Outside reading for extra information: 1. Excellent review of Reaction center structure-function. J. Deisenhofer and H. Michel, "The Photosynthetic reaction center from the purple bacterium Rhodopseudomonas viridis." Science 245: 1463-1473 (Nobel Prize lecture/article).

2. Outstanding, short, and very highly recommended overview of methanogenesis by Ralph S. Wolfe: Alessandro Volta's combustible air. ASM News 62: 529-534.

Carbon dioxide reduction (carbon fixation) is conceptually the inverse of respiration. There is no requirement for light in CO₂ fixation, after all the chemolithoautotrophs use the same carbon reduction pathway as cyanobacteria and higher plants: the Calvin Cycle.

6CO₂ + 18 ATP + 12NADPH --> C₆H₁₂O₆ + 18ADP + 12 NADP⁺

Nearly all autotrophs utilize the Calvin Cycle pathway (see in-class handout for details). Exceptions: green sulfur bacteria, which utilize the reverse TCA path, and Chloroflexus aurantiacus, an enigmatic filamentous organism that uses the hydroxypropanoate pathway (see in class handout for details).

Melvin Calvin received the Nobel Prize for working out the details of the Calvin pathway; it was the first major application of radioisotopes to the solution of a biochemical pathway (used ¹⁴CO₂ to label intermediates of the pathway).

The key enzyme of the Calvin cycle is RuBisCO, suggested to be the most abundant enzyme on Earth (?) :

Ribulose 1,5-Bisphosphate Carboxylase-Oxygenase. It carboxylates the 5-carbon sugar ribulose-1,5-bisphosphate and produces 2 molecules of 3-phosphoglycerate; oxygen is a competitive inhibitor of the reaction, competing with CO₂ for binding. Activation by phosphorylation, followed by reduction produces glyceraldehyde-3-phosphate which can be converted to glucose. Sugar rearrangements in steady-state can regenerate ribulose-bis-phosphate starting material (sugar rearrangement reactions similar to those of the pentose phosphate pathway).

1. Oxygen-evolving

a. cyanobacteria

b. prochlorophytes

2. Purple bacteria

a. purple sulfur bacteria

b. purple non-sulfur bacteria

3. Green bacteria

a. green sulfur bacteria

b. green gliding bacteria

4. Heliobacteria

1. Oxygen-evolving

A. CYANOBACTERIA (G- in wall type, but similar to G+ in many biochemical and genetic properties.

Characterized by presence of Chlorophyll a and water-soluble light-harvesting proteins known as PHYCOBILIPROTEINS. Form unique light-harvesting structure, known as phycobilisomes.

Examples: Synechococcus, Nostoc, Anabaena, Synechocystis,Oscillatoria

B. PROCHLOROPHYTES: very similar to cyanobacteria, and closely related to them. Lack phycobiliproteins, but contain Chlorophylls a +b like higher plants.

Examples: Prochloron, Prochlorothrix, Prochlorococcus (an abundant marine organism)

2. PURPLE BACTERIA (true G-, proteobacteria, like E. coli). Reaction centers related to those of PS II of higher plants

A. PURPLE SULFUR BACTERIA. Autotrophs that utilize H2, H₂S, S⁰, S₂O₃^-2 as electron sources. Contain either Bacteriochlorophyll a or b. More reduced that Chlorophyll a--chemically distinct with distinct absorption properties (longer wavelengths than chlorophyll a, absorbs in the blue and the near-infrared). The characteristic reddish-purple color does not come from Bchl (pale blue-gray) but from carotenoids.

Examples: Chromatium, Ectothiorhodospira, Thiocapsa, Thiopedia,

B. PURPLE NON-SULFUR BACTERIA

Photoheterotrophs under anaerobic conditions; typically chemoheterotrophs under aerobic conditions.

Examples: Rhodobacter, Rhodospirillum, Rhodopseudomonas, Rhodocyclus, Rhodoferax

3. GREEN BACTERIA

A. Green sulfur bacteria. Contain specialized light-harvesting structures, CHLOROSOMES, that contain either Bchl c, d, or e in addition to Bchl a. Typically either green or brown in color. Reaction centers similar to PS I of higher plants. Unique carbon fixation pathway--the reverse TCA cycle. Electrons from H2, S0, S2O3-2, etc.

