eGr12OM BioResoBook(6)(1).pdf

Resource Book G.C.E. (Advanced Level) Biology electricity The role of Enzymes in regulating metabolic reactions An enzyme is a macromolecule, which acts as a biological catal...

Resource Book G.C.E. (Advanced Level) Biology electricity The role of Enzymes in regulating metabolic reactions An enzyme is a macromolecule, which acts as a biological catalyst. Enzymes are produced in living cells/ General characteristics of an enzyme: 1. Most of the enzymes are globular proteins. 2. Enzymes are biological catalysts. They lower the activation energy of the reaction they catalyze (increases the rate of reaction). 3. Most enzymes are heat liable/ sensitive 4. Their presence does not alter the nature or properties of the end products ` of any reaction. 5. Enzymes are highly specific to the substrate (substrate specific) 6. Most enzyme catalyzed reactions are reversible. 7. The rate of enzyme activity is affected by pH, temperature and substrate concentrations. 8. They are not being used up during the reaction. 9. Enzymes possess active sites where the reaction takes place. 10. Some enzymes need non-proteinous components to catalyse the reaction which are known as cofactors. G.C.E. (Advanced Level) Biology Resource Book Fig 2.30 - The relationship between activation energy and the enzyme The mechanisms of enzyme action The reactant and enzyme acts on is referred to as the substrate. The enzyme binds to its substrate forming enzyme-substrate complex. While enzyme and substrate form their complex, catalytic action of the enzyme converts the substrate to the product. Enzyme + substrate Enzyme-substrate complex Enzyme + Product The reaction catalyzed by each enzyme is very specific. The specificity of an enzyme results from its shape. The substrate binds to a specific region of the enzyme. This region is called the active site. The active site is formed by only a few amino acids. Other amino acids are needed to maintain the shape of the enzyme molecule. The shape of the active site is complementary to the shape of the specific substrate of the enzyme, and hence important in the substrate specificity of the enzyme. The shape of the active site of an enzyme is not always fully complementary to its substrate. As enzymes are not rigid structures, the interactions between substrate and active site may slightly change the shape of the active site, so that the substrate and the active site become complementary to each other. This is called induced fit mechanism. The tight fit not only brings the substrate molecules and the active site close to each other, but also ensures the correct orientation of the molecules to help the reaction to proceed and catalyzes the conversion of substrate to product. Thereafter, the product departs from the active site of the enzyme. The enzyme is then free to take another substrate molecule into its active site. Resource Book G.C.E. (Advanced Level) Biology Fig 2.31: Induced fit between an enzyme and its substrate Cofactors Non-proteinuos components which are essential for the catalytic activities of certain enzymes are called cofactors. These cofactors bind to the enzymes in two ways. Some tightly bind and remain permanently and others loosely bind temporarily. Loosely bound cofactors are reversible under certain circumstances. Organic cofactors are called co-enzymes. e.g. derivatives of vitamins e.g. NAD, FAD and biotin Inorganic co-factors – e.g. Zn2+, Fe2+, Cu2+ Factors affecting the rate of enzymatic reactions 1. Temperature 2. pH 3. Substrate concentration 4. Enzyme concentration 5. Inhibitors Temperature Increase in temperature increases molecular motion. Therefore the speed of the moving molecules of both enzymes as well as the substrate will be accelerated. This will enhance the colliding probability for both enzyme active sites and substrate molecules. More collision between the enzyme active sites and substrate molecules generate greater chances for the reaction to occur. This can continue up to a certain G.C.E. (Advanced Level) Biology Resource Book point, after which there is a rapid decline in enzyme activity. This point is referred to as optimum temperature. This may vary from organism to organism. e.g. most of the human enzymes have optimum temperature around the body temperature (35˚C-40˚C). Optimum temperature of bacteria in hot springs is about 70˚C. When the temperature increases beyond the optimum temperature, the hydrogen bonds, ionic and other weak chemical bonds of enzyme active sites may be disrupted. This will result a change in the shape of the active site of enzyme which will alter the complementary nature of the active site of enzyme molecules. Therefore, the complementary binding of enzyme active sites and substrate molecules will be prevented. The above event is called as denaturation of enzyme molecules. Therefore the rate of enzyme catalyzed reaction will start to decline when the temperature increases beyond the optimum temperature and stops completely at certain temperature, although rate of collision will keep on increasing. Fig -2.32 The graph of Rate of reaction (V) vs Temperature(T) pH Enzymes function most efficiently within a certain pH range despite maintaining temperature of the