Biology, 10e Sylvia S. Mader Lectures by Tariq Alalwan, Ph.D. Learning Objectives Define energy, emphasizing how it is related to work and to heat State and apply two energy laws to energy transformations. ti Give reasons why ATP is called the energy currency in cells. Give examples to show how ATP hydrolysis is coupled to energy requiring reactions. Describe a metabolic pathway and how they function. Learning Objectives (cont.) Explain how enzymes lower the energy of activation and speed chemical reactions. List conditions that affect enzyme speed. Explain how a cell can use enzyme inhibition to regulate metabolism. Describe the difference between oxidation and reduction reactions. Enzymes 1
Forms of Energy Energy the capacity to do work; cells continually use energy to develop, grow, repair, reproduce, etc. Divided into two types Kinetic the energy of motion, such as waves, electrons, atoms, molecules, substances and objects Mechanical energy of motion Electrical (e.g. lightning) Thermal (i.e. geothermal energy) Radiant (e.g. visible light, x rays, gamma rays and radio waves) Forms of Energy (cont.) Potential stored energy or energy that is "waiting to happen" Chemical energy stored in the bonds of atoms and molecules l (i.e. foods) Nuclear energy stored in the nucleus of an atom (the energy that holds the nucleus together) Stored mechanical energy (e.g. compressed springs & muscle movement) Laws of Thermodynamics Thermodynamics the study of energy and its transformations governs all activities of the universe First law (conservation of energy): Energy cannot be created or destroyed Energy CAN be transferred or converted from one form to another Enzymes 2
Laws of Thermodynamics (cont.) Second law: Energy cannot be changed from one form to another without a loss of usable energy Heat is a form of energy that dissipates into the environment; heat can never be converted back to another form of energy Carbohydrate Metabolism Carbohydrate Synthesis Cells and Entropy Entropy: A measure of the disorder, or randomness of energy Organized, usable energy has a low entropy Disorganized energy, such as heat, has a high entropy No energy conversion is ever 100% efficient, because much energy is dispersed as heat, increasing entropy The total entropy of the universe always increases over time Examples car rust, dead trees decay, buildings collapse Cells and Entropy Enzymes 3
Energy Flow in Living Things Metabolic Reactions and Energy Transformations Metabolism: Sum of all the biochemical reactions in a cell Reactants participate in a reaction and products form as result of a reaction Two main types of metabolism: Anabolism complex molecules are synthesized from simpler substances (requires energy) Catabolism larger molecules are broken down into smaller ones (releases energy) Exergonic Reactions Free energy (G) the amount of energy that is free to do work after a chemical reaction Change in free energy is noted as ( G); a negative G means that products have less free energy than reactants; the reaction occurs spontaneously from higher to lower free energy Exergonic reactions have a negative G and energy is released Enzymes 4
Endergonic Reactions Endergonic reaction a reaction in which there is a gain of free energy ΔG has a positive value (the free energy of the products is greater than the free energy of the reactants) Requires an input of energy from the environment ATP: Energy for Cells Adenosine triphosphate (ATP) High energy compound used to drive metabolic reactions Constantly being generated ADP + P i + energy ATP Nucleotide consisting of three parts: adenine (nitrogencontaining organic base); ribose (five carbon sugar); and three phosphate groups Hydrolysis of ATP, an exergonic reaction, yields ADP and inorganic phosphate ATP Biological advantages to the use of ATP It provides a common energy currency used in many types of reactions When ATP becomes ADP + P i the amount of energy released is sufficient for biological function with little waste of energy (~ 7.3 kcal per mole) ATP breakdown can be coupled to endergonic reactions that prevents energy waste Enzymes 5
The ATP Cycle ATP and Coupled Reactions Coupled reactions Occur in the same place, at the same time Energy released by an exergonic reaction is captured in ATP That ATP is used to drive an endergonic reaction ATP breakdown is often coupled to cellular reactions that require energy ATP supply is maintained by breakdown of glucose during cellular respiration Coupled Reactions A cell has two ways to couple ATP hydrolysis ATP is used to energize a reactant ATP is used change the shape of a reactant Both can be achieved by transferring P i to the reactant so that the product is phosphorylated Example: Ion movement across the plasma membrane of a cell through carrier proteins Attachment of amino acids to a growing polypeptide chain Enzymes 6
ATP Hydrolysis and Muscle Contraction Metabolic Pathways and Enzymes Enzymes protein catalysts that speed chemical reactions without the enzyme being affected by the reaction Ribozymes RNA molecules that function as biological catalysts; involved in RNA and protein synthesis The reactants of an enzymatically accelerated reaction are called substrates Each enzyme accelerates a specific reaction Each reaction requires a unique and specific enzyme End product will not appear unless ALL enzymes are present and functional Metabolic Pathways Reactions usually occur in an orderly sequence Products of an earlier reaction become reactants of a later reaction Such linked reactions form a metabolic pathway Begins with a particular reactant, Proceeds through several intermediates, and Terminates with a particular end product A is Initial Reactant E 1 E 2 E 3 E 4 E 5 E 6 A B C D E F G Intermediates G is End Product Enzymes 7
