Chapter 15

Importance of Energy Changes and Electron Transfer in Metabolism

  • Living organisms are built of complex structures

    • complex structures of low entropy is only possible when energy is spent

  • Sunlight = ultimate source of energy

Metabolism: chemical reactions of biomolecules (biochemical basis of life processes)

  • sum of all chemical reactions in the cell

    • catabolism: breakdown of larger molecules (oxidative - releases energy)

    • anabolism: synthesis of larger molecules (reductive - requires energy)

  • potential energy of organic compounds is harvested in chemical steps

    • ATP = storage form of energy in the cell (“energy currency”)

Laws of Thermodynamics Apply to Living Organisms

  • Living organisms cannot create energy from nothing

  • Living organisms cannot destroy energy into nothing

  • Living organisms may transform energy from one form to another

    • must increase the entropy of the universe

  • To maintain organization, living systems must be able to extract useable energy from the surrounding and release useless energy back to surrounding

Standard States for Free-Energy changes (STP)

  • Pure solids and liquid: pure substance

  • Gases: gas at 1 atm

  • Solutions: concentration of 1M

∆G equation can be rewritten to free-energy change under standard conditions

  • ∆G: change in Gibbs Free Energy of the reacting system

    • amount of energy to do work at constant temperature and pressure

    • <0 = exergonic (release free energy into universe, spontaneous)

    • >0 = endergonic (gains free energy from universe, non-spontaneous)

  • ∆H: change in heat content of the reacting system

    • number and kinds of chemical bonds in reactants and products

    • <0 = exothermic (release heat into universe)

    • >0 = endothermic (takes up heat from universe)

  • ∆S: change in entropy of the reacting system

    • measurement of randomness/disorder in reactants and products

    • <0 = products are less ordered than reactants

    • >0 = products are more ordered than reactants

  • Equilibrium constant: measure direction of spontaneous process

    • >1 = negative ∆Gº = reaction proceeds forward (more product)

    • =1 = 0 ∆Gº = reaction is at equilibrium

    • <1 = positive ∆Gº = reaction proceeds in reverse (more reactant)

  • ∆Gº: standard free energy change, assumes concentration of 1M

    • [H+] = 1M, pH = 0 (pH in most cells is near neutral)

    • biochemical reactions - standard concentration = 10^-7 M, pH = 7

      • ∆Gº’: modified standard state for biological reactions

Coupling of Production and Use of Energy

  • metabolism: coupling of energy-producing and energy-requiring reactions

  • energy cannot be used directly - shuttle into easily accessible chemical energy

    • “High Energy” bonds: require or release convenient amounts of energy

      • ATP: essential high energy bond-containing compound

        • phosphorylation of ADP to ATP requires energy

        • hydrolysis of ATP to ADP releases energy

  • Energetically unfavorable (endergonic) reactions are frequently coupled to energetically favorable (exergonic) reactions to drive reaction forward

    • Hydrolysis of ATP -∆Gº’=-30.5 kJ/mol

    • Glucose-6-phosphate produced by phosphorylation of glucose

      • In vivo, glucose is phosphorylated by ATP with net -∆Gº’

Free Energy and Redox

  • it is possible to generate useful work from an oxidation-reduction reaction

    • ∆Gº’=-nƒ∆E’º

    • n: number of electrons transferred

    • ƒ: Faraday constant 96.5 kJ/volt-mole

    • ∆E’º: difference in reduction potential between electron acceptor/donor

      • ∆E’º=∆E’º for electron acceptor, -∆E’º for electron donor

      • Nernst equation

  • Oxidation-reduction: electrons are transferred from a donor to an acceptor

    • oxidation: loss of electrons

      • reducing agent: substance that loses electrons

    • reduction: gain of electrons

      • oxidizing agent: substance that gains electrons

    • carbon in most reduced form = alkane

    • carbon in most oxidized form = carbon dioxide (product of catabolism)

    • Pyruvate production in glycolysis

      • Lactate dehydrogenase transfers electrons from lactate to pyruvate

        • can also function in reverse process- pyruvate to lactate

        • utilizes NAD+ coenzyme to produce NADH

          • NAD+: oxidized form of NADH

            • positive charge on nitrogen of nicotinamide

            • absorbance peak at 260-280nm

          • NADH: reduced form of NAD+

            • neutral charge on nitrogen of nicotinamide

            • absorbance peak at 340nm

    • FAD/FADH2: important coenzyme

      • FAD takes on one electron to form FADH semiquinone

      • FADH takes on one electron to form FADH2 (fully reduced)

  • Hydrolysis of thioesters is strongly favorable

    • Acetyl-CoA: donor of acetyl groups in metabolic pathways

      • biosynthesis of fatty acids

      • hydrolysis removes acetyl group, forming acetic acid

      • acetic acid is ionized to form acetate (resonance stabilized)

  • Phosphoryl transfer from ATP

    • nucleophilic attack at the a-phosphorus atom

    • allows andenylyl transfer