Phosphomutases are enzymes that catalyze the intramolecular relocation of phosphate groups. They participate extensively in glycogen metabolism, glycolysis, nucleoside metabolism, and nucleotide-sugar biosynthesis. Under the Enzyme Commission classification system, these enzymes are primarily assigned to EC 5.4.2. Different members recognize specific phosphorylated substrates and connect phosphate-position conversion with energy utilization, nutrient storage, and the supply of glycosylation precursors.

Keywords: phosphomutase; phosphoglucomutase; PGM1; PGM2; PGM3; PMM2; PGAM1; glycogen metabolism; nucleotide sugar; glycosylation

1 Classification and Basic Concepts of Phosphomutases

1.1 EC 5.4.2 Phosphomutases

The Enzyme Commission system primarily classifies enzymes according to the reactions they catalyze. EC 5 comprises isomerases, EC 5.4 comprises intramolecular transferases, and EC 5.4.2 includes phosphotransferases or phosphomutases that catalyze the intramolecular relocation of phosphate groups.

This subclass covers enzymes acting on glucose, amino sugars, mannose, pentoses, phosphoglycerate, and other substrates. Their common feature is alteration of the phosphate position within the substrate rather than simple addition or removal of a phosphate group. Individual members can possess different protein structures, active sites, and catalytic intermediates. EC 5.4.2 therefore describes a shared reaction type rather than a single protein family.

1.2 Major Members of EC 5.4.2

EC 5.4.2 is subdivided into multiple enzyme categories according to the specific substrate and reaction. In the current classification, the former EC 5.4.2.1 has been separated into 2,3-bisphosphoglycerate-dependent and 2,3-bisphosphoglycerate-independent phosphoglycerate mutases. Other members participate in glycogen metabolism, amino-sugar metabolism, pentose metabolism, glycolysis, and phosphonate biosynthesis.

1.3 Metabolic Significance of Phosphate-Position Conversion

Phosphomutase reactions generally do not alter the fundamental carbon skeleton of a metabolite, but the phosphate position determines which downstream enzymes can recognize it. Glucose-1-phosphate (G1P) and glucose-6-phosphate (G6P) share the same six-carbon skeleton. G1P is primarily connected to UDP-glucose and glycogen metabolism, whereas G6P lies at the intersection of glycolysis, the pentose phosphate pathway, and glycogen metabolism.

Conversion of N-acetylglucosamine-6-phosphate (GlcNAc-6-P) into N-acetylglucosamine-1-phosphate (GlcNAc-1-P) enables entry into UDP-N-acetylglucosamine (UDP-GlcNAc) biosynthesis. Conversion of mannose-6-phosphate (Man-6-P) into mannose-1-phosphate (Man-1-P) supports the production of glycosylation precursors such as GDP-mannose. Conversion of ribose-1-phosphate into ribose-5-phosphate (R5P) allows the pentose carbon skeleton to re-enter pentose phosphate and nucleotide metabolism. Phosphomutases therefore regulate the ability of metabolites to enter different downstream reactions by altering their phosphate-position states.

2 Representative Phosphomutases and Their Metabolic Functions

2.1 Phosphoglucomutase 1

(1) Reversible Conversion Between G1P and G6P

Phosphoglucomutase 1 (PGM1) is one of the principal phosphoglucomutases in humans and catalyzes the reversible conversion between G1P and G6P. G1P is preferentially associated with glycogen and UDP-glucose metabolism, whereas G6P connects glycolysis, the pentose phosphate pathway, and glucose export. PGM1 lies between these two metabolic pools, allowing glucose-derived carbon to be redistributed between storage and utilization according to cellular metabolic requirements.

(2) Glycogen Synthesis and Glycogenolysis

After glucose enters a cell, hexokinase or glucokinase converts it into G6P. During glycogen synthesis, PGM1 converts G6P into G1P, which is subsequently converted into UDP-glucose. UDP-glucose serves as the activated glucose donor used by glycogen synthase to elongate glycogen chains. During glycogen mobilization, glycogen phosphorylase primarily releases G1P, which PGM1 converts into G6P so that glycogen-derived glucose units can enter glycolysis or glucose-export pathways.

