Bacterial Photosynthesis, - Kenneth Stedman, 15 Nov. 2002

Photosynthesis myths

• Only plants are phototrophs
• All phototrophs are green
• Photosynthesis produces Oxygen
• All photosynthesis uses a multiprotein complex
• Photosynthesis is pretty well characterized

Energy = Life

What do they look like?

• Phototrophs in a hot (60ºC) spring in the crater of Mount Saint Helens. (eruption 1980)

Oxygenic Photosynthesis

• Plants
• Algae
• Cyanobacteria
• Oxygen is a “Side-product”

Anoxygenic Photosynthesis

• Mostly reduced sulfur-containing chemicals as reductants
• Many Bacteria

Catching Light

• Pigments
  • Chlorophyl
    • Mg-tetrapyrrole
    • Usually in membranes
    • Highly variable

Bacteriochlorophyl a

• Adsorbs other wavelengths than chlorophyll a (and many other Bacteriochlorophyl types)

Photosynthetic Membranes

• Thylakoid- membrane
  • Eukaryotes
• Other membranes
  • Prokaryotes
  • Chlorosome

Other Pigments

• Phycobilins
  • Phycobiliproteins - Light antenna in cyanobacteria and red Algae
• Outside: Phycoerthyrin (550nm
• Middle: Phycocyanin (620nm)
• Inner: Allophycocyanin (650nm)
• Chlorophyll a
• More Phycobilisomes at low Light

What’s Next?

Structures

• Reaction center (Nobel prize)
• “Special Pair” Bacteriochlorophyl a
• 2 Bacteriochlorophyl molecules
• 2 Bacteriopheophytins
  • Bacteriochlorophyll a without Mg
• 2 Quinones
• 2 Carotenoides

Reaction center

• Deisenhofer, J., Huber, R., Michel, H. Nature (1985) 318, 618-624
• Deisenhofer, J., Epp, O., Sinning, I. and Michel, H. (1995) Crystallographic refinement at 2.3 Å resolution and refined model of the photosynthetic reaction centre from Rhodopseudomonas viridis. J. Mol. Biol. 246, 429-457.

Cyclic Photophosphorylation

• “Special Pair” very electropositive
• With light very electronegative
• Electron passed to a Bph and a Quinone
  • (10^12 sec)
• Then into the electron transport chain
  • H+ Gradient
  • ATP Synthesis
• Need extra Reductant (H2S) for NAD(P)H production

Other anoxygenic phototrophs

Halobacteria

• Extremely halophilic Archaea have a 2D crystal of the enzyme Bacteriorhodopsin in their “Purple membrane”

Bacteriorhodopsin Activity

• Under irradiation Bacteriorhodopsin pumps H+ across the Membrane.
• This makes ATP
• Need to isomerize Retinal for this process

Bacteriorhodopsin structure

• Proteorhodopsin Activity
• A bacteriorhodopsin gene was found in cloned DNA from an uncultivated marine bacteria
• This gene can be functionally expressed in E.coli!
• This activity is ubiquitous in the Oceans

Summary

• Photosynthesis = Light > Carbohydrates
• Light > (Chemical) Energy
  • Chlorophyll a / Bacteriochlorophyll
  • Other Pigments
• Antenna/Reaction centers
  • Membranes
  • e- Transport
• Bacteriorhodopsin/Proteorhodopsin

Photosynthesis myths

• Only plants are phototrophs
  • At least 50% of photosynthesis is bacterial
• All phototrophs are green
  • Phototrophs come in all colors
• Photosynthesis produces Oxygen
  • Lots of anoxygenic photosynthesis
• All photosynthesis uses a multiprotein complex
  • Bacteriorhodopsin/Proteorhodopsin are single enzymes
• Photosynthesis is pretty well characterized
  • Many uncharacterized phototrophs in Oceans