| Factors affecting enzymatic reactions
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| Figure 5.1 Reaction profile for enzymatic and nonenzymatic reactions. The basic principles of an enzyme-catalyzed reaction are the same as any chemical reaction. When a chemical reaction proceeds, the substrate must gain activation energy to reach a point called the transition state of the reaction, at which the energy level is maximum. Since the transition state of the enzyme-catalyzed reaction has a lower energy than that of the uncatalyzed reaction, the reaction can proceed faster. ES complex, enzyme-substrate complex; EP complex, enzyme-product complex. |
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| In the case of an inorganic catalyst, the reaction rate generally increases with the temperature of the system. Enzymes function as catalysts at body temperature, but display a temperature optimum in vitro. Because the three-dimensional structure of a protein involves weak bonding interactions,
such as hydrogen and hydrophobic bonds, it can be disrupted by protein denaturation at high temperature.
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Every enzyme has a pH optimum, because ionizable amino acids , such as histidine, glutamate, and cysteine, participate in the catalytic reactions. Since the extremes of pH in our body are 6.9 and 7.7, many enzymes function optimally in this range. However, there are important exceptions. Pepsin, which is secreted by gastric cells and functions in gastric juice, has a pH optimum of 1.5-2.0; trypsin and chymotrypsin have alkaline pH optima, consistent with their digestive activity in pancreatic juice; lysosomal enzymes typically have acidic pH optima. Changes in pH affect the ionic charge of amino acid side chains of enzymes, and can have a dramatic effect on enzymatic activity. In addition, various molecules, including substrates, products, intermediates, and regulatory molecules, also affect the rate of enzymatic reactions.
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