Examples: Chlorobium, Pelodictyon, Prosthecochloris

B. Green gliding bacteria. Contain chlorosomes with Bchl c + Bchl a. However, have reaction centers like purple bacteria (quinone acceptors) and have another carbon fixation pathway--the hydroxypro-panoate pathway (see handout).

Examples: Chloroflexus, Oscillochloris, Heliothrix

4. HELIOBACTERIA

True G+ bacteria, closely related to Clostridium sp. Have reaction centers related to green sulfur bacteria and higher plant photosystem I RC. Form heat-resistant endospores, and are photoheterotrophs under all conditions--no known autotrophs to date. Contain Bchl g (unique, related to Chl a), and are green in color.

Examples: Heliobacillus, Heliobacterium, Heliophilum

1. Chromophores absorb light energy, and energy migrates to "reaction centers". Very rapid, usually ~1-100 picoseconds.

2. PHOTOCHEMISTRY = Light-driven electron transport reaction. Absolutely dependent upon Chlorophylls. Occurs at special protein complex known as REACTION CENTER (also referred to as PHOTOSYSTEMs in cyanobacteria and higher plants). The primary oxidized species in all of photosynthesis is a special pair of chlorophylls, the "special pair."

3. Dark electron transport reactions follow initial charge separation event. No involvement of light. Very similar to respiration.

4. PHOTOPHOSPHORYLATION, ATP synthesis, occurs via chemiosomotic coupling.

5. Other dark biochemical reactions: CO₂ fixation produces biomass).

Chromoproteins: proteins with light-absorbing prosthetic groups.

1. CHLOROPHYLLS

Proteins that bind chlorophylls have 2 functions:

1. Antenna--absorb photons, transfer energy (not electrons) to reaction centers.

2. Reaction centers perform PHOTOCHEMISTRY -- UNIQUE!! Light absorption leads to electron transfer from donor chlorophyll to an acceptor. Charge separation occurs.

It should be noted that most chlorophyll proteins also bind carotenoids--for protection (see below).

2. CAROTENOIDS

Also have 2 functions:

1. Antenna--harvest light energy, transfer the energy to reaction centers for photochemistry

2. Photoprotection.

P + light --> ³P* + O₂ --> ¹O₂ (singlet oxygen)

³P* + carotenoids --> P

¹O₂ + carotenoids --> O₂ + carotenoid

¹O₂ + carotenoids --> oxidized carotenoid

3. PHYCOBILIPROTEINS.

Unique to cyanobacteria. Only function as antenna proteins. Have linear tetrapyrroles as chromophores (heme ring cleaved to form linear molecule). Very high absorbtivity for visible light. Form PHYCOBILISOMES.

Site of photochemistry, and hence, are Bchl/Chl containing proteins. Differ in organization from antenna proteins by having appropriate electron acceptors near a pair of chlorophylls. Light absorption at special pair causes oxidation of chlorophyll, reduction of acceptor = charge separation = electron tranfer.

Two types occur in nature:

1. Type I Reaction centers.

All have Fe-S centers as electron acceptors and produce very strong reductants. Can directly reduce NAD(P)H

Examples: Photosystem I (PS I) of cyanobacteria, higher plants; green-sulfur bacterial RC; heliobacterial RC

2. Type II Reaction Centers

All have quinone acceptors and CAN NOT reduce NAD(P)⁺ directly--requires reverse electron flow.

Examples: Photosystem II (PS II) of cyanobacteria and higher plants; purple bacterial and Chloroflexus sp. RC

In most cases EXCEPT cyanobacteria/

prochlorophytes: reaction center used primarily to produce a proton gradient for ATP synthesis and to provide driving force for reverse electron flow. Light-driven electron transport is CYCLIC, and no net oxidation/reduction can take place. Reductant for CO₂ fixation comes from organic compounds ("non-sulfur" bacteria) or from inorganic sources ("sulfur" bacteria). Defines ecological niches for these organisms.

The presence of 2 RCs, PS II and PS I allows NON-CYCLIC electron transport to occur. PS II oxidizes water, producing a reduced quinone and oxygen as products. ET chain delivers electrons to oxidized PS I and creates a proton gradient for ATP synthesis. PS I can reduce Fe-S protein ferredoxin, which in turn can reduce NADP⁺ to form NADPH. Electron flow from H₂O to NADP⁺ follows the so-called "Z-scheme." CYCLIC ET can also occur around PS I, producing only a proton gradient for ATP synthesis. For some details, see in-class handout).