Glucose → G6P → G1P → UDP-glucose → Glycogen

Glycogen → G1P → G6P → Glycolysis or glucose export

PGM1 does not unidirectionally promote either glycogen synthesis or glycogen breakdown. Instead, it maintains bidirectional exchange between the G1P and G6P pools, dynamically connecting glycogen storage with glucose utilization.

(3) UDP-Glucose and Nucleotide-Sugar Metabolism

G1P is also an important precursor for UDP-glucose formation. In addition to its role in glycogen synthesis, UDP-glucose is metabolically connected to nucleotide sugars such as UDP-galactose. These activated sugars participate in multiple glycosylation processes. PGM1 therefore connects glycogen storage, central carbon metabolism, and nucleotide-sugar supply. Consequently, the effects of PGM1 dysfunction can extend beyond altered G1P/G6P balance to disturbances in UDP-sugar metabolism.

2.2 β-Phosphoglucomutase

β-Phosphoglucomutase (β-PGM), classified as EC 5.4.2.6, catalyzes the reversible conversion between β-D-glucose 1-phosphate and β-D-glucose 6-phosphate. The enzyme is primarily found in certain microorganisms. Although its overall reaction also involves phosphate-position conversion in glucose phosphates, its protein structure and catalytic mechanism differ substantially from those of human PGM1.

β-PGM uses an active-site aspartate residue for phosphate transfer and completes its catalytic cycle through a β-D-glucose 1,6-bisphosphate intermediate. It therefore represents an important example of the mechanistic diversity among phosphomutases.

2.3 PGM3 and Hexosamine Metabolism

(1) Conversion of GlcNAc Phosphates

Phosphoglucomutase 3 (PGM3) primarily functions as a phosphoacetylglucosamine mutase and catalyzes the reversible conversion between GlcNAc-6-P and GlcNAc-1-P. This reaction occurs in the later portion of the hexosamine biosynthetic pathway and represents an important step through which phosphorylated GlcNAc enters nucleotide-sugar biosynthesis.

(2) UDP-GlcNAc and Glycosylation

GlcNAc-1-P can be further converted into UDP-GlcNAc. UDP-GlcNAc is an important activated amino sugar that participates in the formation of N-glycan structures and intracellular protein O-GlcNAc modification. The hexosamine biosynthetic pathway integrates metabolic inputs from glucose, glutamine, acetyl-CoA, and UTP, allowing the PGM3-associated node to connect nutrient availability with the supply of glycosylation substrates:

Hexosamine metabolism → GlcNAc-6-P → GlcNAc-1-P → UDP-GlcNAc → Glycosylation

2.4 PMM2 and Mannose Metabolism

(1) Conversion Between Man-6-P and Man-1-P

Phosphomannomutase 2 (PMM2) catalyzes the reversible conversion between Man-6-P and Man-1-P. After mannose enters the cell and is converted into Man-6-P, PMM2 must convert it into Man-1-P before it can enter the biosynthesis of activated sugars such as GDP-mannose.

(2) GDP-Mannose and the Supply of Glycosylation Precursors

Man-1-P is an important precursor for GDP-mannose biosynthesis. GDP-mannose and related mannose donors subsequently participate in the formation of N-linked oligosaccharide precursors. Although PMM2 does not directly transfer sugars onto glycans, it occupies an upstream position in the supply of glycosylation substrates:

Mannose → Man-6-P → Man-1-P → GDP-mannose and related precursors → Glycosylation

PMM2 directly connects basic mannose metabolism with glycan-precursor biosynthesis. Reduced PMM2 activity can therefore restrict the normal supply of glycosylation substrates at the sugar-phosphate level.

2.5 PGM2 and Pentose Phosphate Metabolism

(1) Phosphate-Position Conversion in Ribose Phosphates

Although phosphoglucomutase 2 (PGM2) contains “phosphoglucomutase” in its name, one of its important physiological activities is phosphopentomutase activity. It can catalyze the conversion between ribose-1-phosphate and R5P, as well as between the corresponding deoxyribose phosphates.

Phosphorolysis of nucleosides produces ribose-1-phosphate. The phosphopentomutase reaction converts it into R5P, which can enter other pentose metabolic pathways. This mechanism allows the sugar component released during nucleoside degradation to be reused.

