Enzymes in Biological Systems and Industry

Biological catalysts make reactions fast enough

Living organisms depend on chemical reactions: food must be broken down, useful molecules made and harmful substances removed. Many of these reactions would happen too slowly at the temperatures inside an organism without enzymes.

An enzyme is a biological catalyst, usually a protein. Like other catalysts, it increases reaction rate without being used up or permanently changed by the reaction. It can therefore act repeatedly on further reactant molecules.

Enzymes provide an alternative reaction pathway with a lower activation energy — the minimum energy needed for a reaction to occur. At the same temperature, more reacting particles have enough energy to react. This allows biological reactions to proceed rapidly without heating cells to damaging temperatures.

How an enzyme acts on its substrate

The reactant on which an enzyme acts is called its substrate. The enzyme has a region called the active site, where the substrate binds temporarily and the chemical reaction takes place.

The shape of the active site fits particular substrates. This helps explain why enzymes are specific: an enzyme that acts on one substance will not necessarily act on another. The lock-and-key model represents this fit, with the enzyme as the lock and the substrate as the key.

After the reaction, the products leave the active site. The enzyme is available to catalyse another reaction. Some enzymes break a large molecule into smaller molecules; others help join molecules or rearrange them.

Enzymes in living organisms

Amylase is a digestive enzyme found in saliva and produced by the pancreas. It catalyses the breakdown of starch into smaller, soluble sugars. This helps convert a large food molecule into substances that can be processed further during digestion.

Catalase, found in cells including liver cells, breaks down hydrogen peroxide. Hydrogen peroxide can damage tissues, so its rapid removal protects cells. The reaction produces water and oxygen:

$$2\mathrm{H_2O_2} \xrightarrow{\text{catalase}} 2\mathrm{H_2O} + \mathrm{O_2}$$

Catalase is shown above the arrow because it helps the reaction happen but is not an overall reactant or product.

Using enzymes to make useful products

Industry also uses enzymes to speed up useful chemical changes. Faster reactions can increase the amount of product made in a given time. Enzymes can allow processes to operate under milder conditions, reducing the energy needed for heating and therefore lowering operating costs.

In fermentation, enzymes inside yeast cells convert sugars into ethanol and carbon dioxide without oxygen:

$$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$$

This process is used to produce alcoholic drinks. Yeast is the microorganism; its enzymes are the catalysts that make the chemical reactions happen.

Another example is invertase, which catalyses the breakdown of sucrose into glucose and fructose. The resulting mixture is called inverted sugar syrup. Here, the enzyme helps make a useful sugar product rather than being part of the product itself.

Conditions must suit the enzyme

The advantages of enzymes depend on maintaining suitable conditions. Each enzyme has an optimum temperature and pH at which it works fastest.

At low temperatures, enzyme-catalysed reactions are slow. Warming usually increases their rate at first, but excessive heat changes the enzyme’s three-dimensional shape. This is denaturation: the active site no longer fits the substrate properly, so enzyme activity falls. Unsuitable pH can also alter the active site and reduce activity.

Industrial operators therefore control temperature and pH to keep their chosen enzyme effective. Simply making a process hotter is not always a way to make it faster.

Catalytic action

• Enzymes are biological catalysts: they increase reaction rate without being used up or permanently changed.
• They lower activation energy, allowing more particles to react at the same temperature.
• A specific substrate binds to the ; products leave and the enzyme can act again.