Fundamentals of Molecular Biology. Part 1: Cell Molecular Biology - A. N. Ogurtsov 2011

Photosynthetic Carbon Fixation
The Calvin Cycle in the Chloroplast Stroma

Plant METABOLISM/14.html">Chloroplasts perform A wide variety of metabolic transformations. In addition to CO₂ fixation, which is discussed in detail below, chloroplasts synthesize virtually all Amino Acids, Fatty acids, carotenes, Pyrimidines, and Purines. Among all these Metabolic Pathways in plant Cells, The Biosynthesis of sugars from CO₂ is the most thoroughly studied.

Carbon fixation begins in the chloroplast stroma via a specialized metabolic pathway known as the Calvin cycle, named after Melvin Calvin, who discovered it. The Calvin cycle converts CO₂ into three-carbon compounds utilizing energy released from the Hydrolysis of ATP and The oxidation of NADPH generated during stages 2 and 3 of Photosynthesis.

Plants in which carbon assimilation occurs almost exclusively via the Calvin cycle to form three-carbon compounds are designated as C₃ plants (in contrast to C₄ plants, discussed below). The reaction in which one CO₂ molecule adds to a ribulose-1,5-bisphosphate molecule, yielding two molecules of 3-phosphoglycerate (Figure 187), is catalyzed by the enzyme rubisco—ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39)—located in the chloroplast stroma.

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Figure 187 - Schematic representation of the initial carbon fixation reaction catalyzed by ribulose-1,5-bisphosphate carboxylase: a - CO₂ molecule; b - ribulose-1,5-bisphosphate molecule; c - reaction intermediate stabilized by the enzyme-substrate complex; d - products (two 3-phosphoglycerate molecules)

Rubisco consists of eight identical large subunits and eight identical small subunits, with a massive aggregate Molecular Weight of ~500 kDa. Only one type of subunit is encoded by chloroplast DNA, while the remaining subunits are encoded in the nuclear genome. Because the catalytic turnover rate of rubisco is relatively low, chloroplasts must maintain a high Abundance of the enzyme to ensure an adequate rate of carbon fixation. Rubisco accounts for approximately 50% of total chloroplast protein and is considered the most abundant enzyme on Earth.

Because CO₂ is initially incorporated into three-carbon compounds, the Calvin cycle (Figure 188) is frequently referred to as the C₃ pathway of carbon fixation (as opposed to the C₄ pathway, which is discussed below).

Quantitatively, for every 6 molecules of CO₂, rubisco produces 12 molecules of 3-phosphoglycerate (totaling 36 carbon atoms).

Subsequently, the enzyme phosphoglycerate kinase converts these into 12 molecules of 1,3-bisphosphoglycerate, consuming 12 ATP molecules in the process.

Next, glyceraldehyde-3-phosphate dehydrogenase converts them into 12 molecules of glyceraldehyde-3-phosphate, while oxidizing 12 molecules of NADPH.

Of the resulting 12 glyceraldehyde-3-phosphate molecules, 2 molecules (containing 6 carbon atoms) are exported to the plant Cytosol via specific phosphate-triose phosphate antiporters, where they are converted into 1 molecule of fructose-1,6-bisphosphate.

The remaining 10 molecules of glyceraldehyde-3-phosphate (30 carbon atoms) retained in the stroma undergo regeneration into 6 molecules of ribulose-1,5-bisphosphate, thereby completing the Calvin cycle (Figure 188, top).

The fixation of six CO₂ molecules and the net production of two glyceraldehyde-3-phosphate molecules require the consumption of 18 ATP and 12 NADPH molecules, which must first be synthesized during the light-dependent reactions of photosynthesis.