Enzymes
ENZYMES Enzyme is protein in nature, it acts as an organic catalyst which speeds up many chemical reactions in organisms. Enzyme works on a substance called substrate and change it into product. Enzyme possesses a specific region called active site which has a distinct shape to which the substrate binds
Examples of Enzymes Enzymes are usually named by adding ase to the name of the substrate. E.g. maltase digests maltose, lipase digests lipids, carbohydrase digests carbohydrates and protease digests proteins etc.
Properties of Enzymes Rapid in action Enzymes increase the rate of metabolic reaction by lowering the activation energy barrier, thereby allowing reactions to proceed readily. The enzyme merely speeds up the reaction.
Activation energy Most chemical reactions do not occur spontaneously (energy must be added for a chemical reaction to start). There is an energy barrier. One way to activate a reaction is to provide a sufficient amount of energy (e.g. through heating) for the substrate to overcome the energy barrier. This amount of energy to activate a reaction is called the activation energy.
Activation Energy However, heat will kill the cell. Enzymes can lower the activation energy and so allow reactions to take place at lower temperatures.
Properties of Enzymes
Properties of Enzymes Specificity Enzymes are specific in reaction they catalysed, i.e. a given enzyme will catalyze only one reaction (in both directions).
Properties of Enzymes A small amount is enough. Enzymes are not destroyed by the reaction they catalyze and can be used again, so small amounts of enzymes is enough. (They are reusable)
Properties of Enzymes An enzyme can work in either direction i.e. speed up both forward and backward reactions. A + B C + D Metabolic reactions are reversible and direction in which they proceed depends on the relative amount of substrate and products present.
Mechanism of enzyme reaction All enzymes are protein molecules, each maintaining a specific three-dimensional structure. Substrate binds to the active site of the enzyme and a temporary structure called the enzyme-substrate complex is formed.
Mechanism of enzyme reaction The action of an enzyme can best be explained by using lock and key hypothesis, with the substrate acting as a lock and the enzyme as the key. The substrate molecules combine with the enzyme to form an enzyme-substrate complex.
The Induced-fit Hypothesis The recent interpretation is that active sites of enzyme molecules may not necessarily be exactly right in shape to fit the substrate molecules in the beginning. The enzyme molecules have a certain extent of flexibility in shape. On binding with the enzyme molecules, the substrate molecules induce the enzyme molecules to change in shape. The conformations of the active sites will then fit the shapes of the substrate molecules.
Factors affecting Enzyme action ph ALL enzymes are sensitive to ph. The concentration of hydrogen affects the stability of tertiary structure of protein molecules; or change the ionization of the amino acids residues at the active sites of the enzymes. Every enzyme has its own ph in which it functions most efficiently
ph Factors affecting Enzyme action
Factors affecting Enzyme action Temperature Below 5 o C, the rate of reaction is very low because enzymes are inactivated at this temperature. Between 5 o C and 40 o C, the rate increases steadily whereas 40 o C is the optimum temperature for many enzymes and it is the temperature at which the reaction proceeds most rapidly. The higher the temperature, the higher the chances for molecular collisions to take place.
Factors affecting Enzyme action If the temperature is further increased above the optimum temperature, then a decrease in the rate of reaction occurs despite the increasing frequency of collisions. This is because the secondary and tertiary structures of the enzyme have been disrupted, and the enzyme is said to be denatured.
Factors affecting Enzyme action Concentration of enzyme Provided that the temperature and other conditions are suitable for the reaction, and there are excess substrate molecules, the rate of reaction is directly proportional to the enzyme concentration. If the amount of substrate is restricted, it may limit the rate of reaction. The addition of further enzyme cannot increase the rate.
Factors affecting Enzyme action Concentration of enzyme
Factors affecting Enzyme action Concentration of enzyme
Factors affecting Enzyme action Concentration of substrate The rate of reaction increases with increasing concentration of substrate molecules and the concentration of enzyme being kept constant. However, when the substrate concentration reaches a certain level the system becomes saturated (i.e. ALL enzyme molecules are used up). When this point has been reached, the only way to increase the rate of reaction is to raise the concentration of enzyme.
Factors affecting Enzyme action Concentration of substrate
Factors affecting Enzyme action Concentration of substrate
Competitive inhibition Inhibition
Competitive inhibitor they compete with the normal substrate for the active site of enzyme. The degree of inhibition depends on the relative concentrations of substrate and inhibitor.
Regulation of enzyme activity Negative feedback inhibition Biochemical pathways usually involve a number of linked reactions. Each reaction is catalysed by a specific enzyme. Regulatory site
Regulation of enzyme activity accumulation of one of the products formed near the end of the chain inhibits the action of an enzyme used in one of the earlier reactions. This is possible when the enzyme has a regulatory site (not the active site) to which the end product will attach.
Regulation of enzyme activity Attachment of the end product to the regulatory site changes the shape of the active site This makes the active site unfit for the substrate to bind and thus inhibits the subsequent reactions Negative feedback ensures that reactions are used efficiently and prevent the excess manufacture of end products. The control of enzyme action helps to maintain a stable internal environment in living organisms.
Regulatory sites are not just used to turn off reactions, they can be used to speed up the reaction too: The end product of a chain of chemical reactions may attach to the regulatory site in such a way that it improves the fit of substrate to the active site enzymes that show these kinds of regulatory activity are called allosteric enzymes
Applications of enzymes Biological Washing Powders: With proteases and lipases
Applications of enzymes Meat fibres, particularly beef, are sometimes too tough to eat after cooking. The effective component of meat tenderizer is papain, a protease isolated from papaya, which helps to loosen the meat fibres by partially breaking down the proteins inside.
Applications of enzymes Contact lenses have to be cleaned regularly because a film of fat or protein tends to be deposited on them. Such deposits can act as substrates for micro-organisms which may damage the lens or the eye. After the contact lens has been rubbed with or immersed in the hydrogen peroxide solution, any remaining peroxide must be destroyed because it could damage the eye. This is achieved by adding a solution of catalase to catalyze the decomposition.
Learning objectives: the protein nature of enzymes. the role of enzymes as catalysts in lowering activation energy through the formation of enzyme-substrate complex the concept of active site and enzyme specificity. the induced-fit model of enzyme action. the effect of temperature, ph, enzyme concentration and substrate concentration on the rate of enzyme reactions. the effects of cofactors, reversible inhibitors (competitive and non-competitive) and irreversible inhibitors on the rate of enzymatic reactions. end-product inhibition. the application of enzymes, e.g. biological washing powder and meat tenderizer.