A mutase is an enzyme that moves a chemical group from one position to another within the same molecule. Unlike enzymes that break molecules apart or join them together, a mutase rearranges what’s already there, shifting a functional group (most often a phosphate) from one spot on a molecule to a different spot on that same molecule. Mutases belong to the isomerase class of enzymes (EC 5), which covers all enzymes that convert a molecule into a different structural form of itself.
How Mutases Work
The core job of a mutase is intramolecular group transfer. Picture a molecule with a phosphate group attached at position 3. A mutase detaches that phosphate and reattaches it at position 2, producing a subtly different molecule that can now participate in the next step of a metabolic pathway. The molecule’s chemical formula stays the same, but the rearrangement changes its shape and reactivity.
At the molecular level, many mutases use a two-step mechanism. The enzyme’s active site grabs the functional group from the substrate and temporarily holds it on one of its own amino acid residues, forming an intermediate. The substrate then rotates or shifts slightly within the active site, and the enzyme transfers the group back to the new position. In phosphoglycerate mutase from certain bacteria, for example, manganese ions in the active site help shuttle a phosphate group onto a serine residue, hold it briefly, then place it on a different carbon of the substrate. Metal ions and positively charged amino acids like arginine and lysine are common features in mutase active sites, where they stabilize the reaction’s transition states through electrical charge interactions.
Phosphoglycerate Mutase in Energy Production
The most well-known mutase in human metabolism is phosphoglycerate mutase 1 (PGAM1), a key enzyme in glycolysis, the pathway your cells use to break down glucose for energy. PGAM1 converts 3-phosphoglycerate into 2-phosphoglycerate by moving a phosphate group from the third carbon to the second. This small shift is necessary for the molecule to continue through glycolysis and ultimately produce ATP, the cell’s energy currency.
PGAM1 sits at a strategic point in glycolysis. Most of the intermediate molecules that cells siphon off to build fats, amino acids, and nucleotides are generated upstream of this step. By controlling how much 3-phosphoglycerate and 2-phosphoglycerate are present in the cell, PGAM1 helps balance energy production against the raw materials needed for growth. This is why cancer cells often ramp up PGAM1 levels: higher activity gives them a metabolic advantage, simultaneously fueling rapid energy production and supplying building blocks for new cell growth.
Phosphoglucomutase in Glycogen Metabolism
Another important mutase is phosphoglucomutase, which moves a phosphate group between the 1 and 6 positions of glucose. When your body breaks down glycogen (its stored form of glucose), the first step releases glucose-1-phosphate. This form can’t enter glycolysis directly. Phosphoglucomutase converts it to glucose-6-phosphate, which can then be burned for energy through glycolysis, fed into the pentose phosphate pathway for other metabolic needs, or, in the liver, converted back into free glucose and released into the bloodstream. The reaction works in both directions, so the same enzyme also helps build glycogen when glucose is abundant.
Methylmalonyl-CoA Mutase and Vitamin B12
Methylmalonyl-CoA mutase (MCM) stands out because it requires vitamin B12 as a cofactor. Specifically, it uses adenosylcobalamin, an active form of B12, to generate highly reactive free radicals that power the rearrangement of its substrate. MCM converts L-methylmalonyl-CoA into succinyl-CoA, a molecule that feeds into the citric acid cycle for energy production. This reaction is the final step in breaking down odd-chain fatty acids, certain branched amino acids, and cholesterol.
MCM is the only enzyme in this subfamily of B12-dependent mutases found in mammals. Other members, like glutamate mutase and methyleneglutarate mutase, operate only in bacteria. The discovery of adenosylcobalamin as a cofactor for glutamate mutase in the late 1950s was actually what first revealed the biological role of vitamin B12.
Bisphosphoglycerate Mutase and Oxygen Delivery
Red blood cells contain a specialized mutase called bisphosphoglycerate mutase, which produces 2,3-bisphosphoglycerate (2,3-BPG). This compound is one of the most important regulators of how hemoglobin binds and releases oxygen. When 2,3-BPG levels rise, hemoglobin releases oxygen more readily to tissues. When levels drop, hemoglobin holds onto oxygen more tightly.
The concentration of 2,3-BPG in red blood cells responds to acidity and oxygen levels. At high altitude, for instance, 2,3-BPG increases to help your body extract more oxygen from thinner air. The system is sensitive to energy demands as well: changes in how much ATP the cell needs can shift 2,3-BPG levels, which in turn adjusts oxygen delivery. This makes bisphosphoglycerate mutase a quiet but critical player in keeping tissues oxygenated under varying conditions.
What Happens When a Mutase Is Deficient
Because mutases occupy essential steps in metabolism, deficiencies can cause serious disease. The clearest example is methylmalonic acidemia, a genetic condition caused by variants in the MMUT gene that reduce or eliminate methylmalonyl-CoA mutase activity. Without functional MCM, methylmalonic acid and other toxic byproducts accumulate in organs and tissues.
Infants with methylmalonic acidemia typically show vomiting, dehydration, weak muscle tone, lethargy, an enlarged liver, and failure to gain weight. The condition causes recurring episodes of metabolic acidosis, where excess acid builds up in the blood. Long-term complications include intellectual disabilities, movement problems, chronic kidney disease, feeding difficulties, and inflammation of the pancreas. Two forms exist: mut0, where no functional enzyme is produced at all, and mut-, where some residual enzyme activity remains. The distinction matters because the severity of symptoms generally tracks with how much enzyme function is left.
Vitamin B12 deficiency can also impair MCM activity even when the gene is normal, since the enzyme cannot function without its B12-derived cofactor. This is one reason persistent B12 deficiency leads to elevated methylmalonic acid levels in the blood, a lab finding clinicians use to distinguish B12 deficiency from other causes of similar symptoms.