Most enzymes are proteins, some are nucleic acids (RNA). Enzymes have enormous catalytic power. They greatly enhance the rate at which specific chemical reactions take place. Enzymes regulate chemical reactions. An enzyme is biological catalyst that can accelerate specific chemical reaction by lowering the required activation energy but it self remains unaltered and can be used again. The substance on which enzyme acts are called reactants or substrates. Enzyme activity depends upon the concentration of enzyme concentration of substrate, temperature and pH. Many digestive enzymes like Pepsin and trypsin, all enzyme names end with the suffix –ase and are named after their substance.
Enzyme structure:
Enzymes are complex three dimensional globular protein molecules or nucleic acids that vary in size from small, simple with a molecular weight of 10,000 to highly complex molecules with molecular weights upto one million. Many enzymes are pure proteins that are delicately folded and interlinked chains of amino acids. Binding of the substrate to the enzyme changes the enzyme’s shape, a phenomenon called induced fit. The active site’s embrace of the substrate brings chemical groups of the active site into positions that enhance their ability to work on the substrate and to catalyze the chemical reaction. When the reaction is complete, the product of catalyzed reaction is released and the enzyme resumes its previous shape and is ready to catalyze another chemical reaction. For convenience the active enzyme is termed as a holoenzyme. It may in turn comprise of major protein part of enzyme the apoenzyme. A number of other chemical substances help the enzyme to be functional. These chemicals are termed as prosthetic group and may be coenzyme, cofactors.
Enzyme function: When a substrate molecule binds to an enzyme’s active site, an enzyme substrate complex (ES) forms. This is the essential first step in enzyme catalysis and can be summarized as
Enzyme (E) + Substrate (S) ------> Enzyme Substrate Complex (ES) --------> Products (P) + Enzymes (E).
<------ <---------
Once the unstable high energy ES forms, amino acid side groups of the enzyme are placed against certain bonds of the substrate. These side groups stress or distort the substrate bonds lowering the activation energy needed to break the bonds. The bonds break releasing the substrate which now reacts to produce the final product and release the enzyme.
Factors affecting Enzyme activity:
Any condition that alters three dimensional shape of an enzyme also affects the enzyme’s activity. Two factors that affect enzyme activity are temperature and pH.
(1) Temperature:
The shape of prein or nucleic acid is determined largely by hydrogen bonds present in its structure. Temperature changes easily, disrupt hydrogen bonds thus changing the structure of enzyme. In higher vertebrates such as birds and mammals, the body temperature is between 35 and 40°C as such the enzyme present in these animals function best within this narrow temperature range. Below 35°C the bonds that determine protein shapes are not flexible enough to permit the shape change necessary for substrate to fit into a reactive site. Above 40°C, the bonds are too weak to hold the protein in proper position and to maintain its shape. When proper shape is lost, the enzyme is destroyed; this loss of shape is called denaturation.
(2) pH:
Most enzymes also have optimum pH usually between 6 and 8. With a change in pH, the shape of the enzyme can also be altered. When the pH is too low, H+ ions combine with R groups of the enzyme’s amino acids, reducing their ability to bind with substrate. Acidic environments can also denature enzymes not adapted to such conditions. Some enzymes function at low pH. For example pepsin, the enzyme found in stomach of mammals has an optimal pH of about 2. Pepsin functions at such a low pH because it has an amino acid sequence that maintains it’s ionic and hydrogen bonds, even in the presence of large number of H+ ions (low pH). On the other hand trypsin is active in more basic medium (PH9) found in small intertue of mammals. Slawary any lose acts around 7.5 pH. Generally the pH optimum of an enzyme reflects the pH of the body fluid in which the enzyme is found.
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