Insulin does both. It triggers phosphorylation of signaling proteins to relay its message through the cell, and it promotes dephosphorylation of key metabolic enzymes to shift the body toward storing fuel. This dual action is one of the most important concepts in understanding how insulin works, and it’s also one of the most confusing for students encountering it for the first time.
The Short Answer: It Depends on the Target
When insulin binds its receptor on the cell surface, it kicks off a chain of phosphorylation events. The receptor itself gets phosphorylated on specific tyrosine residues (Tyr1158, Tyr1162, and Tyr1163 in the kinase domain). Downstream signaling proteins, including IRS and Akt, also get phosphorylated. This is how the insulin “signal” travels from the cell surface inward.
But the end result of all that signaling is often the opposite: dephosphorylation of metabolic enzymes. Insulin activates protein phosphatases, especially protein phosphatase 1 (PP1), which strip phosphate groups off enzymes involved in glycogen and fat metabolism. So the signaling arm of insulin runs on phosphorylation, while the metabolic arm frequently runs on dephosphorylation. Both happen simultaneously, and both are essential.
How Insulin Phosphorylates Its Signaling Chain
The moment insulin docks onto its receptor, the receptor’s two halves phosphorylate each other in a process called trans-autophosphorylation. This creates binding sites for adapter proteins like IRS-1, which then get phosphorylated on their own tyrosine residues. Phosphorylated IRS-1 recruits and activates an enzyme called PI3K, which converts one type of membrane lipid (PIP2) into another (PIP3). PIP3 acts as a landing pad for Akt, bringing it to the membrane where it gets phosphorylated in two steps: first partially, then fully. Fully active Akt is the central hub of insulin signaling, and it goes on to phosphorylate a wide range of targets throughout the cell.
Every step in this relay is a phosphorylation event. Remove any one of them and the insulin signal dies.
How Insulin Promotes Dephosphorylation of Metabolic Enzymes
Here’s where the confusion usually starts. Many of the enzymes that carry out insulin’s metabolic effects, like building glycogen or ramping up fat storage, are inactive when phosphorylated. To turn them on, insulin needs to remove phosphate groups rather than add them. It accomplishes this primarily through PP1, which is active in fat, liver, and muscle tissue. PP1 doesn’t float freely through the cell. It’s anchored to specific locations by targeting subunits that also determine which enzymes it acts on, giving the system precision.
The classic example is glycogen synthase, the enzyme that builds glycogen from glucose. In its phosphorylated state, glycogen synthase is largely inactive. Insulin activates it by promoting its dephosphorylation through PP1.
Glycogen Synthesis: Both Mechanisms at Once
Glycogen synthesis is the best illustration of how insulin uses phosphorylation and dephosphorylation in tandem. Akt, activated through the phosphorylation cascade described above, phosphorylates an enzyme called GSK-3 (glycogen synthase kinase 3). Phosphorylation of GSK-3 inactivates it. GSK-3’s normal job is to phosphorylate glycogen synthase and keep it turned off. So when insulin shuts down GSK-3 through phosphorylation, glycogen synthase loses the thing that was keeping it phosphorylated. The result: glycogen synthase gets dephosphorylated and switches on, and the cell starts packing glucose into glycogen.
In one pathway, insulin phosphorylates a kinase to inactivate it, which leads to dephosphorylation and activation of a metabolic enzyme. Phosphorylation and dephosphorylation are not opposites here. They’re collaborators.
Shutting Down Glucose Production in the Liver
Insulin also uses phosphorylation to suppress glucose release from the liver. A transcription factor called FoxO1 normally sits in the nucleus and turns on genes that drive gluconeogenesis, the process of making new glucose. When insulin signaling activates Akt, Akt phosphorylates FoxO1 at three specific sites (T24, S256, and S319 in the human version). Phosphorylation at S256 primes the other two sites, and the combined effect forces FoxO1 out of the nucleus. Once expelled, FoxO1 gets tagged for destruction by the cell’s recycling machinery.
With FoxO1 gone, the genes for glucose-producing enzymes are no longer switched on, and the liver stops pumping glucose into the blood. This is a pure phosphorylation effect: Akt adds phosphate groups to FoxO1, and FoxO1 is removed from the game entirely.
Boosting Glycolysis in the Heart
Insulin signaling also phosphorylates and activates an enzyme called PFK-2 (6-phosphofructo-2-kinase), which plays a key role in glycolysis. In heart tissue, Akt and other kinases in the insulin pathway phosphorylate the heart isoform of PFK-2 at two serine residues (Ser-466 and Ser-483). This phosphorylation increases PFK-2 activity, which raises levels of a powerful activator of glycolysis called fructose-2,6-bisphosphate. The net effect: the cell burns more glucose for energy. This is an example where insulin’s metabolic outcome is achieved through phosphorylation, not dephosphorylation.
How the Signal Gets Turned Off
The same phosphorylation that starts insulin signaling also has a built-in off switch. Tyrosine phosphatases, especially one called PTP1B, can strip the phosphate groups off the insulin receptor and IRS proteins, dampening the signal. Serine/threonine phosphatases like PP2A can reverse the phosphorylation of downstream targets. This balance between kinases adding phosphate groups and phosphatases removing them determines how strong the insulin signal is and how long it lasts.
When this balance breaks down, the result can be insulin resistance. If phosphatases that shut off the receptor become overactive, or if the phosphatases that dephosphorylate metabolic enzymes become underactive, cells stop responding properly to insulin even when plenty of it is circulating.
A Simple Way to Remember It
Insulin phosphorylates its signaling proteins (the receptor, IRS, Akt, GSK-3, FoxO1) to relay the message. It dephosphorylates many of its metabolic targets (glycogen synthase and other enzymes) to change what the cell is doing with fuel. Some metabolic targets, like PFK-2 in the heart, are exceptions that get activated by phosphorylation directly. The question “does insulin phosphorylate or dephosphorylate?” doesn’t have one answer because insulin’s power comes from doing both, in a coordinated sequence, to shift the entire cell toward glucose uptake, glycogen storage, and reduced glucose output.