The pH scale is used to measure the acidity or alkalinity of a sample and describes how many hydrogen ions or hydroxides are present in the sample. The change of pH will lead to the ionization of amino acids atoms and molecules, change the shape and structure of proteins, thus damaging the function of proteins. Enzymes are also proteins, which are also affected by changes in pH. Very high or very low pH will lead to the complete loss of the activity of most enzymes. The pH value at which the enzyme is most active is called the optimal pH value. Show
 pH Effects Enzyme Activity The structure of the enzyme has a great influence on the activity of the enzyme. In other words, changes in the structure of the enzyme affect the rate of chemical reactions. When the pH value of the reaction medium changes, the shape and structure of the enzyme will change. For example, pH can affect the ionization state of acidic or basic amino acids. There are carboxyl functional groups on the side chain of acidic amino acids. There are amine-containing functional groups in the side chain of basic amino acids. If the ionized state of amino acids in the protein is changed, the ionic bonds that maintain the three-dimensional shape of the protein will change. This may lead to changes in protein function or inactivation of enzymes. pH Effects Substrates PH not only affects the activity of the enzyme, but also affects the charge and shape of the substrate, so that the substrate cannot bind to the active site, or cannot be catalyzed to form a product. In a narrow range of pH, the structural and morphological changes of enzymes and substrates may be reversible. However, if the level of pH changes significantly, the enzyme and substrate may be denatured. In this case, the enzyme and the substrate do not recognize each other, so there will be no reaction. Optimal pH All enzymes have an ideal pH value, which is called optimal pH. Under the optimum pH conditions, each enzyme showed the maximum activity. For example, the optimum pH of an enzyme that works in the acidic environment of the human stomach is lower than that of an enzyme that works in a neutral environment of human blood. When the pH value deviates from the ideal conditions, the activity of the enzyme slows down and then stops. The enzyme has an active site at the substrate binding site, and the shape of the active site will change with the change of pH value. Depending on the extreme extent of the enzyme and pH changes, these changes may permanently "destroy" the enzyme, or once the conditions return to the desired range of the enzyme, the enzyme will return to normal. Related Services
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What happens in cells and what do cells need?The genetic code of all life on Earth is made from DNA. Enzymes are biological catalysts which speed up chemical reactions. Physical factors affect enzyme activity. At low temperatures, the number of successful collisions between the
enzyme and
substrate is reduced because their
molecular movement decreases. The reaction is slow. The human body is maintained at 37°C
as this is the temperature at which the enzymes in our body work best. This not true of the enzymes in all organisms. Higher temperatures disrupt the shape of the active site, which will reduce its activity, or prevent
it from working. The enzyme will have been denatured. Enzymes therefore work best at a
particular temperature. Proteins are chains of
amino acids joined end to end. This chain is not straight – it twists and folds as
different amino acids in the chain are attracted to, or repel each other. Each enzyme is comprised of proteins made of these twisting and folding amino acids, and therefore the enzyme has a unique shape. This structure is held together by weak forces between the amino acid molecules in the chain. High temperatures will break these forces. The enzyme, including its active site, will change shape and the substrate no longer fit. The rate of reaction will be affected, or the reaction will stop. A graph to show the effect of temperature on enzyme activity: The effect of pHEnzymes are also sensitive to pH. Changing the pH of its surroundings will also change the shape of the active site of an enzyme. Many amino acids in an enzyme molecule carry a charge. Within the enzyme molecule, positively and negatively charged amino acids will attract. This contributes to the folding of the enzyme molecule, its shape, and the shape of the active site. Changing the pH will affect the charges on the amino acid molecules. Amino acids that attracted each other may no longer be. Again, the shape of the enzyme, along with its active site, will change. Extremes of pH also denature enzymes. The changes are usually, though not always, permanent. Enzymes work inside and outside cells, for instance in the digestive system where cell pH is kept at 7.0pH to 7.4pH. Cellular enzymes will work best within this pH range. Different parts of the digestive system produce different enzymes. These have different optimum pHs. The optimum pH in the stomach is produced by the secretion of hydrochloric acid. The optimum pH in the duodenum is produced by the secretion of sodium hydrogencarbonate. The following table gives examples of how some of the enzymes in the digestive system have different optimum pHs:
A graph to show the effect of pH on an enzyme's activity: Suggest an enzyme that would produce a trend as shown in the graph above. Pancreatic protease (trypsin). Effects of concentrationSubstrate concentrationEnzymes will work best if there is plenty of substrate available. As the concentration of the substrate increases, so does the enzyme activity. This means that more substrate can be broken down by the enzymes if there is more substrate available. This does not mean that the enzyme activity does not increase without end. This is because the enzyme can't work any faster even though there is plenty of substrate available. So when the amount of available substrate exceeds the amount of enzymes, then no more substrate can be broken down. The enzyme concentration is the limiting factor slowing the reaction. Enzyme concentrationAs the concentration of the enzyme is increased, the enzyme activity also increases. This means that more substrate will be broken down if more enzyme is added. Again, this increase in enzyme activity does not occur forever. So when the amount of available enzyme exceeds the amount of substrate then no more substrate can be broken down. The substrate concentration is the limiting factor slowing the reaction.
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How is enzyme activity influenced by ph?
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