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Lecture 6 Notes – Metabolism: Energy and Enzymes Questions • What forms of energy are used by a cell? • How is energy used to drive biological processes within cells? • What might happen if insufficient energy is available for cells to function? Intro – chapter focuses on how cells perform work: energy, enzymes, membranes 6.1 Energy and the Cell Energy = the capacity to perform work Two types: 1. Kinetic energy = energy of motion a. Heat is kinetic energy – random movement of particles b. Light is kinetic energy 2. Potential energy = stored capacity to perform work as a result of location or structure a. Chemical energy – energy stored in arrangement of atoms in molecules b. Cell converts potential energy to kinetic energy to perform work Thermodynamics = study of energy transformations that occur in a collection of matter, or system 1. First Law of Thermodynamics = total amount of energy in universe is constant, energy can be transferred or transformed but not created nor destroyed 2. Second Law of Thermodynamics = every energy change results in increased disorder, or entropy, unusable energy is lost to surroundings as heat 6. 2 Chemical reactions store or release energy 1. Reactants vs. products: A + B  C + D 2. Free Energy – ∆G – amount of energy left to do work after a chemical reaction has occurred. 3. Endergonic reactions – require an input of energy equal to the difference in potential energy of reactants and products • Example = photosynthesis – uses energy of sunlight to form organic compounds • ∆G > 0 (draw picture) 4. Exergonic reactions – result in an output of energy equal to the difference in the potential energy of the reactants and products • Example = burning and cellular respiration – chemical energy of reactants is released to form energy-­‐poor products • Cellular respiration: glucose is burned to produce ATP, heat, CO2 and water • ∆G < 0 and reaction is spontaneous (draw picture) 5. Cellular metabolism = sum of endergonic reactions and exergonic reactions in cells 6. Energy coupling = the use of energy released from exergonic reactions to drive endergonic reactions ATP = adenosine triphosphate • Powers all forms of cellular work • Glucose contains too much energy – like using a $100 bill to buy a soda in a vending machine • Adenine (nitrogenous base) + ribose (5 carbon sugar) + three phosphate groups (all negatively charged) • Mutual repulsion of phosphate groups contributes to potential energy stored in ATP (like a compressed spring) • Energy stored in bonds between phosphate groups – bonds broken by hydrolysis • ATP + H2O  ADP + P + energy (exergenic reaction) • ADP = Adenosine diphosphate •
Phosphorylation = transfer of a phosphate group from ATP to some other molecule Most cellular work depends on ATP energizing molecules by phosphorylating them ATP drive all three types of cell work: chemical, mechanical, transport o Chemical work: phosphorylation of reactant molecules drive endergonic synthesis of product molecules o Mechanical work: transfer of phosphate groups to special motor proteins (myosin) in muscle cells cause proteins to change shape and pull on actin filaments, causing cells to contract o Transport work: phosphorylation of membrane proteins – movement across membranes ATP is produced from ADP during cellular respiration – breaking down glucose in exergonic reactions – phosphate is bonded to ADP – ADP is phosphorylated – endergonic reactions (energy storing) ADP + P + energy  ATP + H2O (endergonic reaction) 10 million ATP molecules consumed and regenerated by a cell each second 6. 3 Enzyme functions Metabolic pathway – series of linked reactions – each step catalyzed by enzyme – small steps instead of large jump – maximizes cell efficiency Enzymes = large protein molecules that function as biological catalysts, a chemical that speeds up reactions without being consumed (end with –ase suffix) Ribozymes – special enzymes that use RNA instead of proteins – involved in synthesis of RNA and synthesis of proteins at ribosomes Energy of activation = amount of energy put into an exergonic reaction before reaction occurs (Mexican jumping bean analogy)  Enymes lower activation barrier Specific enzymes catalyze each cellular reaction Enzymes (types of proteins) have unique 3-­‐D shape – shape determines the chemical reaction it catalyzes Substrate = reactant in enzyme-­‐catalyzed reaction Substrate binds to active site on enzyme  this causes a slight change in shape of enzyme called an induced fit  this promotes chemical reaction  substrate changes into product and is released & enzyme is unchanged One enzyme may act on thousands or millions of substrate molecules per second. Factors that affect way enzymes work  Substrate concentration -­‐ increases enzyme activity as substrate concentration increases – until all enzyme active sites are full  Optimal pH – change in pH alters ionization of side chains (R groups) – H+ ions can interfere and denature (alter shape of) enzyme  Salt ions interfere with some of the chemical bonds that maintain protein structure  Temperature -­‐ affects molecular motion – high temps may denature proteins o Changing coat colors in animals  Cofactors = non-­‐protein helpers – may be inorganic, such as ions of zinc, iron, copper Coenzymes = organic cofactors -­‐ often vitamins or made from vitamins Enzyme inhibitors block enzyme action Irreversible -­‐ Inhibitors attach to enzyme be covalent bonds – irreversible – toxins and poisons Reversible -­‐ Inhibitors attach by weak bonds (hydrogen bonds) Competitive inhibitor – binds at active site Noncompetitive inhibitor – binds at some other site Feedback inhibition – type of inhibition whereby enzyme activity is blocked by a product of the reaction catalyzed by the enzyme – prevents too much product Poisons, pesticides, drugs = enzyme inhibitors Cyanide = inhibits production of ATP during respiration Nerve gas sarin = inhibits enzyme acetylcholinesterase – transmission of nerve impulses Pesticide malathion – inhibits enzyme acetylcholinesterase Antibiotic penicillin inhibits enzyme that builds bacterial cell walls Painkillers aspirin and ibuprofen inhibit enzyme to produce pain 6.4 Organelles and the Flow of Energy Photosynthesis – 6CO2 + 6 H20 + energy  C6H12O6 + 6 O2 • Chloroplasts • Hydrogen (H+ and e-­‐) are transferred from water to CO2 forming glucose • Redox reaction = oxidation (loss of electrons) + reduction (gain of electrons) • Requires high-­‐energy electron-­‐carrier molecule or coenzyme called NADP+ (nicotinamide adenine dinucleotide phosphate) • Reduction of NADP+: NADP+ + 2e-­‐ + H+  NADPH Cellular Respiration – C6H12O6 + 6O2  6CO2 + 6H2O + energy • Mitochondria • Glucose loses electrons (oxidized) and oxygen gains electrons (reduced) • Need a coenzyme, NAD+ (nicotinamide adenine dinucleotide) • Reduction of NAD+: NAD+ + 2e-­‐ + H+  NADH • And reduction of FAD, FAD + 2e-­‐ + 2H2  FADH2 Electron Transport Chain • Hindenburg disaster: 2H2 + O2  2H2O + BOOM! • Oxygen has super high electronegativity – as electrons fall toward oxygen energy is released (electrons at lower potential energy level when closer to nucleus) – in mitochondria • In chloroplasts the electrons fall toward a chlorophyll a complex with even higher electronegativity than oxygen • ETC: Series of membrane-­‐bound carriers that pass electrons from one carrier to another via redox reactions – more control of energy release than Hindenburg disaster • High energy electrons in and low energy electrons out • Energy is released as electrons lose energy and energy is capture to make ATP ATP Production • ATP synthesis • Peter Mitchell, 1978 theory of ATP production in mitochondria and chloroplasts • Chemiosmosis – in thylakoids of chloroplasts and in cristae of mitochondria o Electrochemical gradient across membrane powers ATP production o ATP Synthase – enzyme synthesizing complex that makes ATP from ADP + P • Draw it for photosynthesis and mitochondria