Introduction to Enzymes
The following has been excerpted from a very popular Worthington publication which was originally published in 1972 as the Manual of Clinical Enzyme Measurements. While some of the presentation may seem somewhat dated, the basic concepts are still helpful for researchers who must use enzymes but who have little background in enzymology.
Factors Affecting Enzyme Activity
Knowledge of basic enzyme kinetic theory is important in enzyme analysis in order both to understand the basic enzymatic mechanism and to select a method for enzyme analysis. The conditions selected to measure the activity of an enzyme would not be the same as those selected to measure the concentration of its substrate. Several factors affect the rate at which enzymatic reactions proceed - temperature, pH, enzyme concentration, substrate concentration, and the presence of any inhibitors or activators.
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Factor Affecting Enzyme Activity
Each enzyme shows its optimal activity under certain experimental conditions. Therefore, while working with enzymes, all experimental condition must be ideal. Before we consider those factors, it would be worth while to define the enzyme activity to get optimum reaction rates.
Units of enzyme activity This is expressed in terms molecules of substrate transformed per minute per molecule of enzyme. The dimensions of substrate molecules are expressed as micromoles of substrate per micromole of enzyme(i.e., number of enzyme units/micromole of enzyme). This is sometimes referred to as molecular activity also.
Effect of Temperature
A typical effect of temperature on the rate of enzyme catalyzed reaction is shown. That at 0oC, the rate is approximately zero, and as the temperature is raised the rate increase until an optimal activity is reached. At still higher temperature the rate decrease reaching the zero level. According to vant Hoff law, a rise of 10oC will double the rate of a reaction. If we assume that at a given temperature To the rate of reaction is V, the matter of 2V at To+ 10o. This is expressed in terms of ratio of velocities of reaction at two temperature 10o apart and indicated by Q10 law, which holds true for enzyme catalyzed reactions at low temperatures.
The temperature range of most enzymes lies between 30o and 50oC. It is interesting to note that certain microbial enzymes have a higher optimal temperature due to which they resist denaturation. This resistance is imposed by the environment that induces an element of adjustment to new ambient temperature. Nevertheless, most enzymes are completely destroyed at 100oC, but a few such as ribonuclease and lecithinase A can resist even boiling temperature.
Effect of pH
Effect of Activators
The rates of enzyme catalyzed reactions are altered by certain ions which behave as activators. A large number of enzymes requiring nucleoside di- and triphosphates. invariably need divalent metal ions like Mg2+ or Mn2+ , which are occasionally replaceable. Many enzymes require monovalent cations, usually Na+, K+ or NH4+ for maximum catalytic activity . Amylases require CI¯ ions for activity , while carbonic anhydrase would need Zn2+ for maximum function. Usually these ions interact with the substrate so that its binding with the active site will be most favored.
Many enzymes are sensitive to oxidizing and reducing agents and the ability of of oxidation or reduction of an enzyme is known as redox potential. It is the electromotive force, measurable in millivolts, developed by the solution when in physical contact with the platinum electrode as compared to the normal hydrogen electrode at zero potential. The redox potential of en enzyme is either positive or negative owing to its relative oxidizing or reducing ability in comparison to hydrogen.
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