The Arnon-Buchanan cycle: a retrospective, 1966-2016.

Area of Science:

• Biochemistry
• Microbiology
• Metabolic Pathways

Background:

• The Calvin-Benson cycle was initially considered the only route for autotrophic CO2 assimilation after its 1954 description.
• Early 1960s experiments with fermentative bacteria revealed alternative CO2 fixation mechanisms.

Observation:

• Ferredoxin was observed to directly donate electrons for CO2 fixation into alpha-keto acids.
• Reactions were identified where pyruvate and alpha-ketoglutarate could be synthesized from CO2 and reduced ferredoxin.

Findings:

• Discovery that reduced ferredoxin can drive the Krebs cycle in reverse, enabling CO2 fixation.
• The Arnon-Buchanan cycle, initially found in green sulfur bacteria, operates independently of light in various anaerobic chemoautotrophs.
• This pathway is now established in major chemoautotrophic bacterial groups, often replacing the Calvin-Benson cycle.

Implications:

• Establishes the Arnon-Buchanan cycle as a significant alternative to the Calvin-Benson cycle for CO2 assimilation.
• Highlights the metabolic versatility of anaerobic chemoautotrophs.
• Contributes to a revised understanding of microbial carbon fixation pathways.