Enzyme action, the factors and progress curves

Last time

Before we start: last time, in Polysaccharides, lipids and water, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Why reactions need a helper

Sugar can sit in a bowl for years without turning into carbon dioxide. Yet inside your cells the same molecules are broken down in seconds. The difference is an enzyme, a biological catalyst that makes reactions fast at body temperature.

Enzymes and activation energy

An enzyme is a globular protein that acts as a catalyst. It speeds up a reaction and is not used up. Every reaction needs a minimum energy to start, called the activation energy. An enzyme lowers the activation energy. It does this by holding the substrate in the active site, in the right position and under a little strain, so the bonds break or form more easily. An enzyme does not change the amount of product at the end. It only gets there faster.

The active site

The active site is a small region of the enzyme, usually a cleft or pocket, formed by the folding of the polypeptide into its tertiary structure. Its shape depends on the sequence of amino acids, so only one type of substrate has a complementary shape. This is specificity. The substrate binds to form an enzyme substrate complex. The reaction happens, and the products leave. The enzyme is free to be used again.

Lock and key, then induced fit

In the lock and key hypothesis, the active site is already complementary to the substrate. It does not change shape before, during or after the reaction. In the induced fit hypothesis, the active site is not an exact fit at first. When the substrate binds, the active site changes shape to wrap around it. Induced fit explains better how the enzyme puts strain on the substrate bonds, and why it can work on closely related substrates.