CAM carbon shuttle

Crassulacean acid metabolism, the temporal counterpart of the C4 shuttle: the two carboxylations are separated across the day/night cycle in one cell rather than between two cell types. At night, with stomata open, PEP carboxylase fixes bicarbonate into oxaloacetate, which is reduced to malate and stored as malic acid in the vacuole, acidifying it. By day, with stomata shut, malate leaves the vacuole and is decarboxylated, and the released CO2 is refixed by Rubisco behind closed stomata, which is what makes the pathway water-conserving. PEP is regenerated from the C3 product, usually via starch or soluble sugar. The phase switch is enforced by reciprocal phosphorylation of PEP carboxylase: a circadian-controlled kinase phosphorylates it at night, relieving malate inhibition, and dephosphorylation by day prevents futile refixation of the CO2 being released. The Calvin-Benson-Bassham cycle is included by reference. The C4 spatial shuttle is a sibling module (MODULE:c4_carbon_shuttle); the two share most of their enzymology and differ in what separates the carboxylations.

Species-neutral at this boundary. Mesembryanthemum crystallinum is the orienting exemplar for the carboxylation step because it is the classical facultative-CAM model with reviewed entries, and the tonoplast transporters are Arabidopsis exemplars of families that are not CAM-specific: ALMT9 and TDT carry the malate flux in C3 plants too, and no reviewed CAM-species entry exists for either. Naming them is a family-level orientation, not a claim that these proteins run CAM. The same applies to the PPCK exemplars Q9SPK4 and Q93VK0, which are likewise Arabidopsis and likewise stand for the family rather than for CAM-running proteins. The PP2A phosphatase that the description credits with the daytime half of the switch is named but deliberately not modelled: its identity in CAM species is not established well enough to ground an annoton, and the switch is satisfied by the kinase's absence by day. The four classical CAM phases (I-IV) are a description of flux over time rather than four distinct enzyme sets, so they are described in prose and encoded as the night/day parts plus the phosphorylation switch, not as four parts. Stomatal control is a precondition and a separate biology, so it is named but not modelled. Decarboxylation subtypes (malic-enzyme versus PEP-carboxykinase CAM) exist as in C4 but GO has no CAM-specific children for them, so they are modelled as a variant set with no subtype concepts. Facultative/inducible CAM and the CAM-idling and CAM-cycling variations are realizations, not parts.