Enzymes

In short: Enzymes are proteins that speed up chemical reactions in living things without being used up. Each enzyme has an active site that fits its substrate, and it works best at an optimum temperature and pH: too hot, and it denatures and stops working.

Starting point: what you already know

• A chemical reaction turns reactants into products.
• Reactions go faster when particles collide more often or with more energy (Year 10 chemistry, rates of reaction).
• A catalyst speeds up a reaction without being used up.

What is an enzyme?

An enzyme is a protein that acts as a biological catalyst. It speeds up a reaction in a living thing and is not used up, so it can be used again and again.

• The substance an enzyme acts on is its substrate.
• The part of the enzyme the substrate fits into is the active site.
• Most enzymes work on only one substrate. They are specific.

Enzymes speed reactions up by lowering the activation energy: the energy needed to get the reaction started. Without enzymes, the reactions in your cells would be far too slow to keep you alive.

How an enzyme works

1. The substrate collides with the active site and binds, forming an enzyme-substrate complex.
2. The reaction happens.
3. The products are released. The enzyme is unchanged and ready again.

Lock and key model: the active site has a fixed shape that matches the substrate, like a key in a lock.

Induced fit model: the active site is flexible. It changes shape slightly as the substrate binds, gripping it more tightly. This is the better model today because it explains how an enzyme can strain the bonds of its substrate.

Factors that affect enzyme activity

Temperature. As temperature rises, particles move faster and collide more, so activity rises. Above the optimum temperature (about 37 degrees for human enzymes), the bonds holding the enzyme's shape break. The active site changes shape and the substrate no longer fits. The enzyme is denatured. This is permanent. At low temperatures enzymes are just slow, not denatured.

pH. Each enzyme has an optimum pH. Pepsin in the stomach works best at about pH 2. Amylase in saliva works best at about pH 7. Too far from the optimum and the enzyme denatures.

Substrate concentration. More substrate means more collisions, so activity rises, until every active site is busy. Then the graph levels off, because the enzyme is saturated.

Enzyme concentration. More enzyme gives more active sites, so a faster rate, as long as there is plenty of substrate.

Quick summary

• Enzymes are proteins that act as biological catalysts. They are specific and reusable.
• The substrate binds to the active site to form an enzyme-substrate complex.
• Induced fit: the active site changes shape slightly to grip the substrate.
• Activity rises with temperature to an optimum, then falls as the enzyme denatures.
• Each enzyme has an optimum pH. Activity levels off when all active sites are busy.

Quiz