environment constant. The narrow range of pH in which a particular enzyme catalyzed reaction takes place is named as the pH range. The pH at which the highest rate of reaction occurs is the optimum pH of the enzyme.The alteration in pH above or below the optimum pH may lead to decline in enzyme activity. This is due to the alteration of chemical bonds involving in formation of enzyme substrate complex. In most enzymes optimum pH range is 6-8, but there are exceptions. Pepsin works best at pH 2 and optimum pH for Trypsin is 8. Resource Book G.C.E. (Advanced Level) Biology Fig 2.33- Rates of reaction of two enzymes at various pH values Substrate concentration Increasing substrate concentration increases the probability of collision between the enzyme and substrate molecules with correct orientation. However the enzyme molecules will be saturated after a particular concentration and therefore there will not be any further increase in the rate of reaction. Enzyme inhibitors Certain molecules or ions selectively bind permanently or temporarily to the enzyme molecules and prevent them from forming enzyme-substrate complex. These substances are called inhibitors. They are either binding reversibly with weak interactions or binding irreversibly through covalent bonds. e.g. Irreversible inhibitors: toxins, poisons Reversible inhibitors- drugs used against microbes Competitive inhibitors Most of these are reversible inhibitors. These chemicals resemble the shape and nature of the substrate. Therefore they compete with the substrate selectively for the active site of certain enzymes. As a result of the above, the number of active sites available for the enzymes may decline and therefore reduces the rate of enzyme catalyzed reactions. The above situation may be reversed by increasing the substrate concentration. e.g. Protease inhibitor of drugs against HIV.-change G.C.E. (Advanced Level) Biology Resource Book substrate products active site enzyme enzyme substrate enzyme Inhibitor complex active site enzyme Inhibitor bind to the enzyme active site Fig 2.34: Competitive inhibitors Non-competitive inhibitors These chemicals do not compete with substrate molecules. They interrupt enzymatic reaction by binding to a part of the enzyme other than the active site. This causes the enzyme molecule to change its shape in such a way that the active site becomes less effective for the formation of enzyme substrate complex. Fig 2.35: noncompetitive inhibitors Regulation mechanism of enzymatic activity in cells Allosteric regulation of enzymes In many cases, the molecules that naturally regulate enzyme activity in a cell behave like reversible non-competitive inhibitors. Regulatory molecules (either activators or inhibitors) bind to specific regulatory sites elsewhere (other than the active site) of the molecule via non-covalent interactions and affect the shape and function of the enzyme. It may result in either inhibition or stimulation of an enzyme activity. Resource Book G.C.E. (Advanced Level) Biology a.) Allosteric activation and inhibition Most enzymes regulated by allosteric regulation are made from two or more subunits. Each sub unit composed of a polypeptide chain with its own active site. The entire complex oscillates between two different shapes one catalyzing active and other inactive. In this two forms regulatory molecules bind to a regulatory site called allosteric site, often located where subunits join. When an activator binds with this regulatory site, stabilizes the shape with functional active sites. Whereas the inhibitor binds with the regulatory site, it stabilizes the inactive form of enzyme. Subunits of enzyme arranged in a way through which they transmit the signals quickly other subunits. Through the interaction of subunits even a single activator or inhibitor molecule that bind to one regulatory site will affect the active site of all sub units. e.g. ADP function as allosteric activator bind to the enzyme and stimulates the production of ATP by catabolism. If the supply of ATP exceed demand catabolism shows down as ATP bind to the same enzyme as inhibitor. b.) cooperativity This is another type of allosteric activation. Binding of one substrate molecule can stimulate binding or activity at other active site. Thereby increase the catalytic activity. e.g. hemoglobin (not an enzyme) is made up of four subunits each with an O2 binding site. The binding of a one molecule of O2 to one binding site increases the affinity for O2 of the remaining binding site. c.) Feedback inhibition In feedback inhibition, a metabolic pathway is stopped by the inhibitory binding of its end product of a process to an enzyme. Thereby limit the production of more end products than required and thus wasting chemical resources. Feedback inhibition Feedback inhibition is an essential process regulates the end products produced in metabolism. e.g. ADP function as allosteric activator and stimulates the production of ATP during the catabolism. In case ATP supply exceeds demand, catabolism slows down as ATP molecules function as allosteric inhibitor. Energy needed for all living processes is obtained directly from ATP. ATP is mainly produced by a process call ed cellular respiration, in living cells.