Lecture Set 16 - Enzymes - Canvas.pdf

Chemical Reactions In principle, chemical reactions can run in both directions. Chemical equilibrium ΔG = 0 At equilibrium, the forward and reverse reactions are balanced. A« B Reactant Product Ø Every reaction has a specific equilibrium point. Ø ΔG is related to the point of equilibrium: the closer...

Chemical Reactions In principle, chemical reactions can run in both directions. Chemical equilibrium ΔG = 0 At equilibrium, the forward and reverse reactions are balanced. A« B Reactant Product Ø Every reaction has a specific equilibrium point. Ø ΔG is related to the point of equilibrium: the closer the reaction is to equilibrium, the more free energy is released. Ø ΔG values near zero are characteristic of readily reversible reactions. Reaction Coupling Ø Many chemical reactions are endergonic (ΔG>0 ) and require energy to be externally applied to occur. However, endergonic reactions can be made to occur when they are coupled to separate, exergonic (ΔG 0 (Energy is Reaction 2: BàC released.) ΔG < 0 (Energy is If the sum of reactions 1 and 2 have an overall negative ΔG, then the reaction proceeds by coupling reactions 1 and 2. The reaction is spontaneous, despite that fact that reaction 1 has a positive ΔG. ATP Hydrolysis Releases Energy The hydrolysis of ATP is highly exergonic (high -DG) ATP Activates Endergonic Processes by Donating Its High-Energy Phosphate Energetic Coupling Example We can see that the ΔG of the hydrolysis of ATP is of a greater absolute value than the ΔG of the formation of glucose 6-phosphate. The overall ΔG is negative. Enzymes Enzymes Are Catalysts Ø Most biologically important reactions occur very slowly in the absence of a catalyst Ø Some reactions are slow because of an energy barrier called the activation energy (Ea). This is the amount of energy required to start a reaction. Ø Catalyst: Accelerates a reaction without being permanently changed in the process. Ø Enzymes: Biological catalysts that speed up and control chemical reaction rates. Properties of Enzymes 1. Most enzymes are proteins (exception – ribozymes) 2. Enzymes lower the activation energy of a reaction 3. Enzymes do not change the nature of a reaction (i.e., DG). 4. Enzymes are very specific for the reactions they catalyze The enzyme-substrate complex is held together by hydrogen bonds, electrical attraction, or hydrophobic interactions (and sometimes covalent bonds) Enzyme-Catalyzed Reactions Enzymes are not consumed by the reaction and can act repeatedly. Reactants that are acted upon by enzymes are called “substrates”. Enzymes Ø Activation Energy: Even though a reaction is energetically favorable (DG < 0), an initial investment of energy is usually required. Ø Enzymes lower the energy barrier for reactions. Ø Enzymes do not change the ΔG for a reaction Enzyme Shape and the Active Site In an enzyme-catalyzed reaction, the substrate binds to the active site of the enzyme. Active Site: The catalytic center of the enzyme. Shape of enzyme active site allows a specific substrate to fit—the “lock and key.” Many enzymes change shape when they bind to the substrate— induced fit. The active site binds the substrate(s) and helps to stabilize the transition state, lowering the activation energy Active Site formation Enzymes orient substrate molecules, bringing together the atoms that will bond. Enzymes can stretch the bonds in substrate molecules, making them unstable. Enzymes can temporarily add chemical groups to substrates. Ø Enzymes can lower the activation energy of a reaction by bringing substrates together Ø Enzymes provide a template upon which the two substrates are brought together in the proper position and orientation to react with each other. Figure 6.14 Catalyzed Reactions Reach a Maximum Rate The rate of a catalyzed reaction depends on substrate concentration. Concentration of an enzyme is usually much lower than concentration of a substrate. At saturation, all enzyme is bound to substrate— maximum rate. Rate can be used to calculate enzyme efficiency: molecules of substrate converted to product per unit time—also called turnover. Ranges from 1 to 40 million molecules per second! Properties of Enzymes o The rate of a catalyzed reaction depends on substrate concentration. o Concentration of an enzyme is usually much lower than the concentration of substrate. o At saturation, all enzyme is bound to substrate—maximum rate. Enzymes do not alter the DG of a reaction Regulation of Enzyme activity Several factors can regulate (activate or inhibit) enzyme activity: Ø Temperature Environmental Factors Ø pH Ø Cofactors- Accessory molecules required for optimal enzyme activity. Ø Ø Ø Ø Inhibitors: Are essential for regulating metabolic pathways. Can be reversible or irreversible Competitive Inhibitor: Looks like substrate and binds to active site of enzyme Non-competitive Inhibitor: Binds to other site on enzyme Regulation of Enzyme Activity Temperature Every enzyme has an optimal temperature. At high temperatures, noncovalent bonds begin to break. An enzyme can lose its tertiary structure and become denatured. Protein unfolding and Denaturation Ø Conditions that affect secondary and tertiary structure include: Ø High temperature Ø pH changes Denaturation: loss of 3-dimensional structure and thus function of the protein Regulation of Enzyme Activity Enzyme activity can be altered by inhibitors or activators. Irreversible inhibition: inhibitor covalently bonds to side chains in the active site—permanently inactivates the enzyme. Example: DIPF or nerve gas Effects of Nerve Gas: excessive muscle contraction followed by paralysis, secretions, seizures and death by respiratory failure. Irreversible Inhibition (Covalent modification) Irreversible inhibitors bind permanently to their target enzyme, often via a covalent bond that influences catalysis Metabolic Pathways Ø Metabolic reactions are organized into pathways that are an orderly series of enzymatically-controlled reactions. Ø The first reaction is the commitment step—other reactions then happen in sequence. Ø Feedback (negative) Inhibition: The final product may inhibit the enzyme needed for the commitment step, which shuts down the pathway—end product inhibition. Metabolic Pathways Thousands of chemical reactions are occurring in cells simultaneously. The reactions are organized in metabolic pathways. Each reaction is catalyzed by a specific enzyme. The pathways are interconnected. Regulation of enzymes and thus the rates of reactions helps maintain internal homeostasis. Control of Metabolic Pathway Ø Feedback inhibition is a common method for regulating metabolic pathways. Ø The end product acts as an inhibitor of an enzyme earlier in the pathway. Ø Thus, when the product is abundant the pathway is turned off. When rare, the pathway is active.