Glycorandomization
Encyclopedia
Glycorandomization, which literally means “diversification of sugar-containing compounds”, is a tool currently used in the pharmaceutical industry to modify the sugar residues of the glycosylated natural products by using unique glycosylation
strategies. It is an efficient and convenient way to prepare libraries of natural product
derivatives that only differ in the attached sugars.
via laborious and time consuming protection and deprotection steps. To generate analogues more efficiently, chemists have recently developed two complementary strategies to achieve glycorandomization: chemoenzymatic glycorandomization and neoglycorandomization. These two methods do not require protection of the sugar or aglycone
. Both methods start with unprotected and unactivated reducing sugar
s, and end with a library of sugars attached to the same aglycone
.
, that relies on the promiscuous enzymes to catalyze the coupling between sugars and aglycons. The three enzymes involved in this strategy possess anomeric kinase
, nucleotidyltransferase
and glycosyltransferase
activities. Altering the substrate specificities of these enzymes will enhance their promiscuity
to catalyze additional reactions, which leads to diverse randomized sugar libraries, like sugar phosphate libraries, NDP sugar libraries and glycosylated sugar libraries. Modifying any one of these three enzymes will eventually lead to final glycorandomization of the natural products. (Figure adapted from reference)
Currently available strategies to alter the substrate specificities include directed evolution
and structure-based engineering. Directed evolution takes advantage of natural selection to produce the desired promiscuous enzyme mutant via mutation
, recombination
, screening and selection. While structure-based engineering modifies the active site
pocket of the wild type enzyme to allow it accommodate unnatural substrates, thus, enhance the substrate flexibility of the enzyme.
between a reducing sugar
and a secondary alkoxylamine containing aglycon to form glycoside
s that are termed as “neoglycosides”. In this chemoselective ligation, the two reactive functional groups selectively form a covalent bond to produce an oxy-iminium intermediate, which then undergoes ring closure immediately under physiological conditions. In this strategy, the only requirement is the installation of the alkoxylamine into the aglycon before glycosylation. (Figure adapted from reference)
ic selectivity of the chemical approach might be scrambled in some cases, depending on the structure of the involved reducing sugars.
Glycosylation
Glycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
strategies. It is an efficient and convenient way to prepare libraries of natural product
Natural product
A natural product is a chemical compound or substance produced by a living organism - found in nature that usually has a pharmacological or biological activity for use in pharmaceutical drug discovery and drug design...
derivatives that only differ in the attached sugars.
Classification
The traditional method for obtaining natural product analogues is by total synthesisTotal synthesis
In organic chemistry, a total synthesis is, in principle, the complete chemical synthesis of complex organic molecules from simpler pieces, usually without the aid of biological processes. In practice, these simpler pieces are commercially available in bulk and semi-bulk quantities, and are often...
via laborious and time consuming protection and deprotection steps. To generate analogues more efficiently, chemists have recently developed two complementary strategies to achieve glycorandomization: chemoenzymatic glycorandomization and neoglycorandomization. These two methods do not require protection of the sugar or aglycone
Aglycone
An aglycone is the non-sugar compound remaining after replacement of the glycosyl group from a glycoside by a hydrogen atom. The spelling aglycon is sometimes encountered .Classes of phytochemicals found in the aglycone and glycosides forms :...
. Both methods start with unprotected and unactivated reducing sugar
Reducing sugar
A reducing sugar is any sugar that either has an aldehyde group or is capable of forming one in solution through isomerisation. This functional group allows the sugar to act as a reducing agent, for example in the Tollens' test or Benedict's test.-Chemistry:...
s, and end with a library of sugars attached to the same aglycone
Aglycone
An aglycone is the non-sugar compound remaining after replacement of the glycosyl group from a glycoside by a hydrogen atom. The spelling aglycon is sometimes encountered .Classes of phytochemicals found in the aglycone and glycosides forms :...
.
Chemoenzymatic glycorandomization
This is a biocatalytic approach, based on enzymatic glycosylationGlycosylation
Glycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
, that relies on the promiscuous enzymes to catalyze the coupling between sugars and aglycons. The three enzymes involved in this strategy possess anomeric kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
, nucleotidyltransferase
Nucleotidyltransferase
Nucleotidyltransferases are transferase enzymes of phosphorus-containing groups, e.g., substituents of nucleotidylic acids or simply nucleoside monophosphates...
and glycosyltransferase
Glycosyltransferase
Glycosyltransferases are enzymes that act as a catalyst for the transfer of a monosaccharide unit from an activated nucleotide sugar to a glycosyl acceptor molecule, usually an alcohol....
activities. Altering the substrate specificities of these enzymes will enhance their promiscuity
Promiscuity
In humans, promiscuity refers to less discriminating casual sex with many sexual partners. The term carries a moral or religious judgement and is viewed in the context of the mainstream social ideal for sexual activity to take place within exclusive committed relationships...
to catalyze additional reactions, which leads to diverse randomized sugar libraries, like sugar phosphate libraries, NDP sugar libraries and glycosylated sugar libraries. Modifying any one of these three enzymes will eventually lead to final glycorandomization of the natural products. (Figure adapted from reference)
Currently available strategies to alter the substrate specificities include directed evolution
Directed evolution
thumb|250px|right|An example of a possible round to evolve a protein based fluorescent sensor for a specific analyte using two consecutive FACS sortings...
and structure-based engineering. Directed evolution takes advantage of natural selection to produce the desired promiscuous enzyme mutant via mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
, recombination
Genetic recombination
Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...
, screening and selection. While structure-based engineering modifies the active site
Active site
In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...
pocket of the wild type enzyme to allow it accommodate unnatural substrates, thus, enhance the substrate flexibility of the enzyme.
Neoglycorandomization
This is a chemical approach using a one step ligationLigation
Ligation may refer to:* In molecular biology, the covalent linking of two ends of DNA molecules using DNA ligase* In medicine, the making of a ligature * Chemical ligation, the production of peptides from amino acids...
between a reducing sugar
Reducing sugar
A reducing sugar is any sugar that either has an aldehyde group or is capable of forming one in solution through isomerisation. This functional group allows the sugar to act as a reducing agent, for example in the Tollens' test or Benedict's test.-Chemistry:...
and a secondary alkoxylamine containing aglycon to form glycoside
Glycoside
In chemistry, a glycoside is a molecule in which a sugar is bound to a non-carbohydrate moiety, usually a small organic molecule. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. These can be activated by enzyme...
s that are termed as “neoglycosides”. In this chemoselective ligation, the two reactive functional groups selectively form a covalent bond to produce an oxy-iminium intermediate, which then undergoes ring closure immediately under physiological conditions. In this strategy, the only requirement is the installation of the alkoxylamine into the aglycon before glycosylation. (Figure adapted from reference)
Comparison
Both chemoenzymatic glycorandomization and neoglycorandomization do not require any protected sugars and aglycons. However, the biocatalytic approach is limited by the availability of either the promiscuous enzymes or the engineering techniques to alter the substrate specificities of enzymes. In contrast, the chemical approach is only limited by the efficiency of installing the alkoxylamine into the aglycon. However, the anomerAnomer
In carbohydrate chemistry, an anomer is a special type of epimer. It is one of two stereoisomers of a cyclic saccharide that differs only in its configuration at the hemiacetal or hemiketal carbon, also called the anomeric carbon. Anomerization is the process of conversion of one anomer to the other...
ic selectivity of the chemical approach might be scrambled in some cases, depending on the structure of the involved reducing sugars.