Transcript Document
Energy, Enzymes, and Biological Reactions Chapter 4 Energy Definition: The Capacity to do work Types of Energy: Potential: Stored energy, measured as a capacity to do work. example: stretched spring Kinetic: Energy of motion, released potential energy. example: releasing of a stretched spring Thermal: Energy released as heat Chemical: Potential energy stored in molecules. Measured as Kilocalories (Kcal) aka Calories (C) (1 calorie (c) = heat req’d to raise 1g of H2O 1C) Why do cells need energy? Chemical work, build, rearrange, tear apart compounds Mechanical work, move cilia, flex a muscle Electrochemical work, nerve impulses Where does energy come from? The universe contains a huge, but finite amount of energy The original source of energy for most life on earth is from the sun Energy is governed by the Laws of Thermodynamics First Law of Thermodynamics The total amount of energy in the universe remains constant Energy can be converted from one form to another, but it is never destroyed Second Law of Thermodynamics Entropy tends to increase in a closed system (No energy conversion is 100% efficient) Overall energy flows in one direction from useable (lots of potential energy) to nonuseable (little potential energy) forms So how can life exist? Energy flows from the sun to plants, which lose energy directly or indirectly to other organisms Overall energy flows in one direction and entropy increases as at each step energy is lost Producers builds complex molecules from simpler building blocks using the energy of the sun i.e. – the sun is constantly supplying us with new energy Energy and chemical reactions Reactant(s) → Product(s) Energy is stored in chemical bonds – all molecules contain energy Endergonic reactions: reactions in which the products contain more energy than reactants Exergonic reactions: reactions in which the products contain less energy than the reactants Endergonic Reactions Requires energy input Endergonic Reaction: Photosynthesis Original source of energy for most life on earth glucose - a product with more energy Overall reaction: 6CO2 + 6H2O C6H12O6 + 6O2 + 6O2 Very endergonic – where does the plant get the energy? → SUN Energy in energy-poor reactants Exergonic Reactions Releases energy Exergonic Reaction – Cellular Respiration Breakdown of glucose; very exergonic The source of ATP energy in cells Overall reaction: C6H12O6 + 6O2 6CO2 + 6H2O -686Kcal glucose energy-rich starting substance + 6O2 Energy out 6 6 products with less energy Adenosine Triphosphate (ATP) ATP is the cell’s energy currency nearly all energy in a cell is stored within the ATP molecule Energy releasing rxns→ ATP→ Energy requiring rxns Cells cleave ATP into ADP & Pi releasing energy This energy can be used to do work such as synthesize other molecules or move muscles How is ATP synthesized? ATP are renewable and are recycled by cells: How is the energy from ATP utilized? Reaction coupling: thermodynamically unfavorable reactions (endergonic) are coupled to the favorable reactions of ATP cleavage (exergonic) ATP → ADP + Pi = –7.3Kcal X → → → → Y = +5Kcal Net energy = -2.3Kcal Total reaction still increases entropy and conforms to the 2nd Law of Thermodynamics Chemical Reactions (Rxn) The conversion, accumulation, & disposal of substances by a cell is done through energydriven reactions Parts of a Reaction (Rxn) Reactants: substances that enter into a reaction Intermediates: substances formed in the middle of a reaction Products: end results of a reaction How are cellular reactions defined? Catabolism: breaking down of complex molecules Anabolism: the building up of complex molecules Metabolism: the sum of all these reactions Anabolic and Catabolic Reactions large energy-rich molecules DEGRADATIVE PATHWAYS (CATABOLIC) ADP + Pi ATP energy-poor products ENERGY INPUT BIOSYNTHETIC PATHWAYS (ANABOLIC) simple organic compounds Types of Reaction Sequences A B C D E F LINEAR PATHWAY K CYCLIC PATHWAY J G I BRANCHING PATHWAY N M L H Activation Energy Exergonic reactions are spontaneous Why don’t exergonic reactions happen all the time? Because of Activation Energy (EA) – the energy required to get a reaction started The EA of a reaction can prevent it from occurring or cause it to occur slowly Activation Energy Initial input of energy to start a reaction, even if it is spontaneous