Gluconeogenesis (human) with precursor-entry routes

Extension of the human gluconeogenesis module that makes the choice of non-carbohydrate precursor explicit. The three physiological precursors enter the pathway at different points: lactate (via lactate dehydrogenase) and the glucogenic amino acid alanine (via alanine aminotransferase) are converted to pyruvate and so require the pyruvate carboxylase / phosphoenolpyruvate carboxykinase backbone, whereas glycerol enters lower down — glycerol kinase and cytosolic glycerol-3-phosphate dehydrogenase feed dihydroxyacetone phosphate directly into the triose-phosphate pool, bypassing pyruvate carboxylase and PEPCK entirely. All routes converge on the shared fructose-1,6-bisphosphatase step and the terminal endoplasmic-reticulum glucose-6-phosphatase system (G6PC1 plus the SLC37A4 antiporter). Because glycerol bypasses the carboxylation arm, pyruvate carboxylase is no longer required by every route: the only steps common to all precursor routes are the terminal G6PC1·SLC37A4 system, which is therefore the single universal gate of free-glucose output. The module is built to be evaluated against tissue expression so that, per tissue, one can ask not just whether gluconeogenesis is possible but which precursors a tissue is equipped to use.