Glycogen and gluconeogenesis
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| glycogen/ˈɡlaɪkədʒn/ | A highly branched glucose polysaccharide used for energy storage in animals and fungi. |
| glycogen phosphorylase | glycogen phosphorylase |
| glucose-6-phosphatase | glucose-6-phosphatase |
| glucagon/ˈɡluːkæɡən/ | A hormone from pancreatic alpha cells that helps raise blood glucose concentration. |
| insulin/ˈɪnsjuːlɪn/ | A hormone produced by pancreatic beta cells that helps lower blood glucose concentration. |
| gluconeogenesis | gluconeogenesis |
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.
Which glycogen store keeps blood glucose up between meals?
Độ khó 1/5 · 100 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
The liver's ~100 g is exported as glucose; muscle's larger ~400 g is used only by the muscle 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.
Match each enzyme to its job.
Độ khó 2/5 · 140 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
Two enzymes take glycogen apart and two build it, and hormones decide which pair is active.
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.
Put the route from liver glycogen to blood glucose in order.
Độ khó 3/5 · 200 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
Glucose-6-phosphatase makes the last chemical step, and only the liver (and kidney) has it.
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.
During a fast, glucagon makes muscle release its glycogen as glucose into the blood.
Độ khó 1/5 · 100 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
Muscle has no glucagon receptors and no glucose-6-phosphatase; its glycogen is only for its own use.
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.
Gluconeogenesis uses 4 ATP and 2 GTP per glucose. How many high-energy phosphates is that?
Độ khó 2/5 · 140 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
4 + 2 = 6, against glycolysis's net gain of 2. Making glucose costs three times what breaking it down earns.
Worked example: the overnight budget
- Liver glycogen: about 100 g × 4 kcal per g = 400 kcal. The brain needs about 120 g = 480 kcal a day.
- So the liver bank covers well under a day of the brain's needs, and less once other tissues share it. After 12–24 h of fasting, gluconeogenesis must take over.
A liver holds 80 g of glycogen (4 kcal per g). The brain uses 480 kcal a day. If the brain alone used this bank, how many hours would it last?
Độ khó 2/5 · 140 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
80 × 4 = 320 kcal, and 320/480 × 24 h = 16 h. In reality other tissues share it, so gluconeogenesis starts earlier.
Worked example: the bank that cannot be opened
- (Fictional runner) Cramps and dark urine after sprinting. Muscle glycogen phosphorylase is missing (the McArdle pattern): glycogen is stored but cannot be reached.
- ATP falls, myoglobin leaks out, and the lactate rise that hard exercise should cause never comes. No lactate with maximal effort points to a break before glycolysis.
In McArdle disease, why does blood lactate not rise during a sprint?
Độ khó 1/5 · 100 xu + phần thưởng nhỏ khi hoàn thành lần đầu tiên hôm nay
Without phosphorylase, stored glycogen cannot feed glycolysis, so pyruvate and lactate stay low; the break is before glycolysis.
Three common slips
- One bank. Muscle glycogen never becomes blood glucose.
- "Gluconeogenesis is free." It costs 6 high-energy phosphates per glucose.
- Glucagon on muscle. Muscle has no glucagon receptors; adrenaline mobilises its glycogen.
Recap
- Liver (~100 g) exports glucose because it has glucose-6-phosphatase; muscle (~400 g) keeps it.
- Phosphorylase and debrancher break glycogen down; synthase and branching enzyme build it.
- Gluconeogenesis costs 4 ATP + 2 GTP + 2 NADH per glucose.