Glycorandomization

Concept

Glycorandomization, literally means “diversify the sugar-containing compounds”, is a tool currently used in pharmaceutical industry to modify the sugar residues of the glycosylated natural products by using unique glycosylation strategies. It’s an efficient and convenient way to achieve libraries of natural product derivatives which only differ in the attached sugars.

Classification

The traditional method to obtain the natural product analogues is by total synthesis via laborious and time consuming protection and deprotection. To generate the analogues more efficiently, recently, chemists have developed two complementary strategies to achieve glycorandomization: chemoenzymatic glycorandomization and neoglycorandomization. These two methods do not need any protected sugars and aglycons. Both of them start with unprotected and unactivated reducing sugars and end with a library of sugars attached to the same aglycon.

Chemoenzymatic glycorandomization

This is a biocatalytic approach which relies on the promiscuous enzymes to catalyze the coupling between sugars and aglycons. The three enzymes involved in this strategy are anomeric kinase, nucleotidyltransferase and glycosyltransferase. 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 library, NDP sugar library and glycosylated sugar library. Modifying any one of these three enzymes will eventually lead to final glycorandomization of the natural products. (Figure adapted from 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.

Neoglycorandomization

This is a chemical approach using one step ligation between reducing sugar and secondary alkoxylamine containing aglycon to form glycosides which are termed as “neoglycosides”. In this chemoselective ligation, two reactive functional groups selectively form covalent bond to produce 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.

Comparison

Both chemoenzymatic glycorandomization and neoglycorandomization do not need 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. Without enzymes involved, the chemical approach is only limited by the efficiency to install the alkoxylamine into the aglycon. However, the anomeric selectivity of the chemical approach might be scrambled in some cases, depending on the structure of the involved reducing sugars.

Uses

Glycorandomization is widely used in pharmaceutical industry to alter the glycosylation patterns of the sugar-containing natural products. It provides a fast way to investigate the effect of subtle sugar modification towards the pharmacological properties of the natural products analogues, thus, afford a new alternative to the future drug discovery.