Chlorobi

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Chlorobi
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Chlorobium tepidum

• thermophilic Gram-negative green sulfur bacterium
  
• isolated from a hot spring in New Zealand --
  forms a dense mat
• can fix atmospheric nitrogen
• relies on sulfur compounds for photosynthetic
  processes
• Possesses chlorosomes
• The C. tepidum TLS strain grows optimally at 48C
  with a doubling time of about 2.5 hours

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Type 1 reaction centers

• The electron is transferred from the quinone on
  one side to a Fe4S4 cluster in the middle.
• It is then transferred to one of two Fe4S4
  clusters in an extrinsic protein attached to the
  dimeric reaction center.
• Note that the type 1 reaction centers of
  Chlorobium and Heliobacteria are homodimers.

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Comparing type 1 and type 2 RCsthe polypeptides
Photosystem I RC proteobacterial RC
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Comparing type 1 and type 2 RCs

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Comparing type 1 and type 2 RCs

H2
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Comparing type 1 and type 2 RCs

1-electron chemistry Quinone(s) are
intermediates Both branches converge at FX
acceptor
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Bacterial photosynthesis

• Type 1 RC
• reduces ferredoxin (the quinone contained within
  the RC is not mobile)
• ferredoxin can be used by FdNAD oxidoreductase
  (FNR) to reduce NAD(P)
• An enzyme like complex I can be used to pass the
  electrons from NADH to the mobile quinone pool.
  (There is evidence for a Fdquinone
  oxidoreductase in some organisms.)
• Cyt bc transfers electrons from the quinone to
  cyt c, which re-reduces the primary donor of the
  RC.

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Bacterial photosynthesis

• In order to reduce CO2 (and make carbohydrate),
  it is necessary to have an electron source.
• Anoxygenic photosynthetic bacteria can utilize
  several different electron sources, including H2S
  and Fe2, depending upon the species. (These are
  much easier to oxidize than H2O.)
• Quinone pool is the point of entry. For example,
  the enzyme sulfidequinone oxidoreductase (found
  in several species) catalyzes the reduction of
  quinones using H2S as a reductant H2S Q gt S0
  QH2
• The ways in which bacteria carry out net
  reduction of NADH also differ, depending upon
  their RC.

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Bacterial photosynthesis

• Type 1 RC
• RC reduces ferredoxin
• FdNAD oxidoreductase (FNR) passes electrons to
  NAD
• Type 2 RC
• also contain a NADHquinone oxidoreductase
  (a.k.a. NADH dehydrogenase).
• Oxidation of quinones drives proton pumping.
• When the proton gradient gets large enough, the
  reverse reaction becomes favorable with
  dissipation of the proton gradient.

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PS1 vs. other type 1 RCs

• Different antenna systems
• PS1 (plants algae) LHC1
• PS1 (cyanobacteria) phycobilisomes
• Chlorobi chlorosomes
• Heliobacteria nothing (?)

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PS1 vs. other type 1 RCs

• The other RCs are simpler homodimeric fewer
  subunits
• PS1 PsaA/B/C/D/E/F/I/J/K/L/M/N (G,H)
• Chlorobi PscA2 PscB(2x Fe4S4) PscC2 PscD(2?)
  (FMO3)2
• PscA core
• PscB 2x Fe4S4 extrinsic protein
  (ferredoxin-like)
• PscC membrane-attached cytochrome
• PscD ? (may be involved with binding PscB,
  ferredoxin, and/or FMO protein)
• FMO link between chlorosome baseplate and RC
• Heliobacteria PshA2 PshB(2x Fe4S4) 1-2?

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PS1 vs. other type 1 RCs

• Primary electron acceptor (A0)
• PS1 Chl a
• Chlorobi Chl a
• Heliobacteria 81-OH-Chl a

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PS1 vs. other type 1 RCs

• Use of the quinone(s)
• PS1
• Phylloquinones are intermediates
• Not mobile (a benzoquinone is mobile quinone)
• Not normally doubly reduced
• Chlorobi Heliobacteria
• No evidence for use of quinones in electron
  transfer
• See P A0 ? P FX in 700 ps.
• In these bacteria, the menaquinones are the
  mobile quinone

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Photovoltage effect
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