The bonding of an enzyme to its substrate forms an enzyme-substrate complex. The catalytic action of the enzyme converts its substrate into the product or products of the reaction. Each reaction is extremely specific, distinguishing between closely related compounds, including isomers. For example, the enzyme sucrase will only act on sucrose and will not bind to any other disaccharide. The molecular recognition of enzymes is due to the fact that they are proteins, which are defined as being macromolecules with unique three-dimensional conformations.
Therefore, the specificity of enzymes is a consequence of its shape which results from its amino acid sequence. The enzyme binds to the substrate at a region known as the active site, a groove on the surface of the protein. The active site of an enzyme is formed by a few of the enzyme’s amino acids while the rest of the protein molecules determine the configuration of the active site. The specificity of the enzyme is attributed to the shape of the active site and the shape of the substrate.
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When the substrate enters the active site interactions between its chemical groups and the amino acids of the protein cause the enzyme to change shape slightly in order to fit more tightly around the substrate. This induced fit brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction. The substrate is held in the active site if the enzyme by weak interactions, usually hydrogen bonds. When more substrate molecules are available they are able to access active sites of enzyme molecules more often.
This in turn aids in speeding up the rate of the reaction. However, when concentration reaches a high enough point a problem arises because all enzyme active sites are already engaged. The high concentration of substrate saturates the enzyme and the rate of the reaction is determined by the rate at which the active site converts substrate to product. The only way to overcome saturation and speed up the rate of product formation is to add more enzymes. Each enzyme has a specific pH at which they are most active, turning substrate into products at high speeds.
For most enzymes the optimal pH is within the 6 to 8 range. For example, trypsin is a digestive enzyme in the intestine and has a pH of 8. However, there are exceptions to the range stated above. For example, the enzyme pepsin in the stomach has a pH of 2. Trypsin would not be able to function in the same acidic environment as pepsin; such shifts in pH would denature an enzyme. Temperature shifts could also denature an enzyme. The rate of enzyme reactions increases with increasing temperature maintaining a direct relationship until a certain point is reached.
At high temperatures molecules move faster and more collision occurs increasing the speed of the reaction. If the temperature reaches a point beyond the optimal temperature of the enzyme then the rate of the reaction drops drastically. This is a consequent of the disruption of hydrogen bonds, ionic bonds, and other weak interactions by thermal agitation. These bonds and interactions within the enzyme stabilize active conformation but once they are disrupted the protein molecule denatures. Some chemicals inhibit the action of specific enzymes.
Inhibition is reversible when enzyme inhibitors bind to an enzyme by weak bonds. These enzyme inhibitors resemble normal substrate molecules and compete to enter the active site of an enzyme. They reduce the productivity of enzymes by blocking the substrate from entering its corresponding active site. Enzyme inhibitors can be overcome by increasing the concentration of the substrate so that as active sites become available, more substrate molecules than inhibitors are around to gain entry.