Molecular Basis of Glycosylation Modification and Functional Release in Natural Products

Natural products are widely distributed in plants, fungi, actinomycetes, and certain marine organisms, and their structural diversity and biological complexity constitute an important foundation for natural product chemistry, phytochemistry, and functional-molecule research. Among these compounds, glycosylation is one of the most common and biologically significant structural modification modes. Introduction of a sugar moiety not only alters solubility, stability, transmembrane transport capacity, and subcellular distribution, but also profoundly affects storage form, toxicity masking, tissue transport, and subsequent activity release. Accordingly, glycosylation of natural products should not be regarded as a simple polarity-enhancing modification, but rather as an important molecular mechanism linking molecular storage, metabolic regulation, and functional release.

Keywords: natural products; glycosylation modification; glycosyltransferases; glycosidases; precursor activation; functional release; deglycosylation; structure-activity relationship

1. Why Glycosylation Constitutes a Key Issue in Natural Product Research

1.1 Glycosylation is an important source of structural diversification in natural products

(1) Introduction of sugar moieties markedly expands the structural space of natural products

The same aglycone can generate a series of derivatives with substantially different physicochemical properties and biological behaviors when linked to different sugars, different glycan-chain lengths, different attachment sites, or different glycosidic-bond configurations. Accordingly, glycosylation is one of the major sources of structural diversification in natural products and an important mechanistic basis for the formation of homologous metabolite families.

(2) Glycosylation often determines the actual in vivo form of natural products

In natural systems, many flavonoids, anthraquinones, triterpenes, steroids, and phenolic compounds do not primarily exist as free aglycones, but are instead stored as monoglycosides, diglycosides, or oligosaccharide glycosides in vacuoles, cell wall spaces, or secretory tissues. Therefore, glycosylation is not a secondary modification, but an important determinant of the actual form in which natural products exist in vivo.

1.2 Glycosylation and functional release together form a dynamic regulatory system

(1) Glycosylation does not imply terminal inactivation

After glycosylation, many natural products exhibit reduced reactivity, weakened membrane permeability, or decreased direct activity. However, these changes usually correspond to a storage state, a masked state, or a transport state rather than irreversible inactivation. Their biological significance lies more in converting highly reactive aglycones into forms better suited for accumulation and controlled deployment.

(2) Actual functional output depends on the timing of deglycosylation-mediated release

Once the glycosidic bond is cleaved in a specific tissue, under a specific enzymatic environment, or in response to particular stress conditions, the aglycone can be released again and may subsequently exert membrane-binding, enzyme-inhibitory, redox-active, or signaling-regulatory effects. Therefore, the biological activity of natural products is often not determined by either the glycosylated or aglycone state alone, but by the dynamic interconversion between the two.

2. Structural Basis of Glycosylation Modification

2.1 The type of glycosidic bond determines product stability and releasability

(1) O-glycosidic bonds are the most common linkage mode

The most common glycosylation form in natural products is O-glycosylation, in which the sugar moiety is attached through an oxygen atom to a hydroxyl site on the aglycone. Flavonoid glycosides, phenolic acid glycosides, anthraquinone glycosides, and many triterpenoid saponins occur in this form. These structures are widely formed and broadly distributed, but are also more susceptible to acidic conditions or glycosidase-mediated cleavage.

(2) C-glycosides, N-glycosides, and S-glycosides have distinct stability characteristics

Some natural products form C-glycosides, in which the sugar moiety is connected to the aglycone skeleton through a carbon-carbon bond. Such structures are generally more stable than O-glycosides and are less readily hydrolyzed by common glycosidases. N-glycosides and S-glycosides are less widely distributed, but have distinctive importance in nitrogen-containing natural products and sulfur-containing metabolites. Accordingly, glycosidic-bond type is itself a key variable determining the ease or difficulty of functional release.

2.2 Sugar type and glycan-chain length determine molecular physicochemical properties

(1) The sugar moiety is not limited to glucose

Common sugar residues in natural product glycosylation include glucose, rhamnose, galactose, arabinose, xylose, and glucuronic acid. Differences among these sugars in stereochemical configuration, acidity, polarity, and steric bulk directly influence glycoside solubility, ionization behavior, and recognition by transport proteins or target molecules.

(2) Glycan-chain elongation further alters steric shielding and recognition behavior

Monoglycosides differ substantially from diglycosides and polyglycosides in polarity, flexibility, and degree of steric shielding. Extension of the glycan chain generally enhances aqueous distribution but may simultaneously weaken direct interaction between the aglycone and hydrophobic targets. Thus, glycan-chain length does not simply increase hydrophilicity, but systematically reshapes the spatial recognition properties of the molecule.

