ENZYMES • Enzymes are defined as biological catalysts. • A catalyst is any substance that speeds up • (changes) the rate of a chemical reaction without itself being changed by the reaction. • The catalysts for biochemical reactions that happen in living organisms are called enzymes. • Enzymes are usually proteins, though some ribonucleic acid (RNA) molecules act as enzymes too. • Those that work inside of living cells are called intracellular enzymes while those that work outside living cells are called extracellular enzymes. • The substances on which enzymes act to form products are called substrates. • The part of an enzyme where the substrate fits during an enzyme-catalyzed reaction is called the active site while the other parts of the enzyme are called allosteric sites. • They catalyze both forward and reverse reactions. • They are specific meaning each enzyme acts on only one substrate or a narrow range of related substrates. • Their activity is affected by temperature, PH, substrate concentration, enzyme concentration, inhibitors and cofactors (coenzymes and activators). • Enzymes perform the critical task of lowering a reaction's activation energy that is, the amount of energy that must be put in for the reaction to begin. • Enzymes work by binding to reactant molecules and holding them in such a way that the chemical bond-breaking and bondforming processes take place more readily. • To clarify one important point, enzymes don’t change a reaction’s ∆G value. • That is, they don’t change whether a reaction is energy-releasing or energy-absorbing overall. • That's because enzymes don’t affect the free energy of the reactants or products. • Instead, enzymes lower the energy of the transition state, an unstable state that products must pass through in order to become reactants. • The transition state is at the top of the energy "hill" in the diagram above. Lock-and-key model and induced-fit hypothesis • The lock-and-key model and the induced-fit hypothesis are two potential models for how substrates may bind in the active site of an enzyme. • The lock-and-key model suggests that the substrate is completely complementary in shape to the active site, so that it fits in 'perfectly' just like the way a key (the substrate) fits into a lock (the enzyme). • There is no change in shape of the active site when the substrate binds. • This model was proposed by Emil Fisher in 1894. Induced-fit hypothesis • It's important to remember that the inducedfit hypothesis is similar to the lock-and-key model. • It says that the substrate and active site are not completely complementary, but there is still some complementarity. • In this, an enzyme changes shape slightly when it binds its substrate, resulting in an even tighter fit. • This adjustment of the enzyme to snugly fit the substrate is called induced fit. • This model was proposed by Daniel E. Koshland in 1958. • Also called ‘a hand in a glove model’. • From the diagrams above note that the active site of an enzyme changed slightly to fit the shape of the substrate. • As the products are leaving the active site of an enzyme it retained the original shape. Intracelluler Enzymes • Intracellular enzymes or endoenzymes are a type of enzymes functioning inside the cell. • They are responsible for undergoing metabolic reactions inside the cell of both eukaryotes as well as prokaryotes. • Thus, intracellular enzymes carry out both photosynthesis and cellular respiration inside the cell. • Moreover, these enzymes are responsible for carrying out DNA replication, protein synthesis etc. • Intracellular enzymes are also responsible for the digestion of food inside food vacuoles in unicellular organisms. • This process is known as intracellular digestion. • Generally, lysosomes contain these intracellular enzymes. • In addition, the digestive enzymes in lysosomes are responsible for the cell death of old cells • Furthermore, intracellular enzymes break down large polymers into smaller chains of monomers. • For example, the enzyme endoamylase breaks down large amylose molecules into dextrin chains, which are shorter. • In contrast, exoenzymes break down monomer subunits of large polymers, starting from the ends. Extracellular Enzymes • Extracellular enzymes or exoenzymes are the enzymes which act outside the cell. • Generally, the number of extracellular enzymes is less than the number of intracellular enzymes. • Moreover, they are responsible for extracellular digestion, which occurs in the alimentary canal of animals. • Here, different types of accessory organs secrete digestive enzymes into the lumen of the alimentary canal through which the food passes. • By mixing with these enzymes, carbohydrates, proteins, lipids, and nucleic acids in the food are digested into their monomer units known as monosaccharides and disaccharides, amino acids, fatty acids, and nucleotides, respectively. • Moreover, extracellular enzymes secreted by decomposers to the outside environment are responsible for the digestion of decaying organic matter. • Furthermore, decomposers play a key role in ecosystems, recycling nutrients. • In addition, these organisms can absorb nutrients, which are the products of the extracellular digestion through their cell