**7. Phosphorus in the regulation of key enzymes involved in the CBB cycle**

The Calvin-Benson-Bassham (CBB) cycle consists of a series of enzyme-catalyzed reactions that assimilate CO2 into organic molecules, such as glucose and other sugars. These reactions can be grouped into three main stages: (1) carboxylation, in which CO2 is fixed to a five-carbon molecule called ribulose-1,5-bisphosphate (RuBP) by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco); (2) reduction, in which the resulting six-carbon molecule is split into two three-carbon molecules of 3-phosphoglycerate (3-PGA) and subsequently reduced to glyceraldehyde-3-phosphate (G3P) using the energy provided by ATP and NADPH; and (3) regeneration, in which the remaining G3P molecules are used to regenerate RuBP through a series of reactions that also involve the consumption of ATP [55].

P plays a crucial role in the regulation of several key enzymes involved in the CBB cycle, including Rubisco, phosphoribulokinase (PRK), and GAPDH. For example, Rubisco, the most abundant enzyme in the chloroplast and responsible for the carboxylation of RuBP, requires a divalent metal ion, such as Mg2+, for its catalytic activity [56]. In the dark, the Mg2+ ion is replaced by a proton, leading to the formation of a stable complex between Rubisco and a molecule of RuBP. The addition of inorganic phosphate (Pi) can reverse this inhibition by promoting the release of the proton and the re-activation of the enzyme [57]. Therefore, the activity of RuBisCO is regulated by the phosphorylation and dephosphorylation of its activase, RuBisCO activase [58]. The phosphorylation status of RuBisCO activase is modulated by a protein kinase and a phosphatase, which are sensitive to the ATP/ADP ratio in the chloroplast stroma [58]. This regulation ensures that RuBisCO activity is adjusted according to the energy availability within the cell.

Similarly, PRK, the enzyme responsible for the phosphorylation of ribulose-5-phosphate (Ru5P) to RuBP, is also regulated by the availability of Pi. The enzyme is inhibited by the binding of a molecule of ADP, which competes with the substrate, ATP, for the same binding site. The addition of Pi can relieve this inhibition by promoting the formation of ATP from ADP and Pi, thereby allowing the enzyme to resume its catalytic activity [27, 59]. Finally, GAPDH, the enzyme that catalyzes the reduction of 1,3-bisphosphoglycerate (1,3-BPG) to G3P using NADPH as the reducing agent, is also sensitive to changes in the availability of Pi. The enzyme forms a complex with another enzyme, phosphoribuloseglycerate kinase (PGK), and the two enzymes work together in a coupled reaction to convert 3-PGA to G3P. The formation of this complex is dependent on the presence of Pi, which acts as a stabilizing factor and ensures the efficient transfer of phosphate groups between the two enzymes [60, 61].
