Transmembrane ion gradients

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Transmembrane ion gradients

• Most commonly used mechanism for energy
  production in microbes
• H gradient across a membrane proton motive
  force (PMF)
• Na grad can also be used (SMF) when external Na
  is high
• Grads formed by respiration, photosynthesis,
  enzyme pumps and scalar reactions

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How ion grads make ATP (Fig. 6.5)

• F1F0 ATP synthases interconvert electrochemical
  and chemical energy
• F0 is proton pump
• F1 is ATPase/ATP synthase
• 3 or 4 protons flow in for 1 ATP produced

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ATPase structure (Brock, Fig. 5.21)
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Uses for ion grads

• Make ATP
• Coupled transport
• Maintenance of turgor
• Maintenance of pH
• NAD(P)H production (reverse respiration)
• Powering flagella to turn

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Respiration (Fig. 6.6)

• Transfer of e-s through ETS (membrane bound e-
  carriers)
• Hs are transferred across membranes when H
  donors and e- acceptors are adjacent
• Final e- acceptors O2, NO3-,fumarate, others

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Electron transfer carriers -- Diffusible (Brock
Fig. 5.10)

• Reduction potentials for NAD(P)/NAD(P)H
  NAD/NADH are -.32 V, i.e., good electron donors
• Act as coenzymes (freely diffusible) in many
  biochemical reactions
• NAD/NADH usually used in catabolic rxns
• NADP/NADPH usually used in anabolic rxns
  photosynthesis

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Electron carriers often found in membrane proteins

• Flavin mononucleotide (l) nonheme iron center (r)

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Membrane electron carriers -- Porphyrin rings

• Is constructed of 4 pyrroles
• Metal varies - Fe2/3 found in heme Mg2 in
  chlorophyll
• Cytochromes have hemes
• Cytochromes have varying reduction potentials

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PMF generation during aerobic respiration

• Protons are transferred outside membrane when
  electron carriers are next to H carriers
• Electron carriers cytochromes, iron-S proteins
• H carriers NAD(P), quinones, FMN
• Proks vary in which carriers they use and which
  initial donors and final acceptors are used

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ETS and relation to reduction potential

• Carriers are located in membrane according to
  relative reduction potentials

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Alternate electron donors Brock, Fig. 5.9)
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Photosynthesis

• Light energy activates electrons to flow through
  an ETS to generate PMF/ATP and NADPH (separate
  reaction system for ATP)
• Cyclic photophosphorylation produces PMF/ATP in a
  process similar to respiration
• Non-cyclic photophosphorylation transfers e-s to
  NADP and requires an electron donor.

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Chlorophylls

• Chlorophylls (in oxygenic organisms) and
  bacteriochlorophylls (anoxygenic) are porphyrins
  (Mg2)
• Found in membranes associated with light
  harvesting pigments

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Structures of Bchls with different Absmax
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Light harvesting pigments (Brock Fig. 17.9)

• Carotenoids are membrane bound
• Functions are both protective and photosynthetic

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Role of carotenoids in photophosphorylation

• Carotenoids absorb light at different wavelengths
  than associated Chl or Bchl and transfer energy
  within the reaction centers, therefore extending
  the range of useable light

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Accessory pigments

• Phycocyanin is protein containing a typical
  phicobilin (open tetrapyrroles derived from
  porphyrins) found in cyanobacteria
• These pigments broaden wavelengths of absorbable
  light

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Oxygenic vs. anoxygenic photosynthesis

• Light activates electrons that generate ion grads
• Ion grads generate ATP
• Reducing power comes from different sources in
  these 2 patterns

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Cyclic photophosphorylation

• Activation by light is of chlorophyll (Bchl) e-s
• PMF is generated
• ATP synthases generate ATP
• No extra electron donor

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Non-cyclic photophosphorylation (Fig. 6.9)
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Electron donors for photosynthesis
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Classification of organisms based on O2
utilization

• Utilization of O2 during metabolism yields toxic
  by-products including O2-, singlet oxygen (1O2)
  and/or H2O2.
• Toxic O2 products can be converted to harmless
  substances if the organism has catalase (or
  peroxidase) and superoxide dismutase (SOD)
• SOD converts O2- into H2O2 and O2
• Catalase breaks down H2O2 into H2O and O2
• Any organism that can live in or requires O2 has
  SOD and catalase (peroxidase)

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Other ways to generate ion grads

• Enzyme pumps can generate grads when enzymatic
  activity is coupled to ion transport on membranes
• Some enzyme pumps are photo-activated
• Scalar reactions produce ion gradients without
  transport (e.g., O. formigenes consumes H while
  decarboxylating oxalate)