wall. • Other organisms such as plants can also absorb these nutrients from their roots. • Similarities Between Intracellular and Extracellular Enzymes: • Intracellular and extracellular enzymes are the two types of digestive enzymes that occur in cells. • Both occur in eukaryotes as well as prokaryotes. • They differ by their location of the action. • Based on their action, they have different important functions in the cell. • However, their main function is to undergo digestion of food particles. • Both are protein molecules made up of chains of amino acids. Factors Affecting the Rate of Enzyme activity – Concentration of Enzyme • Increasing enzyme concentration will elevate the chemical reaction rate, as long as there is substrate available for binding. • Once all of the substrate is bound, the reaction will no longer speed up, because there will be nothing for additional enzymes to bind to. • This property is used for determining the activities of serum enzymes during the diagnosis of diseases. 2.Concentration of Substrate • In the presence of a given amount of enzyme, the rate of enzymatic reaction increases as the substrate concentration increases until a limiting rate is reached. • After which further increase in the substrate concentration produces no significant change in the reaction rate. • At this point, so much substrate is present that essentially all of the enzyme active sites have substrate bound to them. • In other words, the enzyme molecules are saturated with substrate. • The excess substrate molecules cannot react until the substrate already bound to the enzymes has reacted and been released (or been released without reacting). Effect of Temperature • The protein nature of the enzymes makes them extremely sensitive to thermal changes. • Enzyme activity occurs within a narrow range of temperatures compared to ordinary chemical reactions. • As you have seen, each enzyme has a certain temperature at which it is more active. • This point is called the optimal temperature, which ranges between 37 to 40C°. • The enzyme activity gradually lowers as the temperature rises more than the optimal temperature until it reaches a certain temperature at which the enzyme activity stops completely due to the change of its natural composition. • On the other hand, if the temperature lowers below the optimal temperature, the enzyme activity lowers until the enzyme reaches a minimum temperature at which the enzyme activity is the least. • The enzyme activity stops completely at 0C°, but if the temperature rises again, then the enzyme gets reactivated once more. 4.Effect of pH • The potential of hydrogen (pH) is the best measurement for determining the concentration of hydrogen ion (H+)in a solution. • It also determines whether the liquid is acidic, basic or neutral. • Generally, all liquids with a pH below 7 are called acids, whereas liquids with a pH above 7 are called bases or alkalines. • Liquids with pH 7 are neutral and equal the acidity of pure water at 25 C°. • Enzymes are protein substances that contain acidic carboxylic groups (COOH–) and basic amino groups (NH2). • So, the enzymes are affected by changing the pH value. • Each enzyme has a pH value that it works at with maximum efficiency called the optimal pH. • If the pH is lower or higher than the optimal pH, the enzyme activity decreases until it stops working. • For example, pepsin works at a low pH (it is highly acidic), while amylase works at a high pH ( it is basic). • Most enzymes work at neutral pH 7.4. Enzymes Inhibition • Enzyme activity can be inhibited in various ways: • Competitive inhibition occurs when molecules very similar to the substrate molecules bind to the active site and prevent binding of the actual substrate. • Penicillin, for example, is a competitive inhibitor that blocks the active site of an enzyme that many bacteria use to construct their cell walls. Non competitive inhibition • Occurs when an inhibitor binds to the enzyme at a location other than the active site. • Sometimes noncompetitive inhibition, the inhibitor is thought to bind to the enzyme in such a way as to physically block the normal active site. • Other times, the binding of the inhibitor is believed to change the shape of the enzyme molecule, thereby deforming its active site and preventing it from reacting with its substrate. • This latter type of noncompetitive inhibition is called allosteric inhibition the place where the inhibitor binds to the enzyme is called the allosteric site. • Frequently, an end-product of a metabolic pathway serves as an allosteric inhibitor on an earlier enzyme of the pathway. • This inhibition of an enzyme by a product of its pathway is a form of negative feedback. • Allosteric control can involve stimulation of enzyme action as well as inhibition. • An activator molecule can be bound to an allosteric site and induce a reaction at the active site by changing its shape to fit a substrate that could not induce the change by itself. • Common activators include hormones and the products of earlier enzymatic reactions. • Allosteric stimulation and inhibition allow production of energy and materials by the cell when they are needed and inhibit production when the supply is adequate.
