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ENZYME DEFINITION ENZYME CLASSIFICATION Enzyme Specificity How Enzyme Lower Ea Meaning of Ea The energy difference Molecules collide with each other....

ENZYME DEFINITION ENZYME CLASSIFICATION Enzyme Specificity How Enzyme Lower Ea Meaning of Ea The energy difference Molecules collide with each other. between reactants and a Reactant absorb sufficient energy to overcome activation energy. high-energy intermediate, Reactants are converted to an activated condition, transition state. the transition state (T*), The reactant can be converted into products. which is formed during The rate of reaction increase. the conversion of reactant to product. Enzyme Function Provides catalytic groups which enhance the formation of transition state. Stabilises the ‘T’ state and enhances product formation. Provides an alternate reaction pathway by lowering the energy of activation (Ea). Therefore, rate of reaction accelerate. Enzyme does not alter the free energy of reactants or products and does not change the equilibrium of the reaction. Form > Catalytic site Protein folding > Active site Site which reaction take place > > Binding site Site which substrate bind Change in conformation Binding of of enzyme enzyme form > ES complex Lock and Key Model The active site of enzyme is rigid and has have fixed shape. The substrate must be fully complementary to the conformation of active site of enzyme. Induced-fit Model The conformation of active site is not fully complementary to the shape of substrate. However, the active site is more flexible. When substrate bind on the active site, the conformation of active site of enzyme will change. This allow the substrate fit precisely to the active site of enzyme. Properties of Enzyme Reusable - conformation of active site do not alter during the reaction. Specificity - each enzyme catalyse different reaction-active site recognise and bind to the correct substrate. Catalytic action can be regulate - to ensure rate of product formed not in excess of amount needed Catalytic efficiency - enzyme speed up reaction 10 to 10 faster than uncatalysed reaction. 38 Factors affecting enzyme catalysed reactions: Substrate concentration 1. Michaelis Menten plot 2. Hyperbolic 3. Describes relationship between reaction velocity (Vo) and substrate concentration [S]. 4. V max 5. Km V max The rate of an enzyme catalysed reaction increases with substrate concentration until a maximal velocity (Vmax) is reached. When [S] < Km, the reaction velocity is first order where the reaction velocity is proportional to substrate concentration. When [S] > Km, the reaction velocity is zero order where the reaction velocity is constant and independent of substrate concentration. Vmax is achieved at high substrate concentration when all binding sites on enzyme molecules are saturated with substrate. The velocity of a reaction is the number of substrate molecules converted to product per unit time. Reaction velocity is expressed as µmol per minute. Increase of [ E ] will increase Vmax. Km (Michaelis constant) Km is the substrate concentration at which half of the maximum velocity is reached (Vmax/2). Low Km reflects high affinity of enzyme for substrate. Example: Km of glucokinase for glucose is 10 mmols/L while Km of hexokinase for glucose is 0.05mmols/L. This mean 50% molecules of hexokinase are saturated even at a low concentration of glucose. Hexokinase has more affinity for glucose than glucokinase. Increase of [ E ] do not affect Km. The Lineweaver-Burk plot Factors affecting enzyme catalysed reactions:Temperature Velocity increase with temperature until a peak velocity is reached. More substrate molecules having sufficient energy to pass over the energy barrier and form the products of the reaction. Velocity decrease with further elevation of temperature as a result denaturation of the enzyme (temperature- induced). Bell-shaped curve  Factors affecting enzyme catalysed reactions: pH Concentration of protons [H+] affect reaction velocity in several ways. Optimum pH Each enzyme has their own optimum pH for maximal enzyme activity. Catalytic activity requires enzymes and substrates in specific ionized/unionized (protonated/ deprotonated) state in order to interact. Extremes pH Enzyme not able to maintain its tertiary structure. Can lead to denaturation of enzyme Causing the conformation of active site change. Optimum pH for different enzymes varies Pepsin –optimum pH is 2 Trypsin –optimum pH is 7-8 , NAD, FAD  Prosthetic Group Co-substance Non-protein moiety which is permanently Coenzyme that only transiently or transiently associated with the enzyme. associate with the enzyme FAD, FMN Pyridoxal phosphate in Transaminases, Decarboxylation NAD+, NADP, CoA Heme with Globin. Metalloenzyme — Permanently Metal-activated enzyme — Loosely  Inhibitor v V Irreversible Reversible Bind enzyme through Bind enzyme through covalent bond. non-covalent bond. V - v Suicide Non-competitive Competitive Hypercholesterolemia Enzyme: HMG CoA reductase Substrate: HMG CoA Inhibitor: Pravastatin Mechanism Statin drugs are the cholesterol-lowering agents. It competitively inhibits the rate-limiting (slowest) step in cholesterol biosynthesis. This reaction is catalyzed by hydroxymethylglutaryl coenzyme A reductase (HMG CoA reductase) Statins, such as atorvastatin (Lipitor) and pravastatin (Pravachol), are structural analogs of the HMG CoA (natural substrate for this enzyme) It compete effectively with HMG CoA by binding at the active site of HMG CoA reductase. This inhibit de novo cholesterol synthesis, thereby lowering plasma cholesterol levels. - e ① ②