Enzymes and the active site

Last time

Before we start: last time, in Biological molecules and DNA structure, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Speeding up life

Without help, digesting one meal would take weeks, because the reactions are far too slow at body temperature. Enzymes make them happen in seconds. They work because each enzyme has an active site with exactly the right shape for one substrate.

What is an enzyme?

An enzyme is a protein that functions as a biological catalyst. A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction. Because it is not used up, one enzyme molecule can be used again and again, thousands of times a second.

Why life needs enzymes

Enzymes are needed in all living organisms to keep reactions fast enough to sustain life. Digestion, respiration and photosynthesis are all controlled by enzymes. Some enzymes break large molecules down, like amylase on starch. Others build large molecules up.

The key words

The substrate is the molecule the enzyme acts on. For amylase, the substrate is starch. The products are what is made. For amylase, the product is maltose, a sugar. The active site is the part of the enzyme where the substrate binds.

Watch an enzyme work

The substrate fits into the active site, like a key into a lock, forming an enzyme-substrate complex. The reaction happens, the products leave, and the enzyme is ready again, unchanged.

Complementary shapes

The shape of the active site is complementary to the shape of its substrate. They fit together exactly. A different molecule has a different shape, so it cannot fit. So each enzyme is specific: it catalyses only one reaction. Amylase digests starch, but not protein.

Three steps

First, the substrate collides with the enzyme and fits into the active site. Second, an enzyme-substrate complex forms, and the reaction takes place. Third, the products leave the active site, and the enzyme is unchanged.

Three digestive enzymes

Learn these three. Each enzyme, its substrate, and the products. The name often tells you the substrate: lipase for lipids, protease for protein.

Surface area helps

Enzymes can only act on substrate they can reach. Breaking food into small pieces, or fat into small droplets, gives a larger surface area. More substrate meets the enzyme, so digestion is faster.