(2) Connection Between Nucleoside Degradation and Nucleotide Biosynthesis

R5P can be further converted into 5-phosphoribosyl-1-pyrophosphate (PRPP). PRPP is an important activated ribose donor in both de novo purine and pyrimidine nucleotide biosynthesis and nucleotide-salvage pathways. The PGM2-associated node can therefore redirect ribose carbon skeletons released by nucleoside degradation into nucleotide metabolism:

Nucleosides → Ribose-1-P → R5P → PRPP → Nucleotide metabolism

PGM2 consequently participates not only in phosphate-position conversion in pentoses but also in the recovery and redistribution of carbon skeletons during nucleoside turnover.

2.6 PGAM1 and Glycolysis

(1) Conversion Between 3-PG and 2-PG

Phosphoglycerate mutase 1 (PGAM1) is a 2,3-bisphosphoglycerate-dependent phosphoglycerate mutase that catalyzes the reversible conversion between 3-phosphoglycerate (3-PG) and 2-phosphoglycerate (2-PG). Enolase subsequently converts 2-PG into phosphoenolpyruvate (PEP), which pyruvate kinase converts into pyruvate with the concomitant production of ATP:

3-PG → 2-PG → PEP → Pyruvate

(2) Metabolic Positioning of the 3-PG/2-PG Node

PGAM1 lies in the later portion of glycolysis and maintains continuous carbon flow from 3-PG toward 2-PG, PEP, and pyruvate. Because 3-PG is also located near branches such as serine biosynthesis, changes in PGAM1 activity may influence not only glycolytic progression but also the distribution of 3-PG-associated intermediates. Unlike PGM1, which connects glycogen storage with glucose utilization, PGAM1 primarily regulates carbon flow within a single central metabolic pathway.

3 Key Catalytic Mechanisms of Phosphomutases

3.1 PGM-Class Phosphosugar Mutases

(1) Participation of a Phosphorylated Serine

PGM1 and other α-D-phosphohexomutases, including PGM3 and PMM2, share a similar basic catalytic framework. In PGM1, the active site contains a serine residue that can be phosphorylated. The catalytically active enzyme initially carries a phosphate group and transfers it to the position of the substrate that is not already phosphorylated.

(2) Phosphate-Position Conversion Through a Bisphosphosugar Intermediate

After G1P enters the PGM1 active site, the enzyme transfers a phosphate group to the substrate, producing a glucose-1,6-bisphosphate intermediate. The intermediate reorients within the active site, after which the other phosphate group is transferred back to the enzyme. This restores phosphorylated PGM1 and releases G6P:

Phosphorylated PGM1 + G1P → Glucose-1,6-bisphosphate → G6P + Phosphorylated PGM1

The original phosphate group on G1P is therefore not simply moved directly from C1 to C6. Instead, phosphate-position exchange is completed through sequential phosphate-transfer reactions between the enzyme and a bisphosphosugar intermediate. Divalent metal ions such as Mg²⁺ also help establish the active-site electrostatic environment and stabilize phosphate groups.

(3) Substrate Recognition Determines Functional Specialization

PGM1, PGM3, and PMM2 use similar catalytic frameworks but differ in how their active sites recognize sugar skeletons and substituents. PGM1 primarily acts on glucose phosphates, PGM3 recognizes GlcNAc phosphates, and PMM2 primarily processes mannose phosphates. The combination of a shared catalytic mechanism and distinct substrate selectivity allows these enzymes to participate separately in glycogen, hexosamine, and mannose metabolism.

3.2 Phosphorylated-Aspartate Mechanism of β-Phosphoglucomutase

β-PGM uses a catalytic system distinct from that of PGM1. Its active-site aspartate residue forms a phosphorylated enzyme intermediate, and Mg²⁺ is required for the reaction. A bisphosphosugar intermediate is also generated during catalysis, but the key catalytic residue and overall protein fold differ from those of PGM1. Comparison of PGM1 and β-PGM demonstrates that similar phosphate-position conversions can be achieved through different protein architectures and catalytic chemistries.