Table 1. Major Types and Structural Features of Glycosylation Modification in Natural Products

Glycosylation Type	Linkage Mode	Typical Features	Stability Characteristics	Common Classes of Natural Products
O-glycoside	Sugar linked through an oxygen atom	Most widely distributed; commonly formed	Relatively susceptible to acid or glycosidase action	Flavonoids, anthraquinones, phenolic acids, triterpenoid saponins
C-glycoside	Sugar linked through a carbon-carbon bond	More structurally stable; more resistant to enzymatic cleavage	Generally higher than O-glycosides	Certain flavonoids and ketone derivatives
N-glycoside	Sugar linked through a nitrogen atom	Less common; structurally distinctive	Dependent on the specific scaffold environment	Nitrogen-containing natural products
S-glycoside	Sugar linked through a sulfur atom	Often associated with defense-related metabolism	Possesses characteristic chemical reactivity	Sulfur-containing natural products and their precursors

3. Enzymatic Basis of Glycosylation Modification

3.1 Glycosyltransferases are the core executors of glycosylation formation

(1) UDP-sugar-dependent glycosyltransferases constitute the principal catalytic system

Glycosylation of natural products is typically catalyzed by glycosyltransferases, among which the most common are enzymes that use activated sugar donors such as UDP-glucose, UDP-rhamnose, and UDP-galactose. These enzymes recognize both the aglycone acceptor and the activated sugar donor, thereby enabling site-specific or regioselective glycosyl transfer.

(2) Acceptor-recognition capability determines the spectrum of glycosylation products

Different glycosyltransferases exhibit different preferences for hydroxyl position, electronic environment, scaffold planarity, and steric hindrance of the acceptor molecule. Therefore, the same aglycone may form monoglycosides or polyglycosides at different positions under the action of different enzymes. This is also an important reason why glycosidic isomers are abundant among natural products.

3.2 Activated sugar donors determine the direction of glycosylation and product type

(1) Activated sugar donors determine the class of sugar residue that can be introduced

Glycosylation of natural products does not involve arbitrary direct attachment of free sugars, but rather depends on activated forms such as UDP-sugars and TDP-sugars. The type and supply level of activated sugar donors directly constrain the class of sugar residue that can be incorporated and the direction of glycan-chain modification.

(2) The metabolic state of sugar donors limits the upper bound of glycosylation capacity

If a given activated sugar donor is abundant intracellularly, the corresponding glycosylated products are more readily accumulated; conversely, other sugar-containing metabolites may become dominant when that donor is limiting. Therefore, glycosylation of natural products is determined not only by glycosyltransferases themselves, but also by the broader metabolic network governing activated sugar donors.

4. Effects of Glycosylation Modification on the Properties of Natural Products

4.1 Glycosylation markedly alters molecular physicochemical behavior

(1) Introduction of sugar moieties usually enhances water solubility

Most natural-product aglycones are relatively hydrophobic. After glycosylation, polarity increases and water solubility improves, which is more favorable for storage, transport, or targeted allocation in cellular aqueous environments. This change is important for vacuolar accumulation in plants, intracellular compartmentalization, and in vitro model studies.

(2) Glycosylation can reduce nonspecific reactivity of certain aglycones

Some aglycones possess strong lipophilicity or high intrinsic reactivity. Glycosylation can reduce nonspecific interactions with membrane components, proteins, or redox systems through steric shielding and electronic effects, thereby decreasing premature reactions or local toxic manifestation.

4.2 Glycosylation reshapes the distribution and storage behavior of natural products

(1) Glycosides are often better suited to exist as reserve-type molecules

Because glycosylation reduces passive transmembrane diffusion and weakens binding to hydrophobic targets, many glycosylated products are better suited for accumulation as reserve-type molecules. Their physiological significance lies not in immediate action, but in release of the active aglycone at the appropriate time.

(2) Glycosylation affects tissue localization and subcellular distribution

After glycosylation, natural products are more readily transported into vacuoles or specific tissue compartments with the aid of transport proteins, thereby generating tissue-specific or organelle-specific accumulation. In this way, glycosylation exerts the dual role of activity masking and spatial isolation.

5. Molecular Basis of Deglycosylation and Functional Release

5.1 Glycosidases are the core trigger nodes for functional release

(1) β-Glucosidase is the most typical deglycosylating enzyme class

Many natural-product glycosides can undergo deglycosylation under the action of glycosidases such as β-glucosidase, thereby releasing the free aglycone. This process may occur after plant tissue injury, during enzymatic processing, in microbial biotransformation, or in other specific biological environments.

(2) Spatial separation between glycosidases and substrates determines the timing of release

In natural systems, glycoside substrates and glycosidases are often spatially separated. For example, the substrate may be stored in vacuoles or secretory structures, whereas the enzyme may be localized in the cytoplasm, cell wall space, or other compartments. Once tissue rupture, stress induction, or metabolic-state changes disrupt this separation, functional release can be triggered.

