PHOTOSYNTHETIC CO2 FIXATION

PHOTOSYNTHETIC OF CO2 FIXATION:

INTRODUCTION :

CO2 is the  principal carbon source of Autotrophs, and the reduction and incorporation of CO2 requires much energy.

 Photoautotrophs obtain energy by trapping light during the light reactions of photosynthesis, and chemolithoautotrophs derive energy from the oxidation of inorganic electron donors. 

Autotrophic CO2 fixation is crucial to life on Earth because it provides the organic matter on which heterotrophs depend. 

Six different CO2-fixation pathways have been identified in microorganisms.

 Eukaryotic autotrophs and most aerobic bacterial autotrophs use the Calvin-Benson cycle, also called the Calvin cycle. Other pathways are used by some obligatory anaerobic and microaerophilic bacteria and archaea. 

CALVIN-BENSON CYCLE:

OVERVIEW:

• The Calvin cycle refers to the light-independent reactions in photosynthesis that take place in three key steps.

• Although the Calvin Cycle is not directly dependent on light, it is indirectly dependent on light since the necessary energy carriers (ATP and NADPH) are products of light-dependent reactions.

• In fixation, the first stage of the Calvin cycle, light-independent reactions are initiated; CO2 is fixed from an inorganic to an organic molecule.

• In the second stage, ATP and NADPH are used to reduce 3-PGA into G3P; then ATP and NADPH are converted to ADP and NADP+, respectively.

• In the last stage of the Calvin Cycle, RuBP is regenerated, which enables the system to prepare for more CO2 to be fixed.

IN DETAIL:

The Calvin-Benson cycle is also called the reductive pentose phosphate cycle because it is essentially the reverse of the pentose phosphate pathway. 

The reactions of the Calvin-Benson cycle occur in the chloroplast stroma of eukaryotic autotrophs. In cyanobacteria, the cycle is associated with inclusions called carboxysomes. These polyhedral structures contain the enzyme critical to the Calvin-Benson cycle and are the site of CO2 fixation.

The Calvin-Benson cycle is divided into three phases: carboxylation phase, reduction phase, and regeneration phase .

During the carboxylation phase,the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO)catalyzes the addition of CO2 to the 5-carbon molecule ribulose 1,5-bisphosphate (RuBP), forming a 6-carbon intermediate that rap- idly and spontaneously splits into two moles phosphoglycerate glycerate (PGA). PGA is an intermediate of the Embden-Meyerhof pathway (EMP). 

In the reduction phase of the Calvin-Benson cycle, PGA is reduced to glyceraldehyde 3-phosphate by two reactions that reverse two EMP reactions. The EMP reactions differ from the Calvin-Benson cycle reactions in that the Calvin cycle enzyme glyceraldehyde 3-phosphate dehydrogenase uses NADPH rather than NADH .

 Finally, in the regeneration phase, RuBP is reformed, so that the cycle can repeat. In addition, this phase produces carbohydrates such as fructose 6-phosphate and glucose 6-phosphate, which are precursor metabolites .This portion of the cycle is similar to the pentose phosphate pathway and involves transketolase and transaldolase reactions.  

To synthesize fructose 6-phosphate or glucose 6-phosphate from CO2, the cycle must operate six times to yield the desired hexose and reform the six RuBP molecules.

6RuBP + 6CO2 → 12PGA → 6 RuBP + fructose-6-p

The incorporation of one CO2 into organic material requires three ATP and two NADPH. The formation of glucose from CO2 may be summarized by the following equation.

6CO2 + 18 ATP + 12 NADPH + 12H+ + 12H2O → glucose + 18 ADP + 18 Pi + 12 NADP+

                                            The Calvin Benson cycle

 

OTHER CO2 FIXATION PATHWAYS:

TCA CYCLE:

The first alternative CO2-fixation pathway discovered was the reductive TCA cycle, which is used by some autotrophs in the bacterial phyla Aquificae, Proteobacteria, Nitrospirae, and Chlorobi. The reductive TCA cycle is so named because it runs in the reverse direction of the normal, oxidative TCA cycle. 

The Reductive TCA Cycle

OTHERS:

The remaining CO2-fixation pathways are used by members of the bacterial phylum Chloroflexi (3-hydroxypropionate bicycle): The reductive acetyl-CoA pathway  is used by some methanogens; the

3-hydroxypropionate/4-hydroxybutyrate pathway is used by members of the order Sulfolobales; and the dicarboxylate/ 4-hydroxybutyrate cycle was discovered in members of orders Thermoproteales and Desulfurococcales. A seventh pathway has been proposed that links CO2 fixation to dissimilatory phosphite oxidation. This proposal is supported by genomic evidence, but has not yet been established biochemically.