Factors affecting enzyme activity

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Factors affecting enzyme activity: Michaelis and Menten derived the equation for the rate of enzymatic reactions based on the intermediate product theory, namely the Michaelis-Menten equation (for details, refer to Chapter 4 on Microbial Physiology in Environmental Engineering Microbiology). According to the Michaelis-Menten equation, the rate of an enzymatic reaction is influenced by the enzyme concentration and substrate concentration, as well as by temperature, pH, activators, and inhibitors. (1) Effect of enzyme concentration on the rate of enzymatic reactions: It can be seen from the Michaelis-Menten equation and the graphical representation of the relationship between enzyme concentration and the rate of enzymatic reactions that the rate of these reactions is directly proportional to the concentration of enzyme molecules. When the concentration of substrate molecules is sufficient, the more enzyme molecules there are, the faster the rate of substrate conversion. But in fact, when the enzyme concentration is very high, this relationship is not maintained, and the curve gradually becomes flatter. According to the analysis, this may be caused by high concentrations of substrate carrying many inhibitors. (2) Effect of substrate concentration on the rate of enzymatic reaction: In biochemical reactions, when the enzyme concentration remains constant, if the initial substrate concentration is low, the rate of the enzymatic reaction is proportional to the substrate concentration; that is, it increases as the substrate concentration rises. Once all the enzymes have bound to the substrates to form intermediate products, even if the substrate concentration is increased, the concentration of intermediate products does not rise, nor does the rate of the enzymatic reaction increase. It can also be concluded that, under the same substrate concentration, the rate of the enzymatic reaction is proportional to the initial concentration of the enzyme. The higher the initial concentration of the enzyme, the faster its catalytic reaction rate. In actual measurements, even when the enzyme concentration is high enough, the rate of the enzymatic reaction does not increase as the substrate concentration rises; instead, it is even inhibited. The reason is that high concentrations of substrate reduce the effective concentration of water, thereby decreasing molecular diffusivity and slowing down the rate of the enzymatic reaction. An excess of substrate accumulates on the enzyme molecules, forming inactive intermediate products that prevent the enzyme molecules from being released, thereby also reducing the reaction rate. (3) Effect of temperature on the rate of enzymatic reactions: Enzymes exhibit the highest activity and the fastest rate of enzymatic reactions within their optimal temperature range. Within the appropriate temperature range, for every 10°C increase in temperature, the rate of enzymatic reaction can increase by 1 to 2 times accordingly. The optimal temperature for enzymes varies among different organisms. For example, the optimal temperature for various enzymes in animal tissues is 37–40℃ ； The optimal temperature for various enzymes in microorganisms ranges from 25 to 60°C, but there are exceptions; for example, the optimal temperature for Aspergillus niger amylase is 62 to 64°C℃ ； The optimal temperature for the glucose isomerase in Bacillus megaterium, Lactobacillus brevis, Aerobacter aerogenes, etc., is 80℃ ； The optimal temperature for the liquefying amylase of Bacillus subtilis is 85–94°C. It can be seen that the enzymes of some Bacillus species have high thermal stability. Both excessively high and low temperatures reduce the catalytic efficiency of enzymes, that is, they decrease the rate of enzymatic reactions. Enzymes with an optimal temperature below 60°C are mostly destroyed and undergo irreversible denaturation when the temperature reaches 60–80°C ； When the temperature approaches 100°C, the catalytic activity of the enzyme is completely lost. (4) Effect of pH on the rate of enzymatic reactions: Enzymes exhibit activity within their optimal pH range; values above or below this optimal pH reduce enzyme activity. It is mainly manifested in two aspects: ① Changing the charge state of the substrate molecules and enzyme molecules, thereby affecting the binding between the enzyme and the substrate ； ②Either too high or too low a pH can affect the stability of enzymes, thereby causing irreversible damage to them. (5) Effect of activators on the rate of enzymatic reactions: Substances that can activate enzymes are called enzyme activators. There are many types of activators, including ① inorganic cations such as sodium ions, potassium ions, copper ions, calcium ions, etc ； ②Inorganic anions, such as chloride ions, bromide ions, iodide ions, sulfate ions, phosphate ions, etc ； ③Organic compounds, such as vitamin C, cysteine, reduced glutathione, etc. Many enzymes only exhibit catalytic activity or have their catalytic activity enhanced in the presence of a specific appropriate activator; this is known as enzyme activation. Some enzymes are synthesized in an inactive state, and such enzymes are called zymogens. It must be activated by an appropriate activator to become active. (6) Effect of inhibitors on the rate of enzymatic reactions: Substances that can reduce, inhibit, or even destroy enzyme activity are called enzyme inhibitors. It can reduce the rate of enzymatic reactions. Enzyme inhibitors include heavy metal ions, carbon monoxide, hydrogen sulfide, hydrocyanic acid, fluorides, ethyl iodide acetate, alkaloids, dyes, p-chloromercuribenzoic acid, diisopropyl fluorophosphate, ethylenediaminetetraacetic acid, surfactants, and others. The inhibition of enzymatic reactions can be divided into competitive inhibition and non-competitive inhibition. Substances similar in structure to the substrate compete to bind to the active site of the enzyme, thereby reducing the rate of the enzymatic reaction; this effect is known as competitive inhibition. Competitive inhibition is a reversible form of inhibition; by increasing the substrate concentration, the inhibition can ultimately be relieved and the enzyme’s activity restored. Substances similar in structure to the substrate are called competitive inhibitors. When an inhibitor binds to a site other than the enzyme’s active site, the substrate can still bind to the active site, but the enzyme does not exhibit activity; this type of inhibition is known as non-competitive inhibition. Non-competitive inhibition is irreversible; increasing the substrate concentration does not relieve the inhibition of enzyme activity. Inhibitors that bind to sites other than the enzyme’s active site are called non-competitive inhibitors. Some substances can act as inhibitors of one enzyme while also acting as activators of another enzyme.