CARBON-FIXATION CYCLE

                          CALVIN CYCLE

In all oxygenic photosynthetic organisms, CO2 reduction and fixation occurs through carbon-fixation cycle called Calvin Cycle. It is also known as reductive pentose phosphate cycle or C3 cycle. It is a series of biochemical reactions which reduce co2 to three carbon sugar. The C3 cycle takes place in the stroma of chloroplasts in eukaryotic photosynthetic organisms. 

In prokaryotic photosynthetic organisms, it occurs in cytosol. The cycle was elucidated by Melvin Calvin and Andy Benson at the University of California, Berkeley, for which a Nobel Prize was awarded in 1961. The carbohydrate produced directly from the Calvin cycle is actually not glucose, but a three carbon sugar : Glyceraldehyde-3-phosphate(G3P). For the net synthesis of one molecule of glyceraldehyde-3-phosphate, three molecules of Co2 are required.

The reaction of the C3 cycle can be divided into three phase: Carboxylation , Reduction and Regeneration of the ribulose-1,5-bisphosphate (RuBP).

Carboxylation

In carboxylation phase, Co2 is incorporated into five-carbon compound ribulose-1,5-bisphosphate (RuBP). The enzyme which catalyzes this step is ribulose-1,5-bisphosphate carboxylase/ oxygenase (usually called Rubisco). The product of the reaction is a six-carbon intermediate which immediately splits to form two molecules of 3-phosphoglycerate. Thus, for every three molecules of Co2 that enter the cycle, there are six molecules of 3-phosphoglycerate formed.

Rubisco is the pacemaker enzyme of the C3. Plant Rubisco is a multisubunit enzyme composed of eight large subunits and eight small subunits. Large subunits are encoded by the chloroplast genome whereas smaller subunits are encoded by nuclear genome. It makes up about 50% soluble protein in chloroplasts. Rubisco is the most abundant protein in chloroplasts and is also said to be the most abundant protein on the earth. Its activity is regulated by Co2, O2, Mg2+ and pH. The optimum pH for Rubisco activity lies around pH 8.

This step begins with the conversion of RuBP into a highly reactive enediolate intermediate. The Co2 molecules combines with the enediolate intermediate to form an unstable six-carbon compound, which is rapidly hydrolyzed to two molecules of 3-phosphoglycerate.

This Co2 molecule combines with the Amino group of lysine of large subunit of Rubisco to form a Carbamate. This negative charged adduct then binds the Mg2+ ion. The metal center plays a key role in binding RuBP and activating it so that it reacts with Co2. 

Reduction 

The next phase of cycle involves reduction of 3-phosphoglycerate into glyceraldehyde-3-phosphate. First, 3-phosphoglycerate is phosphorylated at the expense of one ATP molecule to form 1,3bisphosphoglycerate. This step is catalyzed by phosphoglycerate kinase. The latter molecule is then reduced by NADP: glyceraldehyde-3-phosphate dehydrogenase to form one molecule of glyceraldehyde-3-phosphate. Six molecules of ATP are utilized in phosphorylating six molecules of 3-phosphoglycerate to 1,3-bisphosphoglycerate and six molecules of NADPH are consumed in reducing six molecules of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate. 

Regeneration of RUBP

The net production of fixed carbon in the Calvin cycle is one molecule of glyceraldehyde-phosphate. The other five glyceraldehyde-3-phosphate molecules are processed in the remainder of the calvin cycle reactions to regenerate three molecules of RUBP. 

For three molecules of RUBP that are carboxylated , cleaved, phosphorylated, reduced and dephosphorylated, six molecules of glyceraldehyde-3-phosphate are produced. Of these, one glyceraldehyde-3-phosphate exits the pathway as a product and five of the 3-carbon molecules are recycled back into three 5-carbon molecules of RUBP. It is an active process.

For regeneration of one RUBP, one ATP is required. Thus, total of three ATP are required for regeneration of three RUBP. 

In summary, for the net synthesis of one glyceraldehyde-3-phosphate(G3P) , the Calvin cycle consumes a total of nine molecules of ATP and six molecules OF NADPH.