Practice question

Maintaining blood glucose between meals requires hepatic glucose output via glycogen breakdown and gluconeogenesis. Both pathways converge on glucose-six-phosphate cytosolic that cannot exit cell due to charge; final dephosphorylation occurs inside endoplasmic reticulum lumen by glucose-six-phosphatase complex composed of transporter T1 importing G6P, catalytic subunit facing lumen hydrolyzing to glucose and Pi, transporters T2 and T3 exporting Pi and glucose. Active site luminal protects against futile cycling by cytosolic hexokinase. Enzyme enriched in smooth ER of hepatocytes and kidney cortex proximal tubules, marker for microsome fraction. Hexokinase I-IV phosphorylate glucose in cytosol trapping it, phosphofructokinase commits glycolysis, DNA polymerase replicates nucleus. Deficiency in G6Pase catalytic subunit causes von Gierke disease type Ia glycogen storage with fasting hypoglycemia, lactic acidosis, hyperlipidemia and hepatomegaly due to shunting to lactate and glycogen. Localization integrates ER as metabolic hub coupling carbohydrate metabolism to systemic homeostasis. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.