metabolic diversity

electron donors

• chemoorganotrophy: organic molecules

• chemolithotrophy: inorganic molecules

• phototrophy: uses light energy to reduce compounds and then use these as electron donors

electron acceptors

• respiration: inorganic /organic molecules

• fermentation: organic molecules

gibbs free energy change

electron transport

• electron transfer never occurs randomly: moves from a low reduction to a high reduction potential

• genrates proton motive force

• metabolism is underpinned by the production of 2 sources of energy: reducing energy (NADH, NADPH and FADH2) and ATP

chemoorganotrophy

• a wide range of organic compounds can be used as a source of electrons: carbs, lipids, peptides, aromatic compounds

• produces acetyl-coA and pyruvate (metabolites)

• what makes metabolism complicated is that there is more than 1 way to metabolise glucose (glycolysis, entner-doudoroff and pentose phosphate pathway)

elevtron transport (inorganic) occurs via:

• cytochromes, quinones and iron-sulfur proteins

anyoxygenic respiration is important to exploit a wide range of ecological niches

• aquatic water quality and denitrification

methanogenesis

• acetate: CH₃COO^- + H⁺ + CH₄ + CO₂

• methanol: 4CH₃OH + 3CH₄ + CO₂ +2H₂O

what differentiates anaerobic respiration and fermentation

1. anaerobic respiration:

   • uses inorganic molecules (other than O2) or organic molecules as terminal electron acceptors via a membrane bound respiratory chain

   • ATP produced by oxidative phosphorylation via the PMF

2. fermentation:

   • use of organic molecules as electron acceptors, without use of a respiratory chain

   • ATP produced by substrate-level phosphorylation in the cytoplasm

   • fermentation energy yields are low; cells grow more slowly than when they respire

chemolithotrophy: important properties

• chemolithotrphs use CO₂ as a carbon source to produce organic molecules via calvin cycle, reverse TCA cycle

• they can also use more complex molecules (acetate)

• to fix carbon, they require NADH; this requires consumption of H⁺ for a reverse electron flow process

chemolithotrophy: electron acceptors

• O₂: H₂ + ½ O₂ → H₂O

• SO₄^2-: 4H₂ + SO₄^2- + H⁺ → HS + 4H₂O

• CO₂: 4H₂ + CO₂ → CH₄ + 2H₂O

iron oxidation

• Reduced iron Fe²⁺ can be oxidised to Fe3+ at low pH 2Fe²⁺ + ½ O₂ + 2H⁺ → 2Fe³⁺ + H₂O

• Ferric ions (Fe³⁺) form insoluble ferric hydroxide [Fe(OH)₃] as the pH Is getting lower Fe³⁺ + 3H₂O → Fe (OH)₃ + 3H⁺

nitrogen oxidation

• Ammonia and nitrites can be used as electron donors to produce nitrates (NO^3-)

• Nitrification (NH₄⁺ → NH₂OH → NO₂^- → NO₃^-) occurs in aerobic conditions, carried out by Nitrosomonas and nitrosobacter

• Anammox (NH₄⁺ + NO₂^- → N₂ + 2H₂O) occurs in anaerobic conditions, carried out by planctomycetes

sulfur oxidation

• Several sulfur derivatives can be used as electron donors to produce sulfuric acid (H₂SO₄)

• H₂S → SO → S₂O₃^2- → H₂SO₄

• Use of acid-producing microbes in biomining

phototrophy

• oxygenic photosynthesis and anoxygenic photosynthesis

bacteriorhodospin

• a very abundant light-driven proton pump in archeal membranes.

• Contains a pigment (retinal) that undergoes conformational change once excited by light (trans → cis).

• this triggers the transfer of a proton to Asp85. Deprotonated retinal pushes against helix F, opening a channel on the cytoplasmic side; this induces deprotonation of retinal from Asp96

• Asp96 undergoes reprotonation. Asp96 transfers a proton outside through hydrogen bonding via water molecules and other residues

oxygenic photosynthesis (cyanobacteria)

• no chloroplast

• their photosynthetic apparatus is variable; most often made of thylakoids

• light is captured by light harvesting complexes (contains several pigments which can use light energy at various wavelengths) which channels energy to a reaction centre

the oxygenic z pathway

• 2 distinct photosystems are excited by light

• light provides energy to strip the electron from water yielding H⁺

• the electron flow is used to pump protons outside the cell and reduce NADP⁺

• the H⁺ gradient is used to generate ATP

• NADPH and ATP are used to fix CO₂ and make glucose

green sulfur bacteria

• light is captured by antenna complexes in organelles called chlorosomes

• photon energy is transferred to the PSI reaction centre

• anoxygenic photosynthesis: PSI donates an electron to the ETC, electron transport pumps protons outside the cell and reduces NADP⁺ via ferredoxin. The proton gradient is used to generate ATP. PSI recieves electrons from inorganic sulfur derivatives

purple bacteria

• light is captured by antenna complexes in organelles called chromatophores

• photon energy is transferred to the PSII reaction centre

• PSII donates an electron to a cyclic ETC

• electron transport pumps protons outside the cell, the H+ gradient is used to generate ATP (cyclic photophosphorylation)

• NADH is produced by reverse electron flow; electrons are transferred from reduced ETC components (with a more positive reducing potential). This reaction is not thermodynamically favourable and consumes energy

• electrons transferred to NAD+ by ETC components are replenished by inorganic or organic compounds