A phosphatase is an enzyme that removes phosphate groups from other molecules. This is the opposite of what a kinase does (kinases attach phosphate groups), and together these two enzyme types act as a molecular on/off switch controlling nearly every process in your cells, from growth and metabolism to immune responses and energy use. Phosphate addition and removal is the single most common way cells regulate the behavior of their proteins.
How Phosphatases Work
Every phosphatase performs the same basic job: it breaks the bond between a phosphate group and its host molecule, releasing the phosphate. This process is called dephosphorylation. The freed phosphate group is handed off to a water molecule, which is why the reaction requires water to proceed.
Not all phosphatases accomplish this the same way, though. One major group, the serine/threonine phosphatases, are metalloenzymes. They rely on metal ions like manganese or magnesium sitting in their active center to activate a water molecule, which then strips the phosphate off in a single step. The other major group, tyrosine phosphatases, uses a different strategy: they form a temporary chemical bond between the enzyme itself and the phosphate before releasing it. Same end result, different chemistry under the hood.
The Kinase-Phosphatase Balance
Cells communicate and make decisions largely through phosphorylation, the process of sticking phosphate groups onto proteins to change their shape and behavior. A kinase grabs a phosphate from ATP (the cell’s energy currency) and attaches it to a target protein, switching that protein “on” or changing what it does. A phosphatase reverses this by pulling the phosphate back off.
This back-and-forth is constant. At any given moment, whether a protein is active or inactive depends on the balance between kinase and phosphatase activity pushing it in opposite directions. When that balance tips too far one way, cells can grow uncontrollably, fail to respond to signals, or mismanage their metabolism. About 98% of protein phosphorylation in human cells occurs on serine and threonine residues, with the remaining fraction on tyrosine residues, yet both types play outsized roles in health and disease.
Major Types of Phosphatases
Phosphatases are classified mainly by what they target. The two broadest categories are protein phosphatases (which remove phosphates from proteins) and non-protein phosphatases (which act on other molecules like fats or sugars).
Protein Tyrosine Phosphatases (PTPs)
PTPs make up the largest family of phosphatase genes in humans. They’re defined by a shared signature motif in their active site and include over 100 genes divided into three classes. Class I alone accounts for the vast majority and splits further into “classical” PTPs, which only target tyrosine-phosphorylated proteins, and dual-specificity phosphatases.
Dual-Specificity Phosphatases (DUSPs)
Most phosphatases are picky about which type of phosphorylated amino acid they’ll touch. DUSPs are the exception. They can remove phosphates from both tyrosine and serine/threonine residues on the same protein, and some can even dephosphorylate non-protein targets like signaling lipids. This versatility makes them important regulators of signaling pathways where multiple phosphorylation events need to be reversed at once.
Serine/Threonine Phosphatases
These phosphatases handle the bulk of dephosphorylation work, given that serine and threonine phosphorylation accounts for roughly 98% of all protein phosphorylation. They include the PPP family (like the well-known PP1 and PP2A) and the PPM family, also called PP2C phosphatases. PPM family members function as single-subunit enzymes that require manganese or magnesium ions in their active center to work. The PPP family phosphatases, by contrast, often assemble into multi-protein complexes where regulatory subunits steer the enzyme toward specific targets.
Lipid Phosphatases
Not all phosphatases target proteins. Some remove phosphates from lipid molecules embedded in cell membranes, altering the signals those lipids send. The best-known example is PTEN, a tumor suppressor that dephosphorylates a membrane lipid called PIP3. PIP3 normally activates a signaling cascade that promotes cell growth and survival (the PI3K-AKT pathway). By stripping a phosphate from PIP3, PTEN effectively puts the brakes on that growth signal. When PTEN is lost or mutated, cells lose this brake, which is why PTEN mutations appear in a wide range of cancers.
Why Phosphatases Matter in Disease
For years, kinases got most of the attention in drug development because they were easier to target with small molecules. Phosphatases were considered “undruggable” due to the shape of their active sites. That view has shifted. A phosphatase called SHP2 has become one of the most actively pursued drug targets in cancer research. The breakthrough came with the discovery of a compound called SHP099, which binds SHP2 in an unexpected way, proving that the obstacles previously thought to make phosphatases impossible to drug could be overcome.
Several small molecules targeting both tyrosine phosphatases and serine/threonine phosphatases have entered clinical trials, primarily for cancer. None have reached the market yet, but the pipeline is growing. Beyond SHP2, researchers are also pursuing drugs that target PP2A and PP5, two serine/threonine phosphatases involved in tumor biology.
PTEN loss is one of the most frequent genetic alterations in human cancer. Because PTEN’s lipid phosphatase activity normally suppresses cell growth through the PI3K-AKT pathway, tumors that lose PTEN function gain an unchecked growth advantage. This makes PTEN status an important factor in understanding a tumor’s behavior and choosing treatment strategies.
Alkaline Phosphatase in Blood Tests
If you’ve seen phosphatase mentioned on a blood test, it was almost certainly alkaline phosphatase (ALP). This enzyme is found throughout the body, with high concentrations in the liver, bones, kidneys, and intestines. Doctors order ALP levels primarily to evaluate liver and bone health.
A commonly reported reference range is 32 to 91 IU/L, but “normal” varies more than most people realize. Analysis of national health survey data found that upper limits differ significantly by sex and ethnicity. Among people at a healthy weight, the 97.5th percentile (the upper boundary of normal) ranged from about 94 IU/L in Asian American/Pacific Islander women to about 124 IU/L in Hispanic men and women. These differences are large enough that a result flagged as high for one group could be entirely normal for another.
Elevated ALP can signal bile duct blockage, liver disease, bone disorders, or simply a healing fracture. Mildly elevated levels in isolation, without other abnormal liver markers, often prompt monitoring rather than immediate concern. Growing children and teenagers naturally have higher ALP because their bones are actively forming.