3.3 Phosphorylated-Histidine Mechanism of PGAM1

Mammalian PGAM1 uses an active-site histidine residue for phosphate transfer. 2,3-Bisphosphoglycerate (2,3-BPG) participates in generating catalytically active phosphorylated PGAM1, after which the enzyme completes the reversible conversion between 3-PG and 2-PG through phosphate-transfer reactions.

The phosphorylated serine of PGM-class enzymes, phosphorylated aspartate of β-PGM, and phosphorylated histidine of PGAM1 represent distinct catalytic strategies. These differences demonstrate that the defining commonality of EC 5.4.2 lies in the overall reaction rather than in an identical molecular mechanism.

4 Connecting Functions of Phosphomutases in Metabolic Networks

4.1 Switching Between Stored Carbon and Central Carbon Metabolism

Phosphomutase reactions can reconnect stored carbon sources with central cellular metabolism. The G1P/G6P node controlled by PGM1 is a representative example. During glycogen synthesis, carbon flows from G6P toward G1P and UDP-glucose. During glycogen mobilization, G1P is converted back into G6P.

The resulting G6P can enter glycolysis, the pentose phosphate pathway, or tissue-specific glucose-export pathways. The same glycogen reserve can therefore support energy production, reducing-equivalent generation, and biosynthesis according to cellular requirements.

4.2 Sugar-Phosphate Metabolism and Glycosylation-Precursor Supply

PGM1, PGM3, and PMM2 control the capacity of glucose-, GlcNAc-, and mannose-derived phosphorylated intermediates, respectively, to enter nucleotide-sugar or glycan-precursor biosynthesis. Although these enzymes act on different substrates, they follow a similar metabolic logic:

Basic sugar metabolism → Sugar phosphate → Phosphomutase reaction → Activated sugar/glycan precursor → Glycosylation

This relationship demonstrates that glycosylation is controlled not only by glycosyltransferases but also by the upstream supply of metabolites. When sugar-phosphate conversion is impaired, glycosylation abnormalities may develop because of reduced UDP-sugar or GDP-sugar availability even if the downstream glycosylation enzymes remain functional.

4.3 Nucleoside Turnover and Reuse of Pentose Carbon Skeletons

Phosphopentomutase reactions allow the ribose component generated during nucleoside degradation to re-enter cellular metabolism. Conversion of ribose-1-P into R5P connects nucleoside degradation with the pentose phosphate pathway, PRPP formation, and nucleotide biosynthesis. Cells can therefore recover not only nucleobases but also the pentose carbon skeletons of nucleosides. This process provides a material connection between pentose and nucleotide metabolism when nucleoside turnover increases or nutrient sources change.

4.4 Coordination of the Glycolytic Trunk with Biosynthetic Branches

The 3-PG/2-PG node controlled by PGAM1 lies in the later portion of glycolysis. Whereas 2-PG proceeds toward PEP and pyruvate, 3-PG is positioned near metabolic branches such as serine biosynthesis. PGAM1 therefore participates not only in intermediate conversion within the continuous glycolytic sequence but also in regulating the distribution of 3-PG between the glycolytic trunk and related biosynthetic processes.

Compared with phosphomutases such as PGM1 that primarily connect distinct metabolic modules, PGAM1 represents regulation of intermediate ratios and continuous carbon flow within a central metabolic pathway.

5 Phosphomutases and Disease

5.1 PGM1 Deficiency and PGM1-CDG

(1) Abnormal Glycogen and Glucose Metabolism

Reduced PGM1 function restricts bidirectional conversion between G1P and G6P. During glycogen synthesis, conversion of G6P toward G1P and UDP-glucose is impaired. During glycogen breakdown, released G1P cannot be efficiently converted into G6P for entry into glycolysis. Glycogen storage, glycogen mobilization, and energy utilization can therefore all become abnormal.

(2) Glycosylation Abnormalities

PGM1 also affects nucleotide-sugar metabolism involving UDP-glucose and UDP-galactose. PGM1 deficiency can consequently produce abnormalities in protein glycosylation. Because the disorder combines impaired glycogen metabolism with a congenital disorder of glycosylation (CDG), it is generally classified as PGM1-CDG. Dysfunction at a single G1P/G6P conversion node can therefore propagate simultaneously into glycogen metabolism, central carbon metabolism, and nucleotide-sugar networks.

