NS230 Unit 8 Hormonal Regulation Summary Example

Phosphorylation Switches Storage Off: A Unit 8 Hormonal Regulation Summary of Four Hormones Across Eight Pathways

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NS230: Macronutrient Metabolism

Unit 8 Hormonal Regulation Summary

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The summary describes human regulation from the sources cited; the grid simplifies effects that vary by tissue and condition.

The Four Hormones

Insulin, released by the pancreas when blood glucose rises after a meal, signals the fed state and promotes storage. Glucagon, released by the pancreas when glucose falls, signals the fasting state and acts mainly on the liver to release glucose. Epinephrine, released by the adrenal medulla during stress and exercise, acts on liver, muscle and adipose tissue to mobilize fuel. Cortisol, released by the adrenal cortex, acts more slowly, mainly by changing which enzymes cells make, and supports gluconeogenesis during prolonged fasting and stress (Petersen et al., 2017).

The Organizing Rule

Glucagon and epinephrine bind receptors that raise cyclic AMP inside cells, which activates protein kinase A. Protein kinase A phosphorylates key enzymes. In the liver, the pattern is consistent: phosphorylation turns off enzymes of storage and turns on enzymes of release. Insulin opposes this by activating phosphatases that remove the phosphates and by lowering cyclic AMP (Berg et al., 2019).

Most of the grid follows from one pattern: in the liver, a phosphate added by protein kinase A turns storage enzymes off and release enzymes on, and insulin works largely by taking that phosphate away, so learning which enzymes are phosphorylated explains most of what each hormone does.

The Grid, Pathway by Pathway

1. Glycogen synthesis. Key enzyme: glycogen synthase. Insulin activates it in liver and muscle by promoting dephosphorylation. Glucagon, in liver, and epinephrine, in liver and muscle, inactivate it through phosphorylation.

2. Glycogen breakdown. Key enzyme: glycogen phosphorylase. Glucagon, in liver, and epinephrine, in liver and muscle, activate it through phosphorylase kinase and phosphorylation. Insulin inactivates it by promoting dephosphorylation. In muscle, calcium and AMP also activate it directly, independent of hormones.

3. Glycolysis in liver. Key control: phosphofructokinase-1, regulated by fructose-2,6-bisphosphate. Insulin raises fructose-2,6-bisphosphate, speeding glycolysis. Glucagon, through protein kinase A phosphorylating the bifunctional enzyme that makes and breaks it, lowers fructose-2,6-bisphosphate, slowing glycolysis. Pyruvate kinase in liver is also inactivated by phosphorylation.

4. Gluconeogenesis in liver. Key controls: fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase. Glucagon promotes the pathway quickly by lowering fructose-2,6-bisphosphate, which releases the brake on fructose-1,6-bisphosphatase, and slowly by increasing transcription of phosphoenolpyruvate carboxykinase. Cortisol also increases that transcription. Insulin suppresses both.

5. Fatty acid synthesis. Key enzyme: acetyl-CoA carboxylase. Insulin activates it, partly by dephosphorylation. Glucagon and epinephrine inactivate it through phosphorylation, partly via AMP-activated protein kinase.

6. Lipolysis in adipose tissue. Key enzymes: hormone-sensitive lipase and adipose triglyceride lipase, with perilipin. Epinephrine activates lipolysis through protein kinase A. Insulin strongly suppresses it, which is why fat release stops after a meal.

7. Fatty acid oxidation. Key control: carnitine palmitoyltransferase I, inhibited by malonyl-CoA. When acetyl-CoA carboxylase is inactive, malonyl-CoA falls and fatty acids enter mitochondria for oxidation. Glucagon and epinephrine therefore promote oxidation indirectly; insulin opposes it.

8. Protein synthesis and breakdown. Insulin promotes protein synthesis and restrains breakdown in muscle. Cortisol promotes muscle protein breakdown, supplying amino acids for gluconeogenesis during stress and prolonged fasting.

Fast Versus Slow Control

The grid mixes two kinds of control that operate on very different timescales.

