ENZYMES

Question 1

Catalyst

Answer 1

A substance that increases the rate of a chemical reaction and is not changed by the reaction

Question 2

Enzyme

Answer 2

A protein that functions as a biological catalyst

Question 3

Enzymes and reactions

Answer 3

Most chemical reactions happening in living things are helped by enzymes. The speed at which they can catalyse reactions is sufficient to sustain life. Most enzyme names end in -ase, e.g. lipase, protease. Enzymes usually speed up reactions, but some slow them down. Some enzymes control reactions to build up molecules (synthesise them), e.g. starch phosphorylase: Enzyme molecules are proteins. Each molecule has a special shape where the substrate fits (a complementary shape). Once the enzyme molecule and substrate come into contact, one or more products are formed.

Question 4

Effect of temperature on enzymes

Answer 4

The optimum (best) temperature for enzyme-controlled reactions is around 37 ° C (body temperature). Increasing the temperature above the optimum temperature slows the reaction down

Question 5

Effect of pH on enzymes

Answer 5

The pH of a solution is how acidic or alkaline it is. Most enzymes have an optimum pH (at which they work best) – usually around neutral (pH 7) – but there are some exceptions:

• pepsin, pH 2.0 – in the stomach, with hydrochloric acid; • salivary amylase, pH 6.8 – in the mouth; • catalase, pH 7.6 – in plants, e.g. potato;
• pancreatic lipase, pH 9.0 – in the duodenum. The ‘wrong’ pH slows down enzyme activity, but this can usually be reversed if the optimum pH is restored.

Question 6

Enzymes are

Answer 6

very specific (each chemical reaction is controlled by a different enzyme, so the enzyme has a high specificity). This is because of the shape of the active site. The shape of the active site of protease will be different from the shape of the active site of amylase, for example. This means that protease cannot break down starch and amylase cannot break down proteins. In other words, the enzyme is specific. For an enzyme to catalyse a reaction, the enzyme molecule and the substrate molecule need to meet and join together by means of a temporary bond. This temporary structure is called the enzyme– substrate complex. The product or products are then released and the enzyme molecule can combine with another substrate molecule.

Question 7

Effect of temperature on enzymes 1

Answer 7

Enzymes work very slowly at low temperatures. This is because they have a low kinetic energy, so there are few collisions with the substrate molecules. As the temperature is increased, the reaction rate increases because kinetic energy and, therefore, rate of collisions increases. However, above the optimum temperature for the enzyme, the reaction rate starts to decrease. This is because enzyme molecules start to permanently lose their shape at high temperature. This deforms the active site, so the enzyme and substrate cannot fit together (so no reaction). This effect is called denaturing. Most enzymes are denatured above 50 ° C.

Question 8

effect of ph on enzymes

Answer 8

An enzyme works best at its optimum pH. At a higher or lower pH, the enzyme is less effective and an extreme pH can denature the enzyme – the active site is deformed permanently. This means there is no longer a complementary fit between the enzyme molecule and the substrate.