Microbial Metabolism

I.  Introduction

II.  Energy and Enzymes

    A.  Laws of Thermodynamics

        1.  First law

                Energy can be neither created nor destroyed
 

        2.  Second law

    B.  Free Energy and Reactions

        G =  H - T• S

        G is the change in free energy
        H is the change in enthalpy (heat content)
        T is the temperature in degrees Kelvin
        S is the change in entropy

        reaction occurs spontaneously if the free energy of the system decrease during the reaction (i.e.  G is negative)

        If the bond energy of the products is less than that of the reactants, heat is released and entropy increases.

        What happens when G = 0?
 
 

    C.  ATP

        adenosine triphosphate (ATP)

        energy transferring molecule in living cells

        ATP has a high energy potential ( G = -7.3 kcal/mole)

        chemical structure
 
 

    D.  Oxidation-Reduction Reactions

        oxidation: removal of electrons

        reduction: gain of electrons

        Oxidation-reduction (O/R) are those reactions in which electrons are transferred from a
        donor to an electron acceptor.
 

        The equilibrium constant for the reaction is called the standard reduction potential (E0).

        Redox couples with more negative reduction potentials will donate electrons to couples with
        more positive potentials.
 
 

    F. Enzymes

        1. Biocatalysts: proteins that increase the rate of a chemical reaction without being altered
 

        2. Composition

        3. Characteristics

            a)  enzymes are not chemically altered

            b) enzymes are specific for each reaction

            c) enzymes accelerate reactions by lowering activation energy

            d)  enzymes do not alter equilibration constants
 
 

III.  Energy Generation

    A.  Overview of Metabolism

        1. Metabolism
 

        2. Anabolism
 

        3. Catabolism

            three stages of catabolism

            a) large nutrients are broken down to constituents
 

            b) constituents broken down to simple molecules (acetyl CoA, pyruvate, TCA
                intermediates)
 

            c) carbon molecules fed into the TCA cycle
 

        4. Amphibolic pathways
 
 

    B.  Breakdown of Glucose to Pyruvate

        1. Glycolytic pathway

            Embden-Meyerhoff pathway, glycolysis

            a) 6 carbon stage; requires 2 molecules of ATP
 

            b) 3 carbon stage; 2 molecules of pyruvate and 2 molecules of NADH formed
 
 

        2. Other pathways for the partial oxidation of glucose

            a) pentose phosphate (hexose monophosphate pathway)

                glucose degraded to 6 molecules of CO2, 12 molecules of NADH, and 1 molecule of
                ATP

                Pathway can be a source of energy, but it is more important for biosynthesis.
 

            b) Entner-Doudoroff pathway

                glucose degraded to 1 molecule of ATP, 1 molecule of NADH, and 1 molecule of
                NADPH
 

    C.  Tricarboxylic Acid (TCA) Cycle

        Krebs cycle, citric acid cycle

        gateway reaction

        acetyl CoA is the substrate for the TCA cycle

        1 molecule of acetyl CoA yields 3 NADH, 1 FADH2, 1 GTP, and 2 CO2
 
 

    D.  Electron Transport System (ETS)

        1. Transfer of electrons

            ETS is composed of a series of electron carriers (cytochromes, coenzymes) that transfer
            electrons from electron donors (e.g. NADH) to electron acceptors (e.g. oxygen).
 

        2. ATP formation

            via oxidation phosphorylation
 

        3. How does oxidative phosphorylation occur?

            a) chemiosmosis

                Protons move outward from a cellular membrane and electrons transported inward.

                Protons return across membrane in special channels with ATP synthase and driven by
                proton motive force.
 
 

            b) conformational change hypothesis

                ETS release of energy induces change in ATP synthesizing enzyme.
 
 

        4. Aerobic respiration

            oxygen is final electron acceptor
 

        5. Anaerobic respiration


             a)  nitrification
 
 

            b)  sulfate reduction
 
 

            c)  methanogenesis
 
 

    E.  Fermentation

 

 

    F.  Catabolism of Carbohydrates and Intracellular Reserve Polymers

        1. Carbohydrates
 
 

        2. Reserve polymers
 
 

        3. Lipid catabolism

        4. Protein and amino acid catabolism


 

    G. Photosynthesis

        1. Introduction

            light energy is trapped and converted to chemical energy

            generates oxygen
 
 

        2. Light reactions

            a) eucaryotes and cyanobacteria

                Chlorophylls are the most important pigments for the absorption of light.

                energy derived via ETS in chloroplast membrane

                CO2 is the carbon source.

                Water is the electron source.
 

        b) green and purple bacteria

            bacteriochlorophylls

            anoxygenic

            CO2 or organic molecules used as carbon source.

            H2, H2S, S, and organic matter used as electron donors.
 
 

IV.  Use of Energy Biosynthesis

    A.  Introduction

            biosynthetic pathways
 
 
 

    B. CO2 Fixation

        1. CO2 as sole carbon source
 
 

        2. Calvin cycle

            reductive pentose phosphate cycle or dark cycle of photosynthesis

            a) carboxylation

                CO2 is added to a 5 carbon compound to produce two 3 carbon molecules
 

            b) reduction

                3 carbon compound reduced
 

            c) regeneration

                regeneration of ribulose-1,5-biphosphate
 
 

    C. Anapleurotic Reactions

        replenishes TCA intermediates
 

    D.  Synthetic Reactions

        1.  Amino acids

        2.  Carbohydrates

        3.  Nucleotides

        4.  Lipids

            a)  simple lipids

            b)  compound lipids

            c)  steroids
 

Last updated June 11, 2007.