Without enzymes, the reactions that keep a cell alive would happen far too slowly at body temperature. An enzyme provides an easier pathway with a lower activation energy, so far more reactant molecules have enough energy to react, and the reaction proceeds quickly. The enzyme does not change how much energy the products contain; it only lowers the "hill" the reactants must climb to begin.

Lock and key: enzymes are specific

The lock-and-key idea links enzymes back to the macromolecules standard: an enzyme is a protein, and a protein's shape determines its function. It also explains denaturation, below, because changing the active-site shape stops the substrate from binding.

Temperature and the optimum

Temperature has two opposing effects on enzyme rate:

• Up to the optimum, raising the temperature makes molecules move faster, so the enzyme and substrate collide more often, and the rate increases.
• Above the optimum, the high temperature breaks the bonds holding the enzyme's shape. The active site changes shape and can no longer bind the substrate. This is denaturation, and it makes the rate fall sharply, usually to zero.

For human enzymes the optimum is around body temperature ($37\,^\circ\text{C}$). On an EOC graph, the rise-then-sharp-fall shape is the signature of an enzyme: rising for more collisions, falling for denaturation.

pH and concentration

Each enzyme also has an optimum pH at which its shape and activity are best. For example, the stomach enzyme pepsin works best in acid, while many other enzymes prefer near-neutral pH. Moving far from the optimum pH disrupts the enzyme's shape and lowers the rate (and can denature it).

Raising the substrate concentration increases the rate, because the enzyme meets substrate more often, but only up to a point: once every active site is occupied (the enzyme is saturated), adding more substrate cannot speed things up further, and the curve levels off.

Try this

Q1. Explain how an enzyme speeds up a reaction without being used up. [2]

• Cue. It lowers the activation energy so more molecules can react, providing an easier pathway; because it is a catalyst it is released unchanged at the end and can be reused.

Q2. State what is meant by denaturation and give one cause. [2]

• Cue. Denaturation is a change in the enzyme's shape (especially the active site) so it can no longer bind its substrate; it can be caused by high temperature or a pH far from the optimum.