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Enzymes are biological catalysts. There are optimum temperatures and pHpH: Scale of acidity/alkalinity. pH below 7 = acidic, pH above 7 = alkaline values at which their activity is greatest. Enzymes are also proteins, and usually denatured above about 45ºC.

Enzymes are important in respiration. Aerobic respiration releases energy from glucose.

What are enzymes?

Enzymes are biological catalysts - catalysts are substances that increase the rate of chemical reactions without being used up. Enzymes are also proteinsproteins: organic compounds made up of amino acid molecules. One of the three main food groups, proteins are needed by the body for cell growth and repair. that are folded into complex shapes that allow smaller molecules to fit into them. The place where these substrate molecules fit is called the active site.

The animation shows how this works. In this example, two small molecules join together to make a larger one.

If the shape of the enzyme changes, its active site may no longer work. We say the enzyme has been denatureddenatured: If a protein is denatured, its structure and function is altered. This can be caused by heat, altered pH or by chemical agents.. They can be denatured by high temperatures or extremes of pHpH: Scale of acidity/alkalinity. pH below 7 = acidic, pH above 7 = alkaline. Note that it is wrong to say the enzyme has been killed. Although enzymes are made by living things, they are proteins, and not alive.

Temperature and enzymes

As the temperature increases, so does the rate of reaction. But very high temperatures denature enzymes.

The graph shows the typical change in an enzyme's activity with increasing temperature. The enzyme activity gradually increases with temperature until around 37ºC, or body temperature. Then, as the temperature continues to rise, the rate of reaction falls rapidly, as heat energy denatures the enzyme.

Graph showing pH and enzyme activity. Between pH 4.5 and pH 8, enzyme activity increases steadily. It peaks at pH 8, then decreases fairly rapidly

Temperature and enzyme activity

pH and enzymes

Changes in pH alter an enzyme’s shape. Different enzymes work best at different pH values. The optimum pH for an enzyme depends on where it normally works. For example, intestinal enzymes have an optimum pH of about 7.5. Enzymes in the stomach have an optimum pH of about 2.

Graph showing temperature and enzyme activity. Between 0 and 40ºC, enzyme activity increases steadily. It peaks at 40°C (optimum temperature), then decreases rapidly

pH and enzyme activity


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