Lecture 11: Energy and Enzymes
Lecture 11: Energy and Enzymes 200001
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This 11 page Class Notes was uploaded by Olivia Sutton on Sunday March 20, 2016. The Class Notes belongs to 200001 at Boston College taught by Danielle Taghian in Spring 2016. Since its upload, it has received 16 views. For similar materials see Molecules and Cells in Biology at Boston College.
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Date Created: 03/20/16
Lecture 11: Energy and Enzymes March 4, 2016 Exergonic Reaction Products have less energy than reactants Energy is released Spontaneous Entropy increases Endergonic Reactions Products have more energy than reactants Energy is required Not spontaneous Entropy decreases Energy in a Chemical Reaction Two types of energy o Kinetic ener gy of motion Molec ular level energy is thermal energy o Potential energy Ener gy of configuration or position Molecular level chemical energy is stored Free energy: amount of energy available to do work What Makes a Chemical Reaction Spontaneous? Reactions tend to occur spontaneously if products have lower potential energy and higher entropy than the reactants Entropy: the measure of disorder of a system o The best way to determine entropy is to count the amount of molecules on each side of the chemical equation Glucose is a high potential energy molecule C-H bonds have a lot of energy stored in them because of shared electrons o Relatively equal electro negativities = high potential energy o Unequal electro negativities = low potential energy Free Energy Change Gibbs free-energy change: indicates whether a reaction is spontaneous or requires energy o Delta G = delta H – T(delta)S Delta means change If the free-energy change is less than 0, this indicates a spontaneous reaction (negative delta G) If the free-energy change is greater than 0, this indicates a reaction the requires energy input to occur (positive delta G) If free-energy change is equal to 0, then the reaction that is at equilibrium Entropy becomes more important with the increase of temperature A really negative number would indicate a more explosive reaction! What Determines Whether a Chemical Reaction is Spontaneous? The combined effects of changes in entropy and potential energy Enzymes Regardless of whether a reaction uses ATP, both reactions need enzymes Enzymes do not affect the delta G Catalysts: any molecule that activates the reaction Enzymes are basically protein catalysts o Bring substrates together in a specific orientation Catalysis Acceleration of a chemical change in a reaction Just because a reaction is spontaneous, it might still take it a while to happen Reactants have to find each other, through high concentration, and have to hit in correction orientation The speed in which molecules interact matter in order to overcome the repulsion of their individual electron clouds Substrates = reactants (words are interchangeable) Enzymes also affect the kinetic energy of reactants o There’s no need for more heat or energy for the collision to occur because the enzyme does that job for them Transition state: molecules are breaking some bonds and making other bonds Activation energy: energy needed for the transition state Needs to be over come regardless if the reaction is spontaneous Catalysts lower the activation energy and stabilizes that transition state Free-energy isn’t changed Substratre Binding Glucose is the molecule that’s going to be oxidized in the series of chemical reactions Active site: where substrates bind to enzymes (like a piece fitting into a puzzle) Induced fit: A conformational change occurs after the bondage is complete o Results in tighter binding of the substrates; enzymes literally closes itself off so that no other substrate can enter into it Initiation: reactants are correctly oriented as they bind to the active site Transition state facilitation: interactions b/t the substrate and active site R-groups lowers the activation energy Termination: reaction products are released from the enzyme Enzymes are activated with a phosphate group Enzyme phosphorylation Causes a conformational change in the protein Charge on phosphate is negative Phosphate groups causes amino acid to move o The addition of a phosphate (PO ) m4lecule to R group of an amino acid residue can turn a hydrophobic portion of a protein into a polar and extremely hydrophilic portion of molecule. o In this way, it introduces a conformational change in the structure of the protein via interaction with other hydrophobic and hydrophilic residues in the protein. Do Enzymes Act Alone? No, cofactors and coenzymes help out Enzymes are regulated by molecules that are not part of the enzyme itself Confactors: inorganic ions o Reversibly interact with enzymes Coezymes: organic molecules o Interact with enzymes Inhibition Competitive inhibition: occurs when a molecule similar in size and shape to the substrate competes for the active bind site o This is why concentration matters Allosteric regulation: occurs when a molecule causes a change in enzyme shape by binding to the enzyme at a location other than the active site o Can increase or decrease enzyme activity o Closes the active site (some open the active site even more) Active sites opened too wide could end up having too many enzymes in them What limits the Rate of Catalysis? In enzymatic reactions, the rate of product formation increases linearly for an increase in substrate concentration at low levels, but it levels out at high concentrations All enzymes show this type saturation kinetics High substrate affinity: achieves maximum reaction rate rapidly as substrate concentration increases Low substrate affinity: achieves maximum rate only after reaching high substrate concentrations What Does This Mean? o When substrate concentration is low, the speed of the enzyme catalyzed reaction is linear o At intermediate concentrations, the rate begins to slow down o At high concentration, the reaction rate plateaus at maximum speed All available enzyme molecules are being used, so the reaction is already at maximum speed regardless of concentration How Do Physical Conditions Affect Enzyme Function? Enzymes function best at particular temperatures and pH o Temperature affects movement of substrates and enzymes o pH affects the enzyme’s shape and reactivity ATP Has High Potential Energy The electrons in ATP have high potential energy because the four negative charges in its three phosphate groups repel each other The hydrolysis of ATP is an exergonic reaction. Why? o Entropy of products is higher than reactants o Potential energy is lower in products than in the reactants. Breaking bonds in ATP requires energy and the making of these bonds release energy. Because the bonds are easy to break and the bonds formed release energy, there is a net energy release o The hydrolysis of ATP releases energy for reactions to occur ATP Plays a Central Role in Metabolism Phosphorylation by ATP increase the energy of other molecules The hydrolysis of an ATP molecule in a coupled reaction can change the equilibrium ratio of the products to reactants How Does ATP Drive Endergonic Reactions? Energetic coupling: when a protein is phosphorylated, the exergonic phosphorylation reaction is paired with an endergonic reaction o This increases the free energy of the substrates Enzyme Catalysts vs. Coupled Reactions Catalysts are enzymes that lower the activation energy to speed up a reaction o They cannot make non-spontaneous reactions proceed ATP hydrolysis releases energy that can be coupled to another reaction o This changes the free energy change of the reaction and can make a non-spontaneous reaction into a spontaneous one Both facilitate chemical reactions in a cell