5.2 PGM3-Associated Glycosylation and Immune Abnormalities

Insufficient PGM3 function can restrict the formation of GlcNAc-1-P and UDP-GlcNAc, affecting multiple glycosylation processes that depend on UDP-GlcNAc. Immune receptors, adhesion molecules, and numerous intracellular signaling proteins are regulated by glycosylation. PGM3 abnormalities can therefore be accompanied by changes in immune-cell development and function, illustrating the connection among hexosamine metabolism, glycosylation-substrate supply, and immune homeostasis.

5.3 PMM2-CDG

Reduced PMM2 function restricts the conversion of Man-6-P into Man-1-P and affects the supply of GDP-mannose and N-glycosylation-associated precursors. Pathogenic PMM2 variants cause PMM2-CDG, one of the representative congenital disorders of glycosylation. PGM1-, PGM3-, and PMM2-associated disorders collectively demonstrate that glycosylation abnormalities can originate at the sugar-phosphate and nucleotide-sugar metabolic stages before glycan assembly begins.

5.4 PGAM1 and Tumor Metabolism

Increased PGAM1 expression or activity has been observed in certain tumors and is associated with enhanced glycolysis, cellular proliferation, and metabolic adaptation. PGAM1 controls the 3-PG/2-PG node and can influence both carbon flow through the later portion of glycolysis and the distribution of related metabolic intermediates.

Because tumors differ in genetic background, nutrient state, and metabolic dependencies, PGAM1 is most appropriately regarded as a representative node for studying the redistribution of glycolytic intermediates and tumor metabolic adaptation rather than as a universal metabolic driver across all cancers.

6 Products

6.1 Products for Phosphoglucomutase Research

6.2 Products for PGM3 and Amino-Sugar Metabolism Research

6.3 Products for PMM2 and Mannose-Phosphate Metabolism Research

6.4 Products for Phosphopentomutase and PGM2 Research

6.5 Products for PGAM1 and Phosphoglycerate Metabolism Research

Phosphomutases connect glycogen metabolism, glycolysis, nucleoside metabolism, and nucleotide-sugar biosynthesis through distinct substrate-recognition and phosphate-transfer mechanisms. Although EC 5.4.2 members differ in their substrates and catalytic systems, they collectively regulate metabolic-flux distribution and cellular homeostasis through phosphate-position conversion.

Major Abbreviations

References

[1] Backe PH, Laerdahl JK, Kittelsen LS, et al. Structural basis for substrate and product recognition in human phosphoglucomutase-1 (PGM1) isoform 2, a member of the α-D-phosphohexomutase superfamily. Scientific Reports. 2020;10:5656.

[2] Tegtmeyer LC, Rust S, van Scherpenzeel M, et al. Multiple phenotypes in phosphoglucomutase 1 deficiency. New England Journal of Medicine. 2014;370(6):533-542.

[3] Altassan R, Radenkovic S, Edmondson AC, et al. International consensus guidelines for phosphoglucomutase 1 deficiency (PGM1-CDG): Diagnosis, follow-up, and management. Journal of Inherited Metabolic Disease. 2021;44(1):148-163.

[4] Stray-Pedersen A, Backe PH, Sorte HS, et al. PGM3 mutations cause a congenital disorder of glycosylation with severe immunodeficiency and skeletal dysplasia. American Journal of Human Genetics. 2014;95(1):96-107.

[5] Altassan R, Péanne R, Jaeken J, et al. International clinical guidelines for the management of phosphomannomutase 2-congenital disorders of glycosylation: Diagnosis, treatment and follow up. Journal of Inherited Metabolic Disease. 2019;42(1):5-28.

[6] Nwosu ZC, Ward MH, Sajjakulnukit P, et al. Uridine-derived ribose fuels glucose-restricted pancreatic cancer. Nature. 2023;618:151-158.

[7] Dai J, Finci L, Zhang C, et al. Analysis of the structural determinants underlying discrimination between substrate and solvent in β-phosphoglucomutase catalysis. Biochemistry. 2009;48(9):1984-1995.

[8] Yang GJ, Tao F, Zhong HJ, Yang C, Chen J. Targeting PGAM1 in cancer: An emerging therapeutic opportunity. European Journal of Medicinal Chemistry. 2022;244:114798.