Effect of insulin (digestive period)
During the digestive period, blood glucose concentration rises to 10–12 mmol/L. This serves as a potent signal for insulin secretion by the pancreas. As a result, the insulin-glucagon ratio shifts toward insulin dominance.
Insulin regulates metabolism through three key mechanisms:
- Transport level: accelerates glucose uptake by cells in insulin-dependent tissues (muscle and adipose tissue).
- Genetic level: induces the synthesis of the enzyme glucokinase in the liver, accelerating glucose phosphorylation.
- Covalent modification: activates phosphodiesterase (which decreases intracellular cAMP levels) and triggers the action of glycogen particle phosphoprotein phosphatase (PP1G).
Insulin binding to its receptor initiates a cascade that converts PP1G from an inactive to an active form (utilizing ATP). The active phosphatase removes inorganic phosphates from key enzymes. Consequently, glycogen synthase is dephosphorylated and becomes active, while glycogen phosphorylase is dephosphorylated and loses its activity. This initiates glycogen synthesis and halts its degradation.
Hormonal regulation under stress
During acute stress, blood concentrations of epinephrine increase. The hormone binds to $\alpha_1$-receptors on hepatocyte membranes, triggering the inositol phosphate signaling pathway.
This cascade involves the activation of phospholipase C, which leads to two effects:
- Mobilization of $Ca^{2+}$ ions from the endoplasmic reticulum.
- Activation of protein kinase C (PKC).
Active protein kinase C uses ATP to phosphorylate active glycogen synthase. A phosphorylated form of the enzyme is produced, and glycogen synthesis is completely blocked. Concurrently, epinephrine acts via the adenylate cyclase system: glycogen phosphorylase is phosphorylated and transitions into its active form. The main outcome is the rapid switching of the cell toward glycogen degradation.
Three pathways of glycogen phosphorylase activation
The initiation of glycogenolysis depends on the physiological state of the tissue (rest, work, stress). There are three main mechanisms for activating the enzyme glycogen phosphorylase:
- Allosteric activation (during moderate exercise and rest). During muscle contraction, ATP hydrolysis leads to the formation of AMP. At low cAMP levels, glycogen phosphorylase is in a dephosphorylated (low-activity) state. AMP acts as its allosteric activator, initiating glycogenolysis without hormonal involvement.
- Calcium-dependent activation. A nerve impulse triggers the release of $Ca^{2+}$ ions from the sarcoplasmic reticulum. Calcium binds to the protein calmodulin. The resulting complex activates phosphorylase kinase, which phosphorylates and activates glycogen phosphorylase.
- Epinephrine cascade. During intense muscular work, epinephrine via PKA simultaneously activates the same phosphorylase kinase. Often pathways 2 and 3 operate together.
Changes in hepatic glycogen metabolism
Carbohydrate metabolism in hepatocytes responds sensitively to changes in the body's physiological state. The table illustrates key changes in process rates and substance concentrations.
| Body state | Changes in blood and hepatocytes | Process rate |
|---|---|---|
| Fasting (postabsorptive) | Glucagon $\uparrow$, Insulin $\downarrow$ | Synthesis $\downarrow$, Degradation $\uparrow$ |
| Postprandial (absorptive) | Insulin $\uparrow$, Glucagon $\downarrow$, Glucose $\uparrow$ cAMP $\downarrow$, Glucose-6-phosphate $ bsp\uparrow$ | Synthesis $\uparrow$, Degradation $\downarrow$ |
| Stress | Epinephrine $\uparrow$ cAMP $\uparrow$, $Ca^{2+}$-calmodulin $\uparrow$ | Synthesis $\downarrow$, Degradation $\uparrow$ |
Briefly: answers to topical questions
How do metabolite and second messenger concentrations in hepatocytes change during the postabsorptive period (fasting)?
In hepatocytes during fasting, the levels of cAMP, Acetyl-CoA, ATP, and NADH increase, while the concentrations of fructose-2,6-bisphosphate and oxaloacetate decrease. Changes include:
- cAMP — concentration rises due to the glucagon-triggered adenylate cyclase cascade.
- Fructose-2,6-bisphosphate — its level drops due to the predominance of phosphatase activity of the bifunctional enzyme.
- Acetyl-CoA, ATP, and NADH — actively accumulate as a result of enhanced fatty acid beta-oxidation.
- Oxaloacetate — is reduced to malate and diverted to gluconeogenesis, becoming unavailable for the citrate cycle.
Through which intracellular cascade does glucagon stimulate glycogenolysis in the liver?
Glucagon stimulates glycogen degradation in the liver via the adenylate cyclase cascade. This mechanism includes the following steps:
- Rise in cAMP — occurs as a result of the activation of the adenylate cyclase system following hormone binding to the receptor.
- Activation of protein kinase A — is triggered by increased cAMP levels.
- Enzyme phosphorylation — protein kinase A phosphorylates regulatory enzymes of glycogen metabolism.
Consequently, glycogen phosphorylase transitions into the active phosphorylated form, leading to accelerated glycogen degradation.
Phosphorylation acts as a switch: under stress (epinephrine), the degradation enzyme is ACTIVE, while the synthesis enzyme is INACTIVE. Remember: stress requires phosphorus to "burn" glycogen stores.
Class and exam questions
Why does glycogen break down without hormonal participation during moderate muscular work?
AMP accumulates in muscles due to ATP hydrolysis. AMP acts as a potent allosteric activator of the less active dephosphorylated form of glycogen phosphorylase.
What role does calcium play in the regulation of glycogen metabolism?
$Ca^{2+}$ ions bind to the protein calmodulin. This complex activates phosphorylase kinase, which in turn converts glycogen phosphorylase into an active state.
How exactly does insulin halt glycogen degradation?
It activates phosphoprotein phosphatase. This enzyme removes inorganic phosphate from glycogen phosphorylase, converting it into an inactive state.
What else to study on this topic
- Inositol phosphate signaling pathway
- Adenylate cyclase cascade and the role of cAMP
- Structure and mechanism of action of calmodulin
- Covalent modification of enzymes (phosphorylation)
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