Animal Tissues, Organs and Systems » Enzymes and the Lock and Key Model

What are enzymes?

Living things carry out thousands of chemical reactions every second. Without help, many of these reactions would be far too slow to keep you alive. Enzymes are the helpers that speed them up.

Enzymes are large protein molecules. Each enzyme is a long chain that is folded into a special three-dimensional shape. They are biological catalysts. A catalyst speeds up a reaction without being used up, so the same enzyme molecule can work again and again.

Enzymes are found in every living cell. They are also made in organs such as the pancreas and the small intestine, and they work there too. Where each one is made and what it does is covered in the lesson on digestive enzymes.

Key terms:

• Enzyme: a biological catalyst, made of protein, that speeds up a specific reaction.
• Catalyst: a substance that speeds up a reaction and is not used up.
• Substrate: the molecule that an enzyme acts on.
• Product: what the substrate is turned into.
• Active site: the part of an enzyme where the substrate fits.

One enzyme, one reaction

Enzymes are specific. This means each enzyme catalyses only one kind of reaction. An enzyme that works on one substrate will not work on a different one.

The reason is shape. Every enzyme has a small dent in its surface called the active site. The active site has a shape that matches one substrate and no others. Only a substrate with the right shape can fit into it.

The lock and key model

The lock and key model is a simplified way to picture how an enzyme works. The enzyme is the lock and the substrate is the key. Only the right key fits the lock.

A model is a simplified way of explaining something. The lock and key model cannot show every detail, but it explains why enzymes are specific. A substrate with the wrong shape is like the wrong key. It does not fit, so nothing happens.

Temperature and enzymes

Temperature changes how fast an enzyme works.

• Low temperature: particles move slowly, so substrate molecules meet active sites less often. The reaction is slow.
• Warmer: particles move faster and meet active sites more often, so the rate goes up.
• Optimum temperature: the temperature at which the enzyme works fastest.
• Too hot: the enzyme changes shape and stops working.

When the shape of the active site changes, the substrate no longer fits. The enzyme is denatured. Once an enzyme is denatured it cannot work again, so the rate drops quickly to zero.

pH and enzymes

pH has a similar effect. Each enzyme has an optimum pH where it works fastest. If the pH is too high or too low, the shape of the active site changes and the enzyme is denatured.

Different enzymes have different optimum pH values. An enzyme that works best in an acidic place will not work well in an alkaline one.

Calculating the rate of a reaction

The rate of a reaction tells you how fast it goes. For an enzyme reaction you can measure how much product forms, or how much substrate is used up, in a given time.

Rate = amount of product formed ÷ time taken

Sometimes you only know how long the reaction took to finish. Then a faster reaction has a shorter time, so you can use rate = 1 ÷ time.

Always write the units. If the amount is in cm³ and the time is in minutes, the rate is in cm³ per minute (cm³/min).

Exam-style question

An enzyme catalyses the breakdown of a substrate. A student measures the volume of product made at different temperatures. At 20 °C, 6 cm³ formed in 3 minutes. At 60 °C, no product formed.

(a) Calculate the rate of reaction at 20 °C. Give the unit. [2 marks]

(b) Explain why no product formed at 60 °C. [3 marks]

(c) Use the lock and key model to explain why the enzyme will not break down a different substrate. [2 marks]