5.2 Deglycosylation does not necessarily terminate at the aglycone state

(1) Removal of the sugar moiety may restore or enhance the original activity

For many flavonoids, phenolic natural products, and some anthraquinone compounds, removal of the sugar moiety enhances the ability of the aglycone to bind hydrophobic targets, and antioxidant, membrane-interaction, or enzyme-inhibitory activity may increase. Therefore, the deglycosylation process often constitutes an important prerequisite for activity release.

(2) After deglycosylation, the aglycone may enter new metabolic branches

Certain aglycones do not remain stable for long after release, but may rapidly undergo oxidation, reduction, methylation, sulfation, or further cleavage. Therefore, the true meaning of functional release lies not merely in deglycosylation itself, but in the overall local fate of the released aglycone.

Table 2. Key Molecular Nodes in Glycosylation and Functional Release of Natural Products

Process Stage	Key Enzyme or Molecule	Main Role	Molecular Consequence	Functional Significance
Sugar-donor formation	UDP-sugar-related metabolic enzymes	Provide activated sugar donors	Determine the source of glycosylation substrates	Constrain sugar-residue type
Glycosylation formation	Glycosyltransferases	Attach sugar moieties to aglycones	Generate glycosylated products	Alter solubility and storage form
Compartmental transport	Transport proteins / membrane transport systems	Mediate directed distribution of glycosides	Generate tissue or organelle accumulation	Achieve spatial isolation
Deglycosylation	Glycosidases	Hydrolyze glycosidic bonds	Release aglycones	Trigger functional expression
Subsequent transformation	Oxidases, reductases, conjugating enzymes, etc.	Further metabolize aglycones	Generate new metabolite spectra	Reshape final functional output

6. Patterns of Glycosylation and Functional Release in Representative Classes of Natural Products

6.1 In flavonoid natural products, glycosylation often determines absorption and activity profiles

(1) Flavonoid glycosides are often the more common in vivo form in plants

Aglycones such as quercetin, kaempferol, and luteolin commonly occur in plants as glucosides, rhamnosides, or rutinosides. Glycosylation makes them more suitable for storage and transport, but usually reduces their capacity for direct interaction with hydrophobic targets.

(2) Deglycosylation often constitutes an important prerequisite for activity remodeling

In enzymatic hydrolysis, fermentation, or other biotransformation systems, flavonoid glycosides are converted into aglycones after sugar removal, often showing greater membrane permeability and stronger biological activity in certain assays. Therefore, functional studies of flavonoids should not be limited to comparison of glycosylated versus non-glycosylated states, but should instead analyze the structure-activity transition before and after deglycosylation.

6.2 In saponin natural products, the glycan chain is both an activity-regulating and toxicity-regulating unit

(1) The glycan chain determines interfacial activity and membrane-interaction characteristics

In triterpenoid and steroidal saponins, glycan-chain structure directly affects interactions with membrane cholesterol, proteins, and interfacial systems. Differences in sugar-chain number, linkage sequence, and terminal sugar type can all lead to substantial differences in hemolytic activity, emulsifying properties, and pharmacological behavior.

(2) Deglycosylation often changes the activity spectrum rather than simply the activity intensity

Unlike many flavonoids, removal of sugar chains from saponins does not necessarily lead to a simple increase in activity. More commonly, it results in a shift in activity pattern, such as reduced membrane-disruptive capacity but enhanced specific binding, or reduced overall irritancy while target effects are retained. Therefore, saponin natural products are better understood from the perspective of glycan-chain regulation of activity spectra.

6.3 In anthraquinone glycosides and phenolic glycosides, sugar moieties often serve a stable storage function

(1) Glycosylation helps reduce local irritancy and reactivity

Anthraquinone aglycones often possess relatively high reactivity and some degree of irritancy. After glycosylation, they are better suited for stable storage. Many anthraquinone glycosides therefore function in natural tissues not as immediate effectors, but as precursor reserve forms.

(2) Functional release is often associated with enzymatic hydrolysis or biotransformation conditions

Under enzymatic hydrolysis, microbial biotransformation, or other specific conditions, anthraquinone glycosides can release the corresponding aglycones, which may then further participate in redox and conjugation reactions. Therefore, functional studies of anthraquinones often need to encompass both the glycosylated storage phase and the deglycosylation-release phase.

