1. Enzymes and How They Work

2. Enzymes Enzymes are proteins. They are Biological catalysts (speed up the rate of reactions in living things without themselves being changed). Without enzymes metabolic reactions would take months to occur. Since they are not changed the enzymes can be used over and over again.

3. Enzymes Enzymes speed up reactions by lowering the amount of energy needed to get the reaction to occur; the Activation energy barrier.

5. Enzymes Enzymes are specific – each enzyme will only catalyse one reaction. Enzymes are thought to act by a lock and key method or by an induced fit method. The shape of the enzyme brings the 2 reacting chemicals together at the enzymes active site so that they react together quickly.

6. Lock and Key Hypothesis The specific action of an enzyme with a single substrate can be explained using a Lock and Key analogy first postulated in 1894 by Emil Fischer. In this analogy, the lock is the enzyme and the key is the substrate. Only the correctly sized key (substrate) fits into the key hole (active site) of the lock (enzyme).

7. Lock and Key Hypothesis Smaller keys, larger keys, or incorrectly positioned teeth on keys (incorrectly shaped or sized substrate molecules) do not fit into the lock (enzyme). Only the correctly shaped key opens a particular lock. This is illustrated below.

9. The Induced Fit Model Not all experimental evidence can be adequately explained by using the so- called rigid enzyme model assumed by the lock and key theory. For this reason, a modification called the induced-fit theory has been proposed.

10. The Induced Fit Model The induced-fit theory assumes that the substrate plays a role in determining the final shape of the enzyme and that the enzyme is partially flexible. This explains why certain compounds can bind to the enzyme but do not react because the enzyme has been distorted too much.

11. The Induced Fit Model Other molecules may be too small to induce the proper alignment and therefore cannot react. Only the proper substrate is capable of inducing the proper alignment of the active site.

12. Induced Fit Model

13. Types of Enzymes Enzymes which function within the body cells are called Intracellular Enzymes. Enzymes which function mainly outside the cells e.g. the digestive enzymes are known as Extracellular enzymes. Some enzymes can become dangerous if they start working at the wrong time or in the wrong place, so the body has organised various methods of containment.

14. Types of Enzymes Secreting the Enzyme in a harmless form – E.g. Pepsin is secreted as pepsinogen. It does not become active pepsin until it is acted on by stomach acid. Lysosomes – These are small membrane-bound structures which hold some strong enzymes inside, they are not released until they are needed.

15. Types of Enzymes Arranged on a Membrane – Some enzymes work in an orderly arrangement (as if moving on a conveyor belt). – E.g. the enzymes involved in photosynthesis and respiration.

16. Factors Affecting Enzymes pH – Enzymes have a specific ph at which they work best. This is called the optimum pH. – E.g. the enzymes in the mouth work at neutral, those in the stomach work at pH 1 or 2 and those in the intestine work in alkaline conditions.

17. Effect of pH

18. Factors Affecting Enzymes Temperature – At low temperatures enzyme action is slow. – There is a steady increase in activity as temperature rises, until an optimum temperature is reached. – Above the optimum temperature, the rate of action slows down. – At about 45ºC to 50ºC, the protein becomes denatured (loses its shape).

19. Effect of Temperature

20. Factors Affecting Enzymes Enzyme Concentration – In general an increase in the rate of enzyme concentration will increase the rate of enzyme action.

21. Factors Affecting Enzymes Substrate Concentration – In general an increase in substrate concentration will increase the rate of enzyme action.

22. Enzymes can be Poisoned – E.g. by heavy metal ions such as mercury and lead. – Cytochrome oxidase, an enzyme in the electron transport system of respiration, is poisoned by cyanide. – Cyanide kills because an affected body cannot respire. Factors Affecting Enzyme

23. Enzymes can act as Poisons – If they get into the wrong part of the body. – Several types of snake, bee, and scorpion venom are harmful – they destroy blood cells or other tissue of other animals. Factors Affecting Enzyme

24. Co-factors and Co-enzymes – Some enzymes consist solely of protein, others have 2 parts, a protein and another chemical component called a co-factor. – For some enzymes this co-factor is a metal. – The organic co-factors are called co- enzymes, and most are vitamins. Factors Affecting Enzyme