Catalytic Characteristics of Enzymes
Enzymes are proteins (some may be catalytic RNA) that have unique capacity of speeding up chemical reaction within cells. They accelerate the velocity of the reaction without being altered after the reaction. They function via a variety of mechanisms, but all of them enhance the reaction rate by lowering the activation energy of the reaction. Like the proteins, enzymes can be coagulated by heat, alcohol, concentrated acids and alkaline reagents. Up to now, there have been more than 2,000 enzymes discovered.
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Some characteristics of enzymes are as follows: (1) Enzymes are unique in nature and show varying degree of specificities, while they are highly specific for a particular substrate. The active site of an enzyme shows a strong affinity for a specific substrate, and further slightly changes the conformation of substrate. It is known as an "induced fit" to accommodate the reacting substrate, and this induced fit further leads to a stabilized transition state to lower the activation energy of the reaction. The same catalyst cannot be applied in more than one reaction. Stereo specificity means that enzymes can detect the different optical isomers and react to only one type of isomer. Group specific enzymes refer to those catalyzing a group of substances with specific groups. (2) Enzymes are extremely efficient and possess great catalytic power, transforming about 100 to 10,000 substrate molecules into product per second and proceeding from 103 to 108 times faster than the uncatalyzed reaction. Enzymes do not affect the equilibrium constant in spite of the increase in the conversion rate of substrate into product. The number of substrate molecules converted to product is called the turnover number. Only a small number of enzymes are typically required under normal cellular conditions to catalyze a chemical reaction. |
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(3) Enzymes itself remain unchanged during the reaction. Although the amino acid residues of enzyme may get broken or form covalent bonds with the substrate, it can typically reform those broken bonds or separate with substrates, allowing the enzyme to bind with more substrates.
(4) Enzymes can be allosterically regulated by a variety of means, but the activity of the catalysts cannot be controlled. Sometimes, activators and coenzymes are necessary for enzymatic catalysis, which are helpful to increase enzyme potency due to the presence of a weak bond between the enzyme and a metal ion. Inorganic substances are often known as activators. Additionally, there are also molecules that can allosterically inhibit enzyme function by modifying its conformational shape. The use of many drugs is related to their action as enzyme inhibitors in the body.
(5) The effectiveness of an enzyme catalyst could achieve maximum at its optimum temperature. Both the enhancement and decrease of the optimum temperature result in a decrease in catalytic activity.
(6) Biochemical catalysis of enzyme also depends on the pH of the solution. An enzyme exerts its full potential at an optimum pH ranging between 5-7 pH values.
(7) The catalytic activity of enzymes can be inhibited by competitive inhibitors, noncompetitive inhibitors or irreversible inhibitors. Competitive inhibitors reversibly bind to the active site of the enzyme, thus blocking the substrate from binding to the enzyme. Noncompetitive inhibitors bind to any site of the enzyme other than the active site, enabling the enzyme less active or inactive. Irreversible inhibitors form bonds with enzymes to make them inactive.
(8) Increase in the concentration of the reactants could increase the reaction rate until the enzyme become saturated with the substrate, while increase in the amount of enzyme will continuously enhance the rate of the reaction.
(9) Enzymes could function reversely, signifying that the enzyme does not determine the direction of reaction. It only functions to accelerate reaction rate until equilibrium is achieved.
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