Energy of Activation Molecules often do not react with each other unless activated in some way Energy must be added to at least one reactant to initiate the reaction Energy of activation (E a ) the energy that must be added to cause molecules to react Enzyme Operation: Enzymes operate by lowering the energy of activation Brings the substrates into contact with one another and even participates in the reaction Does not get consumed by the reaction nor does it alter the equilibrium of the reaction, thus it remains intact Energy of Activation (cont.) Enzyme Substrate Complex The lock and key model of enzyme activity The active site is made up of amino acids and has a very specific shape The enzyme and substrate slot together to form a complex, as a key slots into a lock This (ES) complex reduces the activation energy for the reaction Then the products no longer fit into the active site and are released allowing another substrate in Enzymes 8
Enzyme Substrate Complex (cont.) Induced fit model The active site complexes with the substrates by weak interactions, such as hydrogen bonds, etc Causes active site to change shape Shape change forces substrates together, initiating bond Active site returns to its original state after the product(s) is released Some enzymes (e.g., trypsin) actually participate in the reaction Synthesis vs. Degradation Synthesis (Anabolism): Enzyme complexes with two substrate molecules Substrates are joined together and released as single product molecule Degradation (Catabolism): Enzyme complexes with a single substrate molecule Substrate is broken apart into two product molecules Only a small amount of enzyme is needed in a cell because enzymes are not consumed during catalysis Enzymatic action Enzymes 9
Regulation of Metabolism A particular reactant(s) may produce more than one type of product(s) Presence or absence of enzyme determines which reaction takes place If reactants can form more than one product, the enzymes present determine which product is formed Enzymes are sometimes named for their substrates, and usually end in ase Examples lipase (lipid), cellulase (cellulose), lactase (lactose), etc. Factors Affecting Enzyme Activity Substrate concentration Enzyme activity increases with substrate concentration More collisions between substrate molecules and the enzyme When the active sites of the enzyme are filled, with increasing substrate, the enzyme s rate of activity cannot increase any more Amount of active enzyme can also increase or limit the rate of an enzymatic reaction Factors Affecting Enzyme Activity (cont.) Temperature Enzyme activity increases with temperature Warmer temperatures cause more effective collisions between enzyme and substrate Hot temperatures destroy enzymes, how? Denaturation enzyme s shape changes and it can no longer bind to its substrate efficiently However, there are exceptions such as Some prokaryotes living in hot springs Coat color pattern in Siamese cats Enzymes 10
Factors Affecting Enzyme Activity (cont.) Optimal ph Most enzymes are optimized for a particular ph Changes in ph can make and break intra and intermolecular bonds, and/or hydrogen bonds alters the globular shape of the enzyme ph change can alter the ionization of R side chains Under extreme conditions of ph, the enzyme becomes inactive (due to altered shape) Example pepsin (stomach) and trypsin (small intestine) Effect of ph on Enzymatic Reactions Factors Affecting Enzyme Activity (cont.) Enzyme Cofactors Many enzymes require additional help in catalyzing their reaction from a coenzyme or cofactor Cofactor an inorganic ion (e.g., zinc, copper and iron) Coenzyme a nonprotein organic molecule Many of the coenzymes are derived from vitamins Vitamins small organic molecules required in trace amounts for synthesis of coenzymes Enzyme activity decreases if vitamin is not available (i.e. vitamin deficient disorder) Examples : Niacin (pellagra) & riboflavin deficiency Enzymes 11
Enzyme Inhibition Competitive inhibition Substrate and the inhibitor are both able to bind to the active site If a similar molecule is present, it will compete with the real substrate for the active sites Product will form only when the substrate, not the inhibitor, is at the active site This will regulate the amount of product Enzyme Inhibition (cont.) Noncompetitive inhibition Noncompetitive inhibitors are substances which when added to the enzyme changes the enzyme in a way that it cannot accept the substrate The inhibitor binds to another location (allosteric site) on the enzyme and inactivates the enzyme molecule Both competitive and noncompetitive inhibition are examples of feedback inhibition Competitive and Noncompetitive Inhibitors Enzymes 12
Enzyme Inhibition (cont.) The end product of a pathway can inhibit the pathway s first enzyme The metabolic pathway is shut down when the end product of the pathway is bound to an allosteric site on the first enzyme of the pathway Normally, enzyme inhibition is reversible and the enzyme is not damaged Redox Reactions Energy is transferred through the transfer of electrons from one substance to another (redox reactions) One substance loses electrons (oxidation) One substance gains electrons (reduction) Redox reactions often occur in a series of electron transfers Important in cellular respiration, photosynthesis, and many other chemical processes Electron Transfer in Redox Reactions Enzymes 13