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.[1][2][3]
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The traditional method for obtaining natural product analogues is by total synthesis 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 sugars, and end with a library of sugars attached to the same aglycone.
This is a biocatalytic approach, based on enzymatic glycosylation, 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.[2] Modifying any one of these three enzymes will eventually lead to final glycorandomization of the natural products. (Figure adapted from reference[2])
Currently available strategies to alter the substrate specificities include directed evolution and structure-based engineering. Directed evolution[4] takes advantage of natural selection to produce the desired promiscuous enzyme mutant via mutation, recombination, screening and selection. While structure-based engineering[5] modifies the active site pocket of the wild type enzyme to allow it accommodate unnatural substrates, thus, enhance the substrate flexibility of the enzyme.
This is a chemical approach using a one step ligation between a reducing sugar and a secondary alkoxylamine containing aglycon to form glycosides that are termed as “neoglycosides”.[2][6] 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[2])
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 anomeric selectivity of the chemical approach might be scrambled in some cases, depending on the structure of the involved reducing sugars.
Glycorandomization is used in the pharmaceutical industry and academic community to alter glycosylation patterns of sugar-containing natural products. It provides a fast way to investigate the effect of subtle sugar modification on the pharmacological properties of the natural products analogues, thus, affording a new approach to drug discovery.