2-1 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Chapter 2: The chemistry of life

2-2 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Energy and entropy Chemical and energy transformations in cells –metabolism Sequence of chemical reactions –metabolic pathways Energy is the capacity to do work –potential energy is stored energy –kinetic energy is expressed as movement Energy transformations are governed by the laws of thermodynamics

2-3 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Equilibrium A ↔ B + C A reaction is at equilibrium when there is no net change in the concentration of reactant or products Reactions must be out of equilibrium to do work (cont.)

2-4 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Equilibrium (cont.) Equilibrium constant If the reactants and products contain the same chemical energy per molecule, K eq = 1.0 –in this case, there must be a high concentration of reactants or low concentration of products in order to do work If reactants and products contain different amounts of chemical energy, then K eq ≠ 1.0 –reaction will be out of equilibrium when concentration of reactants and products are equal

2-5 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Free energy Free energy (G) represents the maximum amount of useful work obtainable from a reaction Change in free energy (ΔG) is the useable energy (chemical potential) of a reaction –depends on  change in heat content (ΔH) determined by the making and breaking of chemical bonds  change in entropy (ΔS) determined by the molecular organisation of the system  temperature (T) in degrees Celsius above absolute zero ΔG = ΔH – TΔS (cont.)

2-6 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Free energy (cont.) Change in free energy is related to the concentration of reactants and products –R is universal gas constant ΔG = – RTlog e K eq When ΔG < 1.0, energy is released in exergonic reaction –spontaneous reactions When ΔG > 1.0, energy is needed for endergonic reaction

2-7 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 2.4a and b: Exergonic and endergonic reactions (a) (b)

2-8 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Rates of chemical reactions Rate of reaction towards equilibrium (kinetic energy of reaction) is independent of K eq or ΔG –depends on kinetic energy of reacting molecules Activation energy is minimal level of energy necessary to break existing bonds at the moment that molecules collide Rate of reaction can be increased by –heat  raises kinetic energy of molecules –catalysis  reduces activation energy of reactants

2-9 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 2.5a: Energy levels of molecules

2-10 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 2.5b: Energy levels of molecules

2-11 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Enzymes Enzymes are biological catalysts that lower the activation energy in reactants (substrates) enzyme + substrate ↔ enzyme–substrate complex enzyme–substrate complex → enzyme + product (cont.)

2-12 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Enzymes (cont.) Enzymes are specific in their substrates Active site is specialised region formed from folding of polypeptide chains Site lined by R-groups of amino acids –substrate-binding amino acids –arrangement determines specificity of binding enzyme to substrate Catalytic amino acids

2-13 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Enzyme action Substrate fits active site on enzyme molecule Active site changes shape when substrate attaches to it –induced fit Once fitted to active site, substrate is under strain and ready for reaction –transition state –transition state activation

2-14 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 2.10: Stages of an enzyme-catalysed reaction

2-15 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Enzyme activity Rate of enzyme activity affected by factors that change shape of active site so substrate does not bind –pH –temperature –may alter active site irreversibly (denature) Rate of enzyme activity affected by concentration of –substrate –cofactors –coenzymes

2-16 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Metabolism Metabolic reactions result from enzyme activity Enzyme activity is regulated to prevent over- or under-production Short-term control of enzyme activity by modifying structure of enzyme –covalent modification  phosphorylation (addition of phosphate residues) increases or decreases activity –allosteric inhibition or activation  binding of organic molecule to sites on enzyme surface (cont.)

2-17 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Metabolism (cont.) Concentration of enzyme can be increased by synthesis of more enzyme protein Concentration of enzyme can be decreased by specific breakdown of enzyme protein –removed to lysosome –marked for breakdown with polypeptide marker

2-18 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint ATP Energy from reactions in which reduced bonds in fuel molecules are oxidised is conserved in ATP (adenosine triphosphate) Components of ATP –ribose sugar –adenine –triphosphate group  two high-energy covalent bonds link these three phosphate groups (cont.)

2-19 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint ATP (cont.) Biologically useful attributes of ATP –equilibrium constant of the ATP hydrolysis reaction is high  reaction is out of equilibrium at low concentrations of ATP, ADP and P –ATP formed in single steps in the pathways of glycolysis and cellular respiration –ATP is a common intermediate between degradative and synthetic metabolic pathways

2-20 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Classes of enzymes Transferases and ligases are involved in biosynthesis of cellular constituents Hydrolases break down complex molecules Lyases and isomerases are involved in pathways that transform compounds into substrates for oxidoreductases Oxidoreductases trap potential energy by coupling reactions with ATP formation

2-21 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Electron transport pathways Membrane-bound enzymes and cofactors that operate in a sequence Electrons transferred from donor to acceptor –molecule that loses electron is oxidised –molecule that gains electron is reduced Transfer reactions are oxidation–reduction reactions

2-22 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Fig. 2.18: An electron transport chain

2-23 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Oxidation–reduction reactions In oxidation–reduction reactions, the tendency to donate or accept electrons can be measured as the oxidation–reduction (redox) potential –E 0 ′ Redox reaction is thermodynamically favourable if electrons are transferred from a carrier with more negative potential with one to less negative (more positive) potential (cont.)

2-24 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Oxidation–reduction reactions (cont.) Oxidation–reduction systemE 0 ′ (mV) (a) 2H + + 2e - ↔ H 2 – 420 NAD + + 2H + ↔ NADH + H + – 320 FMN + 2H + ↔ FMNH 2 + 2e - – 120 Coenzyme Q ox + 2e - ↔ Coenzyme Q red – 170 Cytochrome b (Fe 3+ ) + e - ↔ Cytochrome b (Fe 2+ ) Cytochrome c (Fe 3+ ) + e - ↔ Cytochrome c (Fe 2+ ) Cytochrome a (Fe 3+ ) + e - ↔ Cytochrome a (Fe 2+ ) ½O 2 + 2H + ↔ H 2 O (a) E 0 ′ is the standard redox potential relative to that of the H 2 electrode at pH 7 (– 420 mV)

2-25 Copyright  2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Energy in fuel molecules Carbohydrates, lipids and proteins provide cells with energy –fuel molecules Energy can only be extracted from certain bonds of fuel molecules –C—C –C—H –C—N Lipids have more energy per C atom than do carbohydrates or proteins because they have more energy-rich C—H bonds