糖原与糖异生

A brain with no pantry

• The brain burns about 120 g of glucose a day and stores almost none.
• Between meals the body lives on two glucose banks with different owners, and in a longer fast it must make glucose from scratch.

Two banks of glycogen

• Glycogen 糖原 is a branched glucose polymer: α1→4 chains with an α1→6 branch every 8–12 residues.
• The liver stores about 100 g and exports glucose to the blood. Muscle stores about 400 g and keeps it for itself.

Taking it apart, building it up

• Glycogen phosphorylase 糖原磷酸化酶 cuts α1→4 bonds using phosphate, not water, so the glucose comes off already phosphorylated and no ATP is spent. Debranching enzyme deals with the α1→6 stubs.
• Glycogen synthase and branching enzyme build it back up when glucose is plentiful.

One enzyme decides the owner

• Liver has glucose-6-phosphatase 葡萄糖-6-磷酸酶: glycogen → glucose-1-P → glucose-6-P → free glucose → blood.
• Muscle lacks it, so its glucose-6-P is trapped and burned in place. You cannot export what you cannot dephosphorylate.

Hormones set the direction

• Glucagon 胰高血糖素 tells the liver to break glycogen down; muscle has no glucagon receptors. Adrenaline acts on both.
• Insulin 胰岛素 tells liver and muscle to store glucose as glycogen.

Making new glucose

• Gluconeogenesis 糖异生 runs glycolysis backwards, using different enzymes around the three one-way gates. Its raw materials are lactate (the Cori cycle), amino acids from muscle and glycerol from fat.
• It costs 4 ATP + 2 GTP + 2 NADH per glucose: 6 high-energy phosphates, against glycolysis's net gain of 2.

Worked example: the overnight budget

Worked example: the bank that cannot be opened

Three common slips

Recap