Glucose-6-phosphatase is absent in skeletal muscle and brain. This means that glycogen breakdown in these tissues cannot release free glucose into the blood because:
- A Muscle glycogen phosphorylase requires AMP as activator, which is absent postprandially
- B Muscle lacks branching enzyme and cannot synthesise normal glycogen
- C G6P cannot be dephosphorylated to glucose, so glucose-1-phosphate from glycogenolysis remains trapped as G6P within muscle cells ✓
- D Glucose transporter GLUT4 exports phosphorylated glucose only
Explanation
Glucose-6-phosphatase is expressed only in the liver, kidneys and intestinal epithelium — not in skeletal muscle or brain. Glycogenolysis in muscle produces glucose-1-phosphate (via glycogen phosphorylase) which is converted to glucose-6-phosphate (via phosphoglucomutase). Without G6Pase, G6P cannot be converted to free glucose; instead it enters glycolysis directly within the muscle cell. This is why muscle glycogen serves as an intracellular energy reserve for muscle itself, not for maintaining blood glucose. This concept is tested in McArdle's disease (myophosphorylase deficiency) and Von Gierke disease (G6Pase deficiency limited to liver/kidney). GLUT4 does not transport phosphorylated glucose.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.