Plant Physiology - Musienko M. M. 2001

Photosynthesis: Physiological, Biochemical, and Ecological Aspects
Key enzymes of the PPF cycle and their light activation

The reduction of carbon dioxide to CARBOHYDRATES is considered the "dark" phase of Photosynthesis, implying that light plays no direct role in the process. However, recent studies have shown that light activates at least five Enzymes of the cycle: ribulose bisphosphate carboxylase, glyceraldehyde-3-phosphate dehydrogenase, fructose bisphosphatase, sedoheptulose bisphosphatase, and phosphoribulokinase. Therefore, referring to a "dark" phase of photosynthesis is arguably misleading, and we retain the term solely for the sake of continuity with traditional textbooks.

Light triggers various changes in The properties of the chloroplast, where these enzymes reside. For instance, light pumps protons from the chloroplast stroma into the thylakoid lumen, resulting in an increased pH. In the dark, conversely, protons diffuse back into the stroma, thereby lowering the pH. Light also drives the pumping of Mg⁺⁺ ions from the thylakoid interior into the stroma. All these processes help activate the aforementioned enzymes, as their optimal concentrations of protons and Mg match those established upon illumination.

Additionally, light reduces compounds known as "light-action mediators," which in turn can activate certain enzymes of the cycle. While not all of them have been identified, they are believed to be Proteins containing the Amino Acids Cysteine and cystine. In the dark, these amino acid residues exist in an oxidized form (possessing a -S-S- disulfide bond), whereas upon illumination, they are reduced and converted into a form with a sulfhydryl bond (-SH-HS-). It is thought that the reduced light-action mediator reacts directly with the inactive form of the enzyme, reducing a critical functional group within it (possibly a -S-S- bond) and thereby switching the enzyme to its active state. Overall, The regulatory mechanisms of CO₂ reduction are based not only on the control of enzymatic activity, but also on METABOLISM/18.html">The Influence of specific metabolite concentrations and their transport between the chloroplast and Cytoplasm.

Three enzymes are known to be unique to The Calvin Cycle: ribulose bisphosphate carboxylase, phosphoribulokinase, and sedoheptulose-1,7-bisphosphate phosphatase. The most prominent among them is ribulose bisphosphate carboxylase (RuBisCO). Purified RuBisCO is a globular, high-molecular-weight hydrophilic protein that is completely Water-soluble and lacks cations or other conjugates. The enzyme consists of large and small subunits that differ in molecular weight and Amino Acid Composition. The enzyme is thought to resemble a cubic Structure composed of 24 subunits. Small subunits account for 23–30% of the total protein composition. The most probable model of the enzyme's quaternary structure consists of 8 large and 8 small subunits with molecular weights of 51–58 kDa and 12–18 kDa, respectively. The Active Site of the enzyme is located on the large subunits. Notably, the enzyme retains its activity even in the absence of small subunits; however, the small subunits determine the enzyme's conformation and are responsible for regulatory Functions. Enzymatic activity has been detected exclusively in aggregated forms of the large subunits. It is believed that the large and small subunits of RuBisCO are synthesized in different cellular compartments: the large subunit on 70S polyribosomes within the chloroplast, and the small subunit on 80S polyribosomes in the cytoplasm. The assembly of the subunits takes place inside the chloroplast.

Thus, The Biosynthesis of the RuBisCO molecule is a complex process that occurs across different cellular Organelles and is controlled by both nuclear and chloroplast genetic systems.

RuBisCO possesses two catalytic sites: one for binding ribulose bisphosphate, and the other for carbon dioxide. The Evolution of the Calvin cycle occurred during geological epochs when the atmosphere was dominated by carbon dioxide rather than oxygen. These conditions shaped The formation of the primary carboxylation enzyme—ribulose-1,5-bisphosphate carboxylase-oxygenase. This enzyme has a relatively low affinity for CO₂ and is therefore capable of catalyzing a side reaction between ribulose bisphosphate and O₂, leading to the formation of phosphoglycolic acid:

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Consequently, RuBisCO is also capable of a different type of catalytic activity: it can act as a ribulose bisphosphate oxygenase. That is, in the presence of O₂, the enzyme catalyzes the Cleavage of ribulose bisphosphate into 3-phosphoglyceric and phosphoglycolic acids. The rates of both reactions mediated by this enzyme depend on the concentrations of CO₂ and O₂ in the chloroplast stroma. As a result, in plants with the C3 cycle, a significant portion of the CO₂ fixed during photosynthesis is lost due to The breakdown of fixation products and the release of CO₂ via processes that consume O₂. This process occurs exclusively in the light and is therefore termed Photorespiration.