Fast, covalent control. Phosphorylation and dephosphorylation change the activity of enzymes already present, within seconds to minutes. Glycogen phosphorylase, glycogen synthase, acetyl-CoA carboxylase and hormone-sensitive lipase are all controlled this way. This lets the body switch between storage and release as meals are eaten and digested.

Slow, transcriptional control. Changes in the amount of an enzyme take hours to days, as genes are turned up or down. Phosphoenolpyruvate carboxykinase, controlled by glucagon, cortisol and insulin, is the classic example. So are the lipogenic enzymes, which increase with high-carbohydrate feeding, and the urea cycle enzymes, which rise with protein intake. Slow control adjusts the body's capacity to a sustained diet or condition.

Allosteric control by metabolites, such as AMP, citrate and fructose-2,6-bisphosphate, is a third kind, often fastest of all.

A Worked Example: The Liver After Breakfast and Before Lunch

At breakfast, insulin rises. In the liver, phosphatases remove phosphates from glycogen synthase, activating it, and from glycogen phosphorylase, inactivating it, so glucose is stored. Fructose-2,6-bisphosphate rises, speeding glycolysis and braking gluconeogenesis. Acetyl-CoA carboxylase is active, so malonyl-CoA rises and fatty acid oxidation slows.

By late morning, glucose has fallen and glucagon rises. Protein kinase A phosphorylates glycogen synthase, inactivating it, and phosphorylase kinase, which activates glycogen phosphorylase, so glycogen is broken down. Fructose-2,6-bisphosphate falls, releasing the brake on gluconeogenesis. Acetyl-CoA carboxylase is phosphorylated and inactivated, malonyl-CoA falls and fatty acids enter the mitochondria to be oxidized.

The same enzymes are present in both moments. What changes is their phosphorylation state, set within minutes by the balance of insulin and glucagon. This is the rule in action.

Why Cortisol Belongs in a Different Column

Cortisol does not act through cyclic AMP or protein kinase A. It enters cells and binds a receptor that changes gene expression, so its effects appear over hours. It increases the amount of gluconeogenic enzymes, promotes muscle protein breakdown to supply amino acids and reduces glucose uptake in some tissues. Its role is to sustain glucose supply during prolonged stress or fasting rather than to switch metabolism quickly.

Tissue Differences

The rule applies most cleanly to the liver. Muscle lacks glucagon receptors, so glucagon has little direct effect on muscle. Muscle glycogen is released to fuel the muscle itself, not the blood, since muscle lacks glucose-6-phosphatase. In adipose tissue, the key hormonal decision is between storing and releasing fatty acids, governed mainly by insulin and epinephrine. These differences mean that one hormone can do different things in different tissues.

Why the Grid Simplifies

The grid treats each hormone as acting alone, but in the body they act together and in proportion. What matters for the liver is often the ratio of insulin to glucagon rather than either level by itself. Small changes in that ratio, such as those between breakfast and lunch, shift the phosphorylation state of many enzymes at once. The grid is a map of directions, not a measure of how far each enzyme moves.

How Fasting Shifts the Balance

In the first day of fasting, glucagon rises, insulin falls and the liver shifts from glycogen breakdown toward gluconeogenesis, which supplies most glucose after the first day (Rothman et al., 1991). Cortisol's slower effects sustain gluconeogenesis over longer fasts.

References

Berg, J. M., Tymoczko, J. L., Gatto, G. J., Jr., & Stryer, L. (2019). Biochemistry (9th ed.). W. H. Freeman.

Petersen, M. C., Vatner, D. F., & Shulman, G. I. (2017). Regulation of hepatic glucose metabolism in health and disease. Nature Reviews Endocrinology, 13(10), 572-587. https://doi.org/10.1038/nrendo.2017.80

Rothman, D. L., Magnusson, I., Katz, L. D., Shulman, R. G., & Shulman, G. I. (1991). Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR. Science, 254(5031), 573-576. https://doi.org/10.1126/science.1948033