7.Research Products Relevant to Studies of Glycosylation Modification and Functional Release in Natural Products

Name	CAS No.	Product Type	Application Stage	Key Use	Use Notes
UDP-glucose disodium salt	28053-08-9	Activated sugar donor	Glycosylation-reaction studies	Common sugar donor for in vitro glycosyltransfer reactions	Suitable for glycosyltransferase catalysis and site-selectivity studies
UDP-galactose sodium salt	137868-52-1	Activated sugar donor	Glycosylation-reaction studies	Used to study the effect of sugar-residue type on product structure	Suitable for comparative studies of sugar-residue type
p-Nitrophenyl-β-D-glucopyranoside	2492-87-7	Enzymatic substrate	Glycosidase-activity assays	Used for determination of β-glucosidase activity	Suitable for development of deglycosylation assay methods
β-Glucosidase	9001-22-3	Enzyme preparation	Deglycosylation studies	Hydrolyzes glycosidic bonds and releases aglycones	Suitable for glycoside-aglycone conversion studies
Rhamnosidase	37288-35-0	Enzyme preparation	Derhamnosylation studies	Cleaves rhamnose-related glycosidic bonds	Suitable for stepwise enzymatic hydrolysis of flavonoid diglycosides
Quercetin	117-39-5	Aglycone standard	Structure-activity analysis	Representative flavonoid aglycone for studying activity changes after deglycosylation	Suitable for paired comparison with quercitrin
Isoquercitrin	482-35-9	Glycoside standard	Glycosylation-state studies	Representative flavonoid glycoside for analysis of glycosylation effects	Suitable for glycoside-aglycone comparative studies
Rutin	153-18-4	Glycoside standard	Studies of multisugar natural products	Used to investigate the influence of diglycoside structure on solubility and activity	Suitable for flavonoid polyglycosylation model studies
Luteolin	491-70-3	Aglycone standard	Post-deglycosylation functional studies	Flavonoid aglycone model for analyzing activity release	Suitable for paired studies with its glucoside
Luteoloside	5373-11-5	Glycoside standard	Flavonoid glycoside studies	Used to analyze sugar-mediated masking and enzymatic release behavior	Suitable for flavonoid glycoside hydrolysis studies
Baicalein	491-67-8	Aglycone standard	Activity-release studies	Used to investigate activity changes after sugar removal	Suitable for combined analysis with baicalin
Baicalin	21967-41-9	Glycoside standard	Glycosylation-state studies	Used to analyze the influence of glycosylation on solubility and release behavior	Suitable for enzymatic hydrolysis and transport studies
Daidzein	486-66-8	Aglycone standard	Isoflavone studies	Used to study biological activity in the aglycone state	Suitable for paired comparison with daidzin
Daidzin	552-66-9	Glycoside standard	Isoflavone glycoside studies	Used to analyze the effects of glycosylation on metabolism and activity	Suitable for intestinal enzymatic hydrolysis studies
Glycyrrhetinic acid	471-53-4	Aglycone standard	Triterpenoid aglycone studies	Used to study changes in activity profile after deglycosylation	Suitable for comparison with glycyrrhizic acid
Glycyrrhizic acid	1405-86-3	Glycoside standard	Saponin glycan-chain studies	Used to analyze the relationship between glycan-chain length and interfacial activity	Suitable for saponin structure-function studies
Hesperidin	520-26-3	Glycoside standard	Flavonoid glycoside studies	Used to study the effects of glycosylation on solubility and enzymatic release	Suitable for citrus flavonoid model studies
Hesperetin	520-33-2	Aglycone standard	Activity-release studies	Aglycone model for analyzing reconstruction of biological activity after deglycosylation	Suitable for comparison with hesperidin
Naringin	10236-47-2	Glycoside standard	Flavonoid diglycoside studies	Used to study the influence of diglycosides on bitterness and conversion behavior	Suitable for enzymatic-pathway studies
Naringenin	480-41-1	Aglycone standard	Post-deglycosylation functional studies	Used to analyze changes in membrane permeability and activity after deglycosylation	Suitable for combined studies with naringin

Glycosylation modification and functional release in natural products together constitute a continuous molecular process jointly determined by glycosyl transfer, compartmental distribution, glycosidic-bond hydrolysis, and subsequent metabolism. Glycosylation confers structural features that make natural products more suitable for storage, transport, and conditional release, whereas deglycosylation enables these molecules to re-enter the stage of functional expression under appropriate spatiotemporal conditions. Research on this process should not remain at the static level of asking whether a molecule is glycosylated or deglycosylated, but should instead be extended to the continuous mechanistic chain defined by sugar-residue type, attachment site, enzymatic nodes, release environment, and downstream metabolic fate. Only within this framework can the molecular basis of glycosylation modification and functional release in natural products be understood with greater accuracy.

For more related articles, please see below:

[1] Molecular Mechanisms of Glycosylation-Mediated Regulation of Cell Surface Signaling