C4 carbon shuttle

The C4 carbon-concentrating shuttle, which raises the CO2 concentration at Rubisco by fixing inorganic carbon twice in two cell types. Mesophyll carbonic anhydrase supplies bicarbonate, PEP carboxylase fixes it into a C4 acid, the acid moves to the bundle sheath and is decarboxylated there, and the released CO2 accumulates around Rubisco while the C3 product returns to the mesophyll for PEP regeneration by pyruvate, phosphate dikinase. The decarboxylation step is the classic three-way variation (NADP-malic enzyme, NAD-malic enzyme, PEP carboxykinase), modelled as a variant set; real species often run more than one route at once. The Calvin-Benson-Bassham cycle that consumes the released CO2 is included by reference rather than restated. Single-cell C4 (Bienertia, Suaeda aralocaspica), which performs the same shuttle between two compartments of one cell, is a realization of this module rather than an exception to it, but its compartment assignments are not modelled here. CAM, which separates the two carboxylations in time rather than in space, is a sibling module (MODULE:cam_carbon_shuttle).

Species-neutral at this boundary: maize proteins are reviewed orienting exemplars for an NADP-ME implementation, not a claim that the module is maize-specific or that these are the only members. Representative members are reviewed (Swiss-Prot) entries except the maize carbonic anhydrase, for which no reviewed entry exists; Q41729 is an unreviewed entry carrying EC 4.2.1.1 and is marked as such. The variant set is deliberately non-exclusive: the NADP-ME, NAD-ME and PCK routes co-occur, and PCK commonly operates as a supplement to a malic-enzyme route rather than alone. Bundle sheath and mesophyll are named in prose and in node labels rather than as cell-type terms, because the module is also satisfied by single-cell C4. Suberized bundle-sheath walls and the resulting low CO2 conductance are a precondition for the shuttle working, not a step of it, and are left out.