A level Biology exam revision resources written by A level Examiners


The activity of enzymes is strongly affected by changes in pH, and temperature. Each enzyme works best at a certain pH, and temperature. The activity of enzymes decreases away from these values. This is due to the importance of the tertiary structure of the enzyme, in it's function. Also the tertiary structure is highly dependant on ionic interactions, and hydrogen bonds. The values for pH, and temperature, which give an enzymes greatest rate of reaction are known as the optimum values.



The diagram above shows as the temperature of the solution containing the substrates and enzyme molecules increases so does the rate of reaction. This occurs because as the temperature increases the kinetic energy of the molecules also increases. As the molecules are moving around faster there are more collisions which lead to reactions between enzyme molecules, and substrate molecules. This effect continues up to the optimum temperature..   Exam Advice

Above the optimum temperature, the rate of reaction decreases, as the increasing kinetic energy causes the hydrogen bonds, holding the tertiary structure of the enzyme together, to break. This causes the active site to change shape. This is known as denaturation. The substrate can therefore no longer fit the active site, and the rate of reaction decreases until all the enzyme molecules are denatured, and the rate of reaction is zero..   Exam Advice



The pH affects the ionic charges, on some of the R groups present in the amino acids, making up the enzyme molecule. Therefore as a result of changing the pH, the tertiary structure will change, and this will alter the active site, and the substrate will no longer be able to fit.


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