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Enzyme biological catalyst that changes the rate of a reaction not consumed by reaction, reusable some enzymes are globular proteins, others are ribose nucleic acids. (Active site + R groups) 1. act as template for substrate orientation, 2. Stresses substrates and stabilizes transition state, 3. Provides favorable microenvironment, 4. Participating directly in catalytic reaction. Induced Fit Model of enzyme-substrate interaction - - energy is needed to contort reactant molecules so that bonds can change initial investment of energy = free energy of activation/ activation energy reactants break when molecules absorb enough energy to become unstable o absorption of thermal energy increases speed of reactant molecules they collide more often, more forcefully thermal agitation of the atoms in molecules (bonds more likely to break) reactant molecules that have absorbed sufficient free energy to react are unstable = transition state Enzymes lower activation energy barrier, enabling reactant molecules to reach transition state at moderate temperatures. An enzyme cannot change the free energy change required for reaction. Protein Structure of an Enzyme HIV Protease 2 subunits (polypeptide chains) & flaps 1. to guard entrance to active site 2. exclude incoming water molecules 3. flexible; allow a substrate to access active site by opening, closing Model of Enzyme action: Lock and Key Hypothesis Enzymes have specific surface configurations produced by 3D folding of polypeptide chain substrate= key whose shape is complementary to enzyme (lock) when enzyme and substrate collide in correct orientation, substrate attached to active site of enzyme short lived enzyme-substrate complex formed (held by weak interactions e.g. H bonds, ionic bonds) products no longer fit into active site, escapes into surrounding medium active site free to receive further substrate molecules 1. 2. 3. specificity enzymes lower activation energy barrier heating, pH curtails enzyme activity (denaturation) - Both enzyme and substrate change shape straining bonds during reaction. Strained form of the substrate (transition state) exist for short while Molecular Basis of enzyme action (lowering activation energy) 1. Proximity Effects - Temporary binding of reactants next to each other on an enzyme increases chance of reaction - uncatalysed reactions depend on random collisions between substrate molecules 2. Strain Effect - Slight distortion of reactants as they bind to enzyme strains bonds which are to be broken - Chance of breakage is increased 3. Orientation Effects – Reactants are held by the enzyme in such a way bonds are exposed to attack 4. Microenvironment effects - Hydrophobic amino acids create a water-free zone in which non-polar reactants may react more easily 5. Acid-base catalysis - Acidic and basic amino acids in enzyme facilitate transfer of reactants to and from reactants. Rate of reaction 1. 2. measure formation of product measure disappearance of substrate [Note graphs and expt e.g. on p.12] Temperature Coefficient = Rate of reaction at (x+10) degrees / (x degrees) At lower temperatures, enzyme is in active state, rate of reaction depends on kinetic energy of molecules. At optimum temp, enzyme is in active state At higher temp, enzyme is irreversibly altered in shape and flexibility of the enzyme molecule. (Thermal agitation breaks hydrogen, ionic bonds and other weak interactions that stabilize the active configuration.) Substrate Concentration Michealis constant = conc. of substrate required to make reaction rate maximum Low Km – HIGH affinity of enzyme for substrate (low substrate conc. needed to Attain maximum velocity) High Km – LOW affinity of enzyme for substrate (high substrate conc. needed) Enzyme Inhibitors 1. 2. Competitive Inhibitors Non-competitive inhibitors a. Reversible b. Irreversible Substrate Binding Conc. Effect of inhibition Rate of rxn Competitive Inhibition Similar in structure and charge to normal substrate Temporarily bound to enzyme at active site Degree of inhibition depends on relative concentration of inhibitor and substrate reversible Still reaches maximum at higher substrate concentration but slower. Non-competitive inhibition Not similar in structure and charge Temporarily bound to enzyme at allosteric site (reversibly or not) Degree of inhibition depends on concentration of inhibitor Reversible or irreversible Rate of reaction reaches lower maximum level. For non-competitive inhibition, enzyme-inhibitor complex at point on enzyme other than active site. Globular structure of enzyme rendering active site unreceptive to substrate. Substrate may still be able to bind to active site but catalysis cannot take place. Effects of inhibitors cannot be overcome by high substrate conc. Rate of reaction continues decreasing with increasing substrate concentration till inhibitor saturation is reached (rate of reaction nil). Allosteric Regulation Cooperativity Allosteric enzymes consist of 2 or morepolypeptide chains or subunits. Each subunit has its own active site. Binding of an allosteric inhibitor stabilizes inactive form of the enzyme. e.g. feedback inhibition Allosteric activator causes induced fit in one subunit – triggering favorable change in conformation in all subunits.