AS Biology Enzymes

Enzyme activity The reactions of cells including photosynthesis and respiration require the participation of substances called enzymes that are produced by living things. Enzymes are biological catalysts that speed up reactions. They increase the rate at which cellular reactions occur. Enzymes show specificity, that is, they only react with a particular substance. The substance whose rate the enzyme catalyses (speeds up) is called a substrate. The activity of enzymes is influenced, among other things, by the temperature, the pH of the medium and the concentration of the substrate. 1. Effect of Temperature on Enzyme Activity Enzyme activity rises steadily with increasing temperature, doubling for each 100 C rise. At temperatures above 40 0 C the enzyme becomes unstable and the active sites become altered by denaturation. At temperatures above 50 0 C, the enzyme shows no activity because its proteins are already destroyed by heat. However, bacteria living in hot springs can tolerate very high temperatures, of even up to about 100 0 C. 2. Effect of pH on Enzyme Reaction Some enzymes act best at different pH of the medium. Pepsin which digests protein in the stomach, acts best at an acidic pH medium of between 1 and 2. Trypsin, an enzyme produced by the pancreas, shows optimum activity in an alkaline medium, usually at a pH that is between 7 and 9. 3. Effect of Substrate Concentration Given a constant enzyme concentration, increasing the concentration of the substrate increases the rate of reaction. The increase in the rate of reaction ceases when all the active sites of the enzyme are occupied. 4. Effect of Enzyme Concentration Given that all the other factors are constant, the rate of reaction of an enzyme is directly proportional to the concentration of the enzyme. If the substrate concentration falls, the reaction will plateau out to a constant rate. Enzyme Inhibition (a) Competitive inhibitors Some substances that are closely related to the normal substrates of certain enzymes have similar shapes. For example, succinic acid the substrate for succinic dehydrogenase is closely related to malonic acid. When both are present, they compete for the same active sites on the enzyme succinic dehydrogenase. By occupying the active sites of the enzyme, malonic acid prevents succinic acid from reacting with the enzyme and the enzyme is inhibited. This is called competitive inhibition. (b) Non-competitive inhibition A molecule can attach itself to an enzyme and cause the shape and the active sites of the enzyme to alter. Because the shape of the active sites is altered, the substrate is unable to react with the enzyme. Cyanide is a poison that acts by altering the sites of certain respiratory enzyme. Study Question Discuss the factors that influence the rate of action of an enzyme. How are certain poisons likely to affect the action of an enzyme?