Catalysts Agents that speed up chemical reactions without getting used up Biological Catalysts: Enzymes Enzymes are protein catalysts (ribozymes are RNA catalysts) They are required in small amounts They are not altered permanently by the reaction They do not change the thermodynamics of a reaction They can only accelerate the rate at which a favorable reaction proceeds Role of Enzymes in Biological Reactions Enzymes accelerate reactions by reducing activation energy Enzymes combine with reactants and are released unchanged Enzymes reduce activation energy by inducing the transition state Enzymes and Activation Energy Enzymes decrease activation energy required for a chemical reaction to proceed Biological Catalysts Example: A phosphatase enzyme can catalyze a rxn in 10 milliseconds Without the enzyme the rxn would take… 1 trillion yrs. (1,000,000,000,000) THE REACTION IS CONSIDERED SPONTANEOUS Enzyme Specificity Enzymes are usually very specific Substrates interact with enzyme’s active site Enzyme Activity: Induced Fit Model Transition State During catalysis, the substrate and active site form an intermediate transition state Fig. 4-12, p. 81 How do enzymes lower EA? Catalytic mechanisms induce transition state Bringing substrates into close proximity Orienting substrates Altering environment around substrates Factors That Affect Enzymes Temperature: increasing temperature speeds up rxns (both enzymatic and non-enzymatic) up to a point (WHY?) High temperatures will destroy the enzyme Enzymes are proteins Proteins get denatured (unfolded) at high temps Factors That Affect Enzymes Concentration of substrate and products: increasing substrate will increase reaction up to a point increased product will slow reaction (known as negative feedback) Concentration of enzyme Increasing concentration increases enzyme activity up to a point Factors That Affect Enzymes pH: [H+] affects enzyme shape, so enzymes work best at narrow ranges of pH Optimal pH – pH at which enzyme can catalyze best For most enzymes, optimal pH is around neutral, depending on the environment in which the enzymes work E.g. Pepsin – digestive enzyme in stomach, optimal pH ~2 Controlling Enzyme Activity Enzymes are very efficient at what they do Because of this they need to be carefully controlled The cells needs to be able to regulate when a reaction occurs The cell also has to be able to regulate how much product is produced from a reaction Enzyme inhibitors Competitive inhibitors Bind to active site of enzyme Prevent substrate from binding Non-competitive inhibitors Also called Allosteric inhibitors Bind to enzyme in a region other than the active site called allosteric site Change the shape of the active site to prevent substrates from binding Enzyme Regulation Enzyme activity is often regulated to meet the needs for reaction products Allosteric regulation occurs with reversible combinations of regulatory molecules with an allosteric site on the enzyme High-affinity state (active form); enzyme binds substrate strongly Low-affinity state (inactive form);enzyme binds substrate weakly or not at all Allosteric Regulation Allosteric activators and allosteric inhibitors Fig. 4-17, p. 84 Feedback inhibition If too much product is created the first enzyme may be shut off by the product becoming an allosteric or competitive inhibitor: Cofactors and Coenzymes Some enzymes need assistance in the form of cofactors Minerals – inorganic cofactors Examples: Potassium, Sodium, Calcium Vitamins – organic cofactors or coenzymes Examples: The specialized nucleotides NAD+ and FAD act as cofactors for enzymatic reactions; NAD+ contains the vitamin niacin and FAD contains the vitamin riboflavin Ribozymes RNA-based catalysts Help remove surplus segments of RNA molecules with cutting and splicing reactions In ribosomes, help join amino acids together when building proteins Some coenzymes accept and hold onto electrons (e-) and protons (H+) during the breakdown glucose Why are these coenzymes required? Enzymes are not used up or modified during a reaction If the enzyme accepted the e- or H+ it would be modified Oxidation/Reduction (Redox) Reactions One compound gains e- or H+ lost by another compound The oxidized compound loses electrons or H+ The reduced compound gains electrons or H+ Reduction acts as a mechanism